Prestressed Concrete Transmission Pole Structures: Recommended Practice for Design and Installation 9780784412114

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ASCE Manuals and Reports on Engineering Practice No. 123

Prestressed Concrete Transmission Pole Structures Recommended Practice for Design and Installation Prepared by the Task Committee on Concrete Transmission Pole Structures of the Committee of Electrical Transmission Structures of the Structural Engineering Institute of the American Society of Civil Engineers Edited by Wesley J. Oliphant, P.E., and Douglas C. Sherman, P.E.

Library of Congress Cataloging-in-Publication Data Prestressed concrete transmission pole structures: recommended practice for design and installation / prepared by the Task Committee on Concrete Transmission Pole Structures, of the Committee of Electrical Transmission Structures, of the Structural Engineering Institute, of the American Society of Civil Engineers; edited by Wesley J. Oliphant, Douglas C. Sherman. p. cm. — (ASCE manuals and reports on engineering practice; no. 123) Includes bibliographical references and index. ISBN 978-0-7844-1211-4 (pbk. : alk. paper) — ISBN 978-0-7844-7679-6 (ebook) 1. Electric lines—Poles and towers—Design and construction. 2. Electric lines—Poles and towers—Installation. 3. Prestressed concrete poles—Design and construction. 4. Prestressed concrete poles—Installation. I. Oliphant, Wesley J. II. Sherman, Douglas C. III. Structural Engineering Institute. Task Committee on Concrete Pole Structures. TA683.5.P65P74 2012 621.319′22—dc23 2012010308 Published by American Society of Civil Engineers 1801 Alexander Bell Drive Reston, Virginia 20191 www.asce.org/pubs Any statements expressed in these materials are those of the individual authors and do not necessarily represent the views of ASCE, which takes no responsibility for any statement made herein. No reference made in this publication to any specific method, product, process, or service constitutes or implies an endorsement, recommendation, or warranty thereof by ASCE. The materials are for general information only and do not represent a standard of ASCE, nor are they intended as a reference in purchase specifications, contracts, regulations, statutes, or any other legal document. ASCE makes no representation or warranty of any kind, whether express or implied, concerning the accuracy, completeness, suitability, or utility of any information, apparatus, product, or process discussed in this publication, and assumes no liability therefor. This information should not be used without first securing competent advice with respect to its suitability for any general or specific application. Anyone utilizing this information assumes all liability arising from such use, including but not limited to infringement of any patent or patents. ASCE and American Society of Civil Engineers—Registered in U.S. Patent and Trademark Office. Photocopies and permissions. Permission to photocopy or reproduce material from ASCE publications can be obtained by sending an e-mail to [email protected] or by locating a title in ASCE’s online database (http://cedb.asce.org) and using the “Permission to Reuse” link. Cover photographs courtesy of Valmont Newmark. Reproduced with permission. Copyright © 2012 by the American Society of Civil Engineers. All Rights Reserved. ISBN 978-0-7844-1211-4 (paper) ISBN 978-0-7844-7679-6 (e-book) Manufactured in the United States of America. 18

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MANUALS AND REPORTS ON ENGINEERING PRACTICE (As developed by the ASCE Technical Procedures Committee, July 1930, and revised March 1935, February 1962, and April 1982) A manual or report in this series consists of an orderly presentation of facts on a particular subject, supplemented by an analysis of limitations and applications of these facts. It contains information useful to the average engineer in his or her everyday work, rather than findings that may be useful only occasionally or rarely. It is not in any sense a “standard,” however; nor is it so elementary or so conclusive as to provide a “rule of thumb” for nonengineers. Furthermore, material in this series, in distinction from a paper (which expresses only one person’s observations or opinions), is the work of a committee or group selected to assemble and express information on a specific topic. As often as practicable, the committee is under the direction of one or more of the Technical Divisions and Councils, and the product evolved has been subjected to review by the Executive Committee of the Division or Council. As a step in the process of this review, proposed manuscripts are often brought before the members of the Technical Divisions and Councils for comment, which may serve as the basis for improvement. When published, each work shows the names of the committees by which it was compiled and indicates clearly the several processes through which it has passed in review in order that its merit may be definitely understood. In February 1962 (and revised in April 1982) the Board of Direction voted to establish a series titled, “Manuals and Reports on Engineering Practice,” to include the Manuals published and authorized to date, future Manuals of Professional Practice, and Reports on Engineering Practice. All such Manual or Report material of the Society would have been refereed in a manner approved by the Board Committee on Publications and would be bound, with applicable discussion, in books similar to past Manuals. Numbering would be consecutive and would be a continuation of present Manual numbers. In some cases of reports of joint committees, bypassing of Journal publications may be authorized.

MANUALS AND REPORTS ON ENGINEERING PRACTICE CURRENTLY AVAILABLE No.

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Hydrology Handbook, Second Edition How to Select and Work Effectively with Consulting Engineers: Getting the Best Project, 2012 Edition Planning and Design Guidelines for Small Craft Harbors, Revised Edition Sedimentation Engineering, Classic Edition Gravity Sanitary Sewer Design and Construction, Second Edition Existing Sewer Evaluation and Rehabilitation, Third Edition Structural Plastics Selection Manual Wind Tunnel Studies of Buildings and Structures Agricultural Salinity Assessment and Management, Second Edition Quality in the Constructed Project: A Guide for Owners, Designers, and Constructors, Third Edition Guidelines for Electrical Transmission Line Structural Loading, Third Edition Design and Construction of Urban Stormwater Management Systems Steel Penstocks, Second Edition Guidelines for Cloud Seeding to Augment Precipitation, Second Edition Quality of Ground Water: Guidelines for Selection and Application of Frequently Used Methods Design of Guyed Electrical Transmission Structures Manhole Inspection and Rehabilitation, Second Edition Inland Navigation: Locks, Dams, and Channels Guide to Improved Earthquake Performance of Electric Power Systems Hydraulic Modeling: Concepts and Practice Conveyance of Residuals from Water and Wastewater Treatment Environmental Site Characterization and Remediation Design Guidance

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Groundwater Contamination by Organic Pollutants: Analysis and Remediation Underwater Investigations: Standard Practice Manual Design Guide for FRP Composite Connections Guide to Hiring and Retaining Great Civil Engineers Recommended Practice for FiberReinforced Polymer Products for Overhead Utility Line Structures Animal Waste Containment in Lagoons Horizontal Auger Boring Projects Ship Channel Design and Operation Pipeline Design for Installation by Horizontal Directional Drilling Biological Nutrient Removal (BNR) Operation in Wastewater Treatment Plants Sedimentation Engineering: Processes, Measurements, Modeling, and Practice Reliability-Based Design of Utility Pole Structures Pipe Bursting Projects Substation Structure Design Guide Performance-Based Design of Structural Steel for Fire Conditions Pipe Ramming Projects Navigation Engineering Practice and Ethical Standards Inspecting Pipeline Installation Belowground Pipeline Networks for Utility Cables Buried Flexible Steel Pipe: Design and Structural Analysis Trenchless Renewal of Culverts and Storm Sewers Safe Operation and Maintenance of Dry Dock Facilities Sediment Dynamics upon Dam Removal Prestressed Concrete Transmission Pole Structures: Recommended Practice for Design and Installation

50 54 60 62 66 67 71 73 74 77 79 81 85 91 92 94 96 97 98 99

101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123

COMMITTEE ON ELECTRICAL TRANSMISSION STRUCTURES TASK COMMITTEE ON CONCRETE POLE STRUCTURES

Cochairs:

Wesley J. Oliphant, P.E.

Douglas C. Sherman, P.E.

Members:

Frank W. Agnew, P.E. Melanie Bragdon, P.E. Byron E. Chandler, P.E. Dana R. Crissey, P.E. William Y. Ford, P.E., Vice Chair Fouad H. Fouad, Ph.D., P.E. Meihuan Z. Fulk, Ph.D., P.E. Bryan J. Hanft, P.E. Jaber K. Jaber

Paul M. Legrand II, P.E. Herbert H. Payne Jr., P.E. Archie D. Pugh, P.E. David H. Seligson, P.E. Kenneth L. Sharpless, P.E., Secretary David D. Villarreal, P.E. John L. Webb, P.E. C. Jerry Wong, Ph.D., P.E.

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BLUE RIBBON REVIEW PANEL

Jon M. Ferguson, P.E., POWER Engineers, Inc. John P. Gervais, P.E., Valmont-Newmark Paul C. Jakob, P.E., Progress Energy Florida Robert Mitchell (Mitch) Currah, P.E., Lower Colorado River Authority (Retired) Garold D. Oberlender, Ph.D, P.E., Oklahoma State University Mahesh Pandey, University of Waterloo Maria Anna Polak, Ph.D., P.Eng., University of Waterloo Morris Stover, P.E., Kiewit Power Engineers Co. F. Blake Tucker, P.E., American Electric Power Kenneth Wright, P.E., Tucson Electric Power Co.

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CONTENTS

PREFACE ..................................................................................................... xiii 1

STRUCTURAL CONFIGURATIONS AND POLE APPLICATIONS ...................................................................... Configurations ...................................................................................... Electrical Utility Applications ............................................................ References..............................................................................................

1 2 7 13

2

INITIAL CONSIDERATIONS ......................................................... Physical Characteristics ...................................................................... Loads ...................................................................................................... Deflection .............................................................................................. Transportation and Erection............................................................... Attachments .......................................................................................... Guying ................................................................................................... Climbing and Maintenance ................................................................ Grounding ............................................................................................. Testing .................................................................................................... References..............................................................................................

15 15 15 16 16 16 17 18 19 20 21

3

MATERIALS ........................................................................................ Concrete Materials ............................................................................... Properties of Concrete ......................................................................... Steel Reinforcement ............................................................................. Miscellaneous Materials...................................................................... References..............................................................................................

23 23 24 24 27 28

4

DESIGN ................................................................................................ Design Considerations ........................................................................ Multisection Concrete Poles ............................................................... Hybrid (Steel and Concrete) Poles ....................................................

31 32 34 36

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CONTENTS

Reverse Taper Poles ............................................................................. Concrete Pole Design as Related to Wood Pole Equivalency ...... Design Criteria ..................................................................................... Design Methodology ........................................................................... References..............................................................................................

37 38 39 42 48

5

CONNECTIONS ................................................................................. Methods of Connection....................................................................... Connection Design Considerations................................................... Connection Failure Modes ................................................................. Installation Considerations................................................................. References..............................................................................................

51 53 59 63 67 67

6

FOUNDATIONS ................................................................................. Design Considerations ........................................................................ Foundation Types ................................................................................ Guy Wires and Anchors...................................................................... References..............................................................................................

69 69 70 72 73

7

MANUFACTURING AND QUALITY ASSURANCE ................ Design and Drawings.......................................................................... Manufacturing Process........................................................................ Quality Assurance................................................................................ References..............................................................................................

75 75 76 80 83

8

ASSEMBLY AND ERECTION ......................................................... 85 Hauling and Access ............................................................................. 85 Handling................................................................................................ 88 Framing ................................................................................................. 88 Field Drilling......................................................................................... 91 Field Cutting ......................................................................................... 93 Erection .................................................................................................. 93 Weight Considerations ........................................................................ 99 Climbing ................................................................................................ 99 Storage ................................................................................................... 99 References.............................................................................................. 100

9

INSPECTION, MAINTENANCE, AND REPAIR ........................ Inspection .............................................................................................. Maintenance and Repair ..................................................................... References..............................................................................................

101 101 103 107

10 STRUCTURE TESTING .................................................................... 109 Foundations and Anchors .................................................................. 110 Material.................................................................................................. 111 Manufacture .......................................................................................... 111 Assembly and Erection ....................................................................... 112

CONTENTS

Test Loads.............................................................................................. Load Application ................................................................................. Loading Procedure .............................................................................. Load Measurement .............................................................................. Deflection .............................................................................................. Failures .................................................................................................. Report .................................................................................................... Special Considerations for Horizontal Testing ............................... APPENDIX I SAMPLE PURCHASER TECHNICAL SPECIFICATIONS FOR SPUN-CAST PRESTRESSED CONCRETE POLES FOR TRANSMISSION AND DISTRIBUTION STRUCTURES ..................................................... APPENDIX II SAMPLE PURCHASER TECHNICAL SPECIFICATIONS FOR STATIC-CAST PRESTRESSED CONCRETE POLES FOR TRANSMISSION AND DISTRIBUTION STRUCTURES ..................................................... APPENDIX III ADDITIONAL INFORMATION FOR PURCHASER’S SPECIFICATION FOR STATIC- AND SPUNCAST PRESTRESSED CONCRETE POLES FOR TRANSMISSION AND DISTRIBUTION STRUCTURES ....... APPENDIX IV METHODOLOGY FOR SELECTING AN APPROPRIATE CONCRETE COMPRESSIVE STRENGTH TO BE USED IN THE DESIGN OF CONCRETE POLES .........

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112 112 113 113 113 114 115 115

119

131

143

149

GLOSSARY ................................................................................................. 151 NOTATION ................................................................................................. 155 INDEX .......................................................................................................... 157

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PREFACE

The purpose of this manual is to provide the reader with a basic knowledge of the principles and methods for the design, manufacturing, and use of prestressed spun- and static-cast concrete poles for overhead utility line structures. This manual is the result of a multiyear collaborative effort by engineers from electric utilities, consulting firms, and manufacturers engaged in the design and application of these structures. Since the publication in 1987 of the ASCE Guide for the Design and Use of Concrete Poles and the 1994 ASCE-PCI Committee Report “Guide for the Design of Prestressed Concrete Poles,” significant advancements and innovations have been realized in concrete pole design and manufacturing technologies. These advancements have propelled the use of concrete poles into an ever-increasing and significant role in the design and construction of overhead utility line structures. Technological advances have occurred in three key areas. First, the types and quality of the raw materials used in the production of highperformance concrete have improved dramatically. Second, advanced manufacturing methods and equipment to produce high-quality, stronger, and longer length poles are being used. Third, with meaningful research and development (R&D) investment in the technology, significant and innovative enhancements to the engineering design technology of concrete poles are being developed. In addition to their use in distribution, subtransmission, or wood replacement applications, concrete poles are being designed and used in major transmission line projects up to and including 345kV and 500kV transmission lines. Following is a brief summary of this manual’s content: Structural Configurations and Pole Applications: A variety of structural and phase wire configurations are needed by utilities to address different power line environments. A number of arrangements are

xiii

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PREFACE

discussed and illustrated for common applications using prestressed concrete poles. Initial Considerations: The purchaser should provide design specifications to assure the manufacturer designs and supplies a product meeting the intended need. Physical characteristics, structural loads, deflection limits, attachments, guying, climbing, grounding, transportation, construction environment, maintenance considerations, and more are discussed to assist in development of pole specifications. Materials: A prestressed concrete pole is primarily high-strength concrete reinforced with prestressed and static steel components. Various other materials are used as attachments for supporting the power lines, grounding, climbing, and more. A structural material criterion is provided. Design: More in-depth information and methodology for prestressed concrete pole design related to loads, connections, multipiece pole consideration, foundation type, material strength, deflection, capacity, linear, and nonlinear behavior is presented. Connections: Properly configuring and designing attachment connections are critical to the design of a prestressed concrete pole. Conductor arms, brackets, insulators, guy attachments, grounding, and climbing provisions are some of the normal connection items requiring design. Many types of connections are illustrated along with further discussion on design considerations. Foundations: Most prestressed concrete poles are directly embedded into the ground and backfilled. However, other options may be required based on the need or environment. These types of foundations are explained and design considerations discussed. Manufacturing and Quality Assurance: Many of the basic principles related to manufacturing are very similar for both static-cast and spun-cast prestressed concrete poles. Production, tolerances, materials, concrete mix, curing, finishing, and quality assurance processes are outlined. Assembly and Erection: Common practice is for concrete poles to be delivered full length. The pole length can exceed 100 ft or more. Some poles may need to be assembled if they are multipiece or framed structures. Handling, hauling, storing, assembling, erecting, climbing, construction environment, and other considerations are addressed. Inspection, Maintenance, and Repair: Inspection of poles is important during the construction phase and through their service life. This portion of the manual discusses the common points of inspection, maintenance aspects, and repair considerations. Structure Testing: A structural load test may be required to verify a pole design. This portion of the manual focuses on test facility

PREFACE

xv

procedures, structure test configurations, loading and deflection criteria, and application and reporting requirements. Supplements: This manual includes an Appendix section with sample purchaser technical specifications for both static- and spun-cast prestressed concrete poles and a methodology for selecting an appropriate concrete compressive strength for design. Other reference pages include a glossary and nomenclature listing. Every effort has been made to ensure the accuracy and clarity of this information. The user is reminded that utility line structures are an integral part of a larger overhead line system. The user is therefore cautioned that the application of these structures should come only after sound engineering judgment has been applied. Furthermore, as an overall treatise covering a wide variety of line applications, this manual cannot conceivably comply with all conditions. The user should bear in mind that often there will be specific local conditions and requirements that may dictate design and usage conditions that differ from those described herein. The task committee is grateful for the dedicated effort of all who participated in the development of this manual. The task committee also wishes to acknowledge the excellent work of prior committees of both ASCE and PCI, which have served to provide an excellent foundation for the development of this manual. We also wish to extend our appreciation to the following contributors for the many photos, illustrations, graphs, and tables that further enhance the value of this manual for the reader. Contributors include Alabama Power Company, American Electric Power Company, CenterPoint Energy, Georgia Transmission Company, University of Alabama–Birmingham, and Valmont Newmark.

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1 STRUCTURAL CONFIGURATIONS AND POLE APPLICATIONS

The two types of prestressed concrete poles predominantly used in the electrical utility industry are centrifugally cast (commonly referred to as spun cast) and statically cast. Spun poles have a round or a polygonal cross section with a hollow core that is created by the spinning process. The size of the hollow core depends on the pole diameter and wall thickness. Statically cast poles have a square or polygonal cross section with a core that is solid or a hollow core formed by a retractable mandrel or a permanent form. Prestressed concrete poles are an engineered product for which unique designs and closely controlled manufacturing processes are performed. Although both centrifugally cast and statically cast poles are referred to as prestressed concrete poles, each has differences in performance, specification, and recommended use. These differences will be highlighted throughout the document. Recent technological advances in centrifugally cast products have produced multiple-section poles, including hybrid poles with concrete and tubular steel sections. These multiple-section poles, using slip-fit or bolted flange splices, make possible longer poles and greater versatility than can be attained by single-piece poles. A word of caution should be mentioned regarding the application of a prestressed pole in a different manner than the originally intended use. An example is a pole that was originally designed as a single-pole tangent structure but is now being considered for reuse as a small angle structure (with reduced wind span length perhaps), or a guyed dead-end structure, or an H-frame leg in which the original pole design may have insufficient embedment length, service load cracking capacity, or ultimate load capacity for the new application. It is therefore prudent that the Manufacturer 1

2

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

or someone with experience in the design of prestressed concrete poles verify the capability of a structure to perform in the new application. Prestressed concrete poles are primarily used in one of the following four structural configurations: • • • •

cantilevered; guyed; framed; or combined.

Electrical utility applications for these configurations include transmission line structures, distribution line structures, and substation structures. This chapter provides details of structural configurations and applications of prestressed concrete poles by electrical utilities.

CONFIGURATIONS Cantilevered Structures The most common use of concrete pole structures is cantilevered poles directly embedded in the earth or supported by a foundation. Typical cantilevered structures are shown in Fig. 1-1 through Fig. 1-4. They can

Fig. 1-1. Single circuit cantilevered braced line post tangent structure Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

STRUCTURAL CONFIGURATIONS AND POLE APPLICATIONS

3

Fig. 1-2. Single circuit cantilevered line post tangent structures Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

be single-pole or multipole structures depending on how the phase conductors are oriented. Cantilevered structures, often called self-supporting structures, are designed to withstand various combinations of vertical and horizontal loads as an unsupported beam-column. Although shear and torsion loads create stresses on the structure, the design of a cantilevered structure is generally controlled by the bending stresses caused by horizontal loads. Horizontal loads are usually the result of wire tension and wind forces on the structure, equipment, and wires. Eccentric vertical loads also contribute to bending stresses. Eccentric vertical loads can be caused by equipment and conductor loads and by the vertical load of the structure in a deflected state. Guyed Structures Another category of prestressed concrete structures is guyed structures. To reduce the bending stresses associated with cantilevered structures, steel wire guys can be installed to transmit the horizontal loads imposed on the structure to the ground. Although guys significantly reduce bending stresses in the structure, the vertical component of the

4

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 1-3. Single circuit cantilevered line post tangent structures Source: Photograph courtesy of Valmont Newmark. Reproduced with permission. guy force induces additional vertical loading on the pole. This vertical load and localized stress-concentrations near guy wire attachments should be considered in the design of the structure. (See ASCE Manuals and Reports on Engineering Practice No. 91, Design of Guyed Electrical Transmission Structures, for more information on this topic.) A guyed transmission structure is shown in Fig. 1-5. The multiple components of a guyed structure are typically analyzed as a system using three-dimensional structural analysis software. The guy wire size, orientation, pretension, and maximum allowable guy load should be specified to the structure Manufacturer. Framed Structures Framed structures are assembled from numerous structural members, including poles, crossarms, crossarm braces, and X-braces. Two or more

STRUCTURAL CONFIGURATIONS AND POLE APPLICATIONS

5

Fig. 1-4. Double circuit cantilevered braced post tangent structure with underbuild Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

poles are connected to one another by the members such that the poles and connecting members act as a moment-resisting system. Framed structures are configured in a manner that allows for horizontal loads to be transmitted to the ground through the total stiffness of the structural support system. The stiffness is achieved through use of the bracing members with pinned connections, moment-carrying connections, or a combination of the two. An example is a braced H-frame structure as shown in Fig. 1-6. However, it should be noted that the structure is cantilevered in the longitudinal direction, perpendicular to the plane of bracing. A symmetrical H-frame structure without bracing may be analyzed as a cantilevered structure, because the pinned pole-to-crossarm connection does not create a moment-resisting connection.

6

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 1-5. Single circuit guyed dead-end structure Source: Photograph courtesy of Georgia Transmission Corporation. Reproduced with permission.

Like guyed structures, framed structures are typically analyzed as a system using three-dimensional structural analysis software. The structure designer should determine the size, orientation, and connection details of all members in the frame. Combined Structures Structures may be designed as a combination of cantilevered, guyed, or framed structures. Examples include a single-pole structure guyed below the bottom conductor, an H-frame structure cantilevered above the crossarm, and an H-frame structure guyed at the bottom of the X-brace. The cantilever nature in the longitudinal direction of a braced H-frame structure can be changed with the addition of guys in this direction. Fig. 1-7 shows a combined structure that resist some loads through guy wires.

STRUCTURAL CONFIGURATIONS AND POLE APPLICATIONS

7

Fig. 1-6. Single circuit braced H-frame tangent structures Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

ELECTRICAL UTILITY APPLICATIONS Transmission and Distribution Structures The structural configurations previously mentioned lend themselves to several common transmission and distribution structure applications. In the past, single-pole, self-supporting, cantilevered structures have been used only for tangent and small line angle applications. However, in recent years, much greater strength capacities have been developed that allow large angle and dead-end structure applications. Many electrical utilities prefer to guy single poles for small and large line angle structures to utilize smaller pole sizes instead of using selfsupporting structures. In some cases, guying is required because the

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 1-7. Two-pole braced guyed dead-end structure Source: Photograph courtesy of Valmont Newmark. Reproduced with permission. loading from long span, large line angle, or dead-end applications exceeds the cantilever capacity of concrete poles. The structures are guyed at some or all of the conductor positions depending on the loading. This approach, however, is becoming more difficult to use in urban and suburban areas because of the cost and difficulty of obtaining guy easements. The braced H-frame is a common structure. The H-frame is often used for long, cross-country transmission lines. Frame action enables this structure to support large horizontal and vertical loads, resulting in longer span lengths and larger wire size than single-pole, cantilever structures. These structures can also be designed to carry significant longitudinal loading by cantilever or guying. H-frame structures are not typically used in urban and suburban areas, because they require a wide right-of-way corridor and create a large footprint. Combinations of cantilevered, guyed, and framed structures can be used to optimize the benefits of each type. Two examples of combined structures are guyed H-frame structures and dead-end structures that are guyed in one direction and self-supporting in the other direction. Guyed H-frames can be used to provide increased longitudinal capacity as an

STRUCTURAL CONFIGURATIONS AND POLE APPLICATIONS

9

anticascading structure in long cross-country lines. Large angle dead-end structures guyed only in one direction are used at locations where the wires on one side are full tension and the wires on the other side are reduced tension (e.g., first span out of a substation). See Fig. 1-8. Substation Structures Cantilevered structures are the most commonly used configurations in substation structures (see Fig 1-9). Bus support structures for higher voltages typically use a three-pole structure, one pole per phase. At 69 kV and below, a single-pole “tee” structure can support all three phases. Single or multipole structures can be used to support disconnect switches, lightning arrestors, potential and current transformers, wave traps, and other electrical equipment. The minimally deflecting cantilevered structures are well suited as switch support structures to meet the tight tolerances of most switches. Cantilevered and guyed structures are used to support

Fig. 1-8. Three-pole dead-end structures free standing in one plane and guyed in another Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 1-9. Concrete pole substation structures Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

shield wires for lightning protection. A three-pole or H-frame structure can be used for a terminal structure. Comparison of Concrete Pole Structures with Alternatives Table 1-1 provides a generalized, qualitative comparison of concrete electrical utility pole structures and common alternative structures. A plus symbol (+) indicates a favorable rating, a minus symbol (−) indicates an unfavorable rating, and a plus/minus symbol (+/−) indicates a neutral rating for a particular parameter. The comparison is made for typical direct embedded, tangent structures with a fixed right-of-way width. The ratings are based on the experience of the committee members. The specific application and experience of the individual user may vary. The following is a brief description of the parameters listed in Table 1-1:

STRUCTURAL CONFIGURATIONS AND POLE APPLICATIONS

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Table 1-1. Comparison of Concrete Pole Structures with Alternatives.

Parameter

1 2 3 4 5

6

7 8 9 10

11 12

13 14 15 16 17

18

Installation Cost Life Cycle Costs (50 years) Strength Capacity Custom Design Effort Structural Rigidity/ Switch Pole Application Saltwater/ Corrosive Environment Insect/Animal Vulnerability Environmental Impact Short Duration Fire Resistance Quality Control and Product Uniformity Delivery Lead Time Geographic Delivery and R-O-W Access Considerations Ease of Handling and Erection Effort Weight and Crane Requirements Field Drilling/ Modifications Inspection and Maintenance Effort Ease of Replacement/ Removal Disposal/Recycle at Replacement

Fiberglass Pole

Direct Embedded Steel Pole

Steel Lattice Tower with Grillage Foundation

+ −

− −

− +

− +

+/− +

− N/A

− +

+ +

+ −

+

−

−

+

+

+

−

+/−

−

−

+

−

+

+

+

+

−

+

+

+

+

−

−

+/−

+/−

+

−

+

+

+

+ −

+/− +

+/− +

− +/−

− +/−

+/−

+

+

+/−

−

−

+

+

+/−

+/−

+/−

+

+

+/−

−

+

−

+/−

+

+/−

+/−

+

+/−

+/−

−

+/−

−

+/−

+

+

Direct Embedded Concrete Pole

Wood Pole

+/− +

Notes: + Favorable; − Unfavorable; +/− Neutral; N/A Not Applicable

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

1. Installation Cost—The cost to engineer, procure, and install the structure 2. Life Cycle Costs—Costs associated with maintenance and replacement of the structure for a 50-year service life 3. Strength Capacity—The ability to manufacture a high-strength capacity structure 4. Custom Design Effort—The effort required to custom design a structure for a specific application 5. Structural Rigidity/Switch Pole Application—The ability to design and manufacture a rigid, low deflection structure for switch installation 6. Saltwater/Corrosive Environment—The ability of the structure to withstand a saltwater/corrosive environment (e.g., coastal or brackish water location, industrial contamination, or active soil conditions) 7. Insect/Animal Vulnerability—The ability of the structure to withstand insect and animal degradation (e.g., termite and woodpecker boring) 8. Environmental Impact—The impact the structure has to environmental quality (e.g., leaching of chemical treatment) 9. Short Duration Fire Resistance—The ability of the structure to withstand brushfire 10. Quality Control and Product Uniformity—Control of structure variation and defects 11. Delivery Lead Time—The time required for a structure manufacturer/provider to deliver the structure after receipt of order 12. Geographic Delivery and R-O-W Access Considerations—The ability to transport the structure to the project site; this is especially important when right-of-way (R-O-W) access is impeded by terrain or soil conditions 13. Ease of Handling and Erection Effort—The effort required to assemble and install the structure 14. Weight and Crane Requirements—The length and weight of the structure dictate the crane requirement 15. Field Drilling/Modifications—The ability to modify structure attachments (e.g., insulator assembly) in the field; this can be done by drilling or welding, as applicable 16. Inspection and Maintenance Effort—The effort required to inspect and maintain the condition of the structure 17. Ease of Replacement/Removal—The effort required to remove and replace an existing structure of the same material type 18. Disposal/Recycle at Replacement—The ease and cost of pole removal and disposal. Consideration to include potential re-use/ salvage value

STRUCTURAL CONFIGURATIONS AND POLE APPLICATIONS

13

REFERENCES ASCE. (1997). “Design of guyed electrical transmission structures.” ASCE Manual of Practice No. 91, Reston, VA.

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2 INITIAL CONSIDERATIONS

To allow the Manufacturer to perform the design function properly, the Purchaser should provide the following information in the design specifications. PHYSICAL CHARACTERISTICS The structure configuration should be made clear to the Manufacturer. Structures can be either single- or multipole. The information should include the pole heights above ground, pole embedment depths, and the location of all wire and equipment attachments. Splices may be necessary as a result of weight or length limitations. If the pole is to be guyed, the location, size, and material of the guy wires at both the pole attachment point and at the groundline should be supplied. Terrain features can sometimes result in long guy wires. Providing this information is best and most clearly communicated with a drawing. LOADS The magnitude and direction of all the loads imposed on the structure should be provided. These loads should comply with the requirements of the National Electrical Safety Code (ANSI 2007) and other applicable codes. Other wind and ice loads required by the Purchaser, and the dead weight loads of any attached equipment, should also be communicated. A load tree or load table in LVT (Longitudinal /Vertical /Transverse) format is the preferred method of supplying the load data. A detailed discussion of the types of design loads to be considered is presented in Chapter 4. 15

16

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

The point of structure fixity at or at some point below the groundline should be specified. Secondary loads imposed on the structure because of foundation rotation should also be examined. The type of foundation used will determine the degree of foundation rotation that should be considered. (Chapter 8 discusses foundations.)

DEFLECTION Any special deflection limitations and the conditions under which these requirements must be met should be provided. Deflection limitations generally are used to maintain proper clearances from the structure and its attachments to other objects and the edge of right-of-way easements. They also may be necessary because of required equipment tolerances, such as those on switches. Additionally, deflection limits are used for aesthetic reasons. A pole that has excessive deflection will appear to be bowed and could be aesthetically unpleasant. Deflections can be controlled by increasing the structure stiffness or by raking the structure. Increasing the structure stiffness to control deflection generally will increase the structure diameter and cost.

TRANSPORTATION AND ERECTION Weight of a concrete pole should be considered when evaluating its use. Thoughtful consideration should be given to how the structure sites will be accessed and how the poles will be delivered and erected. The Manufacturer should consider the loads imposed on the pole by the loading, unloading, hauling, assembly, and erection of the structure (see Fig. 2-1). The Manufacturer should clearly indicate on the pole the proper lifting points for the structure and should provide the user with any special handling procedures that are recommended during the shipment and erection of the structure. If single point pickup is required, this should be communicated clearly to the Manufacturer and may result in additional pole cost. (See Chapter 8 for more details.)

ATTACHMENTS Attachments to concrete poles can include but may not be limited to shield wires, insulators, and structural supports for conductors, guy wires, ground down lead clips, climbing and working ladder clips, supports for equipment (such as switches or distribution equipment), danger signs, and aerial markers.

INITIAL CONSIDERATIONS

17

Fig. 2-1. Flat bed truck unloading pole at the job site Source: Photograph courtesy of CenterPoint Energy. Reproduced with permission.

Location of attachments is driven by the overall functionality of the pole while maintaining all clearance requirements (see Fig. 2-2). The Manufacturer should be provided with hole locations, diameters, and hole patterns required to attach the insulators and other various pieces of equipment to the structure. (Chapter 5 shows the typical connection details for different types of component attachments.)

GUYING The Manufacturer should be provided with all pertinent information regarding the guying of the structure. Such information should include guy size, guy wire modulus of elasticity, rated breaking strength, stranding, maximum allowable load for anchors and guy quantity restrictions, guy angle limits, restrictions imposed by right-of-way constraints, and terrain conditions. (See ASCE [1997] for guidelines.)

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 2-2. Guy stub pole attachments, climbing and grounding Source: Photograph courtesy of Georgia Transmission Corp. Reproduced with permission. The Manufacturer should confirm the structural capacity of all guy attachments. The Manufacturer may suggest alternate guy wire sizes for the Purchaser’s consideration that will optimize the design of the pole. Factored or unfactored guy loads should be provided by the Manufacturer so that the Purchaser may check the anchor requirements.

CLIMBING AND MAINTENANCE The ability to access those areas of the pole where hardware is attached may necessitate the installation of climbing provisions. The two most common types of climbing attachments are steps and ladders. Step bolts for climbing are normally placed approximately 15 in. center to center (based on the normal stride of a lineman). Provisions for working at the attachment locations are provided by placing additional steps at an elevation that will provide easy access to the attachment or equipment. The location of the working steps should be approximately 4 ft. below the attachment. Ladders are attached to the pole using clips. The clips are bolted to the pole using preinstalled threaded inserts. Manufacturing constraints such as location of the spiral reinforcement may limit the location of the inserts. The Manufacturer and Purchaser should coordinate the final placement of the ladders. The steps and ladders should be located on the pole in such a way as to minimize climbing over other appurtenances such as insulators, vangs, and switches, as shown in Fig. 2-3. (See Chapter 9 for a more in-depth discussion on maintenance.)

INITIAL CONSIDERATIONS

19

GROUNDING The grounding of concrete poles can be external, internal, or both to the pole (see Fig. 2-4). When the pole is to be externally grounded, a threaded insert normally is provided for attaching the ground clamp. Internal grounds can be imbedded in the concrete or pulled through the center void as required. Purchasers often use a combination of internal grounding with an external ground pad near the groundline of the pole.

Fig. 2-3. Conductor insulator, bracing, climbing, and grounding attachments to the pole Source: Photograph courtesy of Georgia Transmission Corp. Reproduced with permission.

Fig. 2-4. Internal pole groundwire detail Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Specific grounding requirements should be clearly specified by Purchaser. The Manufacturer should provide an electrical bond between the pole’s internal steel reinforcing prestressed cage and the pole ground.

TESTING Full-scale structure testing may be necessary depending on special load cases, new design methodologies, legal obligations, or requirements of the Purchaser or Manufacturer. When structure load testing is required, the Manufacturer should be provided with the type and height of structure to be tested and the load cases to be examined. Two types of testing are used to determine the flexural behavior and capacity of the poles under static loading: pole testing and structure testing. Pole testing is used to verify the design and the quality of the production of the poles. In this test, poles normally are tested in the horizontal position (see Fig. 2-5). The test will verify the cracking moment, ultimate capacity, and deflection of the pole. Structure testing involves

Fig. 2-5. Horizontal structural concrete pole test Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

INITIAL CONSIDERATIONS

21

testing the framed structure in the actual position and configuration as designed. In this test the loads are applied incrementally and the structure behavior is monitored and recorded. A detailed discussion of structure testing is presented in Chapter 10.

REFERENCES American National Standards Institute (ANSI). (2007). C6 national electrical safety code, New York. ASCE. (1997). “Design of guyed electrical transmission structures.” ASCE Manual of Practice No. 91, Reston, VA.

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3 MATERIALS

Prestressed concrete poles are made of two main materials: concrete and steel reinforcement. This chapter provides information on the applicable standards and specifications of the various components and mechanical properties of the materials. Additional material components for grounding, climbing, and attachments are also discussed. CONCRETE MATERIALS Cement Portland cement should conform to ASTM C150 (ASTM 2011c). Coarse and Fine Aggregate (Sand) Aggregate should conform to ASTM C33 (ASTM 2011b). Maximum size aggregate should be 3/4 in., and not exceed three-fourths of the clear spacing between reinforcing steel and pole surface. Admixtures Water reducing, retarding, and accelerating admixtures should conform to ASTM C494 (ASTM 2011a). Water Water should be free from foreign materials in amounts harmful to concrete and embedded steel.

23

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

PROPERTIES OF CONCRETE Compressive Strength The minimum design 28-day concrete cylinder compressive strength f’c is 5,000 psi, with 6,000 to 12,000 psi strength being more common. The compressive strength of concrete is determined in accordance to ASTM C39 (ASTM 2004). Stress-Strain Behavior A typical stress-strain curve for concrete in compression is shown in Fig. 3.1. STEEL REINFORCEMENT Prestressing Steel Prestressed concrete poles are typically reinforced with either uncoated, stress relieved steel wire (ASTM [2010c], grade 250 or 270) or uncoated

Fig. 3-1. Typical compressive stress-strain curves of concrete Source: Illustration courtesy of University of Alabama at Birmingham. Reproduced with permission.

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25

Table 3-1. Material Properties for Prestressing Steel. Nominal diameter of stand (in)

Breaking strength of stand (min lb)

Nominal steel area of stand (in2)

1/4 (0.250) 5/16 (0.313) 3/8 (0.375) 7/16 (0.438) 1/2 (0.500) 3/5 (0.600)

Seven-Wire Strand GRADE 250, fpu = 250 ksi 9,000 0.036 122 14,500 0.058 197 20,000 0.080 272 27,000 0.108 367 36,000 0.144 490 54,000 0.216 737

7,650 12,300 17,000 23,000 30,600 45,900

3/8 (0.375) 7/16 (0.438) 1/2 (0.500) 3/5 (0.600)

Seven-Wire Strand GRADE 270, fpu = 270 ksi 23,000 0.085 290 31,000 0.115 390 41,300 0.153 520 58,600 0.217 740

19,550 26,350 35,100 49,800

Nominal weight of stands (lb per 1,000 ft)

Minimum Load at 1% extension (lb)

Modulus of Elasticity = 27,500,000 to 28,500,000 psi Source: Table courtesy of University of Alabama–Birmingham. Reproduced with permission.

low relaxation and stress relieved seven-wire strand (ASTM [2010b], grade 250 or 270). The steel is placed inside the form and stressed to the required tension. Strand is also available with coatings, such as epoxy, in accordance with ASTM A882 (ASTM 2010a). Galvanizing may also be used to provide protection in extremely corrosive environments. However, the galvanizing process may result in the prestressing steel having lower breaking strengths and a slightly lower modulus of elasticity. Increased development length should be considered for both epoxy coated and galvanized strands because of the reduction in bond strength. Mechanical properties of commonly used prestressing steel are presented in Table 3-1. Load-elongation curve for a strand is limited to 1% elongation as shown in Fig. 3-2. Supplemental Longitudinal Steel Reinforcement Additional nonprestressed longitudinal steel reinforcement may be needed at points along the length of the pole to increase the ultimate moment

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 3-2. Material properties for prestressing steel Source: Illustration courtesy of University of Alabama at Birmingham. Reproduced with permission. capacity. Deformed steel reinforcing bar (ASTM 2012) or nonprestressed strand (ASTM 2010b) is commonly used and should conform to the applicable ASTM standards. This supplemental longitudinal steel reinforcement is usually placed at the critical sections only and does not typically extend throughout the entire length of the pole. Consideration of the strength of the nonprestressed longitudinal reinforcing steel may be important, because mild steel will yield at strains much less than prestressed reinforcement. If deformed steel reinforcing bar is used and the structure undergoes large deflection, the pole may have nonrecoverable deformation. Spiral Reinforcement Spiral reinforcement surrounding the longitudinal reinforcement (strands and bars) helps to resist radial stresses caused by the wedging effect of the strand at release. It also can have a significant role in controlling or minimizing cracks attributable to torsion, shear, shrinkage, or temperature-induced stresses. The wedging effect from the release of the pretensioning forces causes tensile stresses at every cut-off strand location throughout the pole. Thus,

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27

along the length of transfer (about 50 times the strand diameter), strands produce radial pressure against the surrounding concrete, which could develop longitudinal cracks unless properly contained by adequate spiral reinforcement. The spiral reinforcement generally conforms to ASTM A82 (ASTM 2007). Its size should be in the range of No. 5 to No. 11 gauge wire, depending on the pole use and size. The minimum area of spirals should be computed as 0.1% of the concrete wall area in a unit length increment. More spiral reinforcement is required at the tip and butt segments of the pole to resist the radial stresses that occur at transfer of prestress. Other areas of high shear stress, such as guy wire or other attachments and near the groundline, may need additional spiral reinforcement. The minimum clear spacing of spiral is fourthirds of the maximum size of coarse aggregate and should not be less than 1 in. The maximum center-to-center spacing should not exceed 4 in., unless it is shown through tests that the performance of the pole is not impaired.

MISCELLANEOUS MATERIALS Other materials used in conjunction with prestressed concrete poles include the following: 1. Polyvinyl chloride (PVC) pipe is used as a sleeve for through bolts used for attaching insulators, through vangs, and for alignment holes. 2. Threaded inserts (including grounding and climbing lugs) should be die-cast zinc alloy, stainless steel (ASTM 2011d), or galvanized steel that conform to ASTM B240 (ASTM 2010d). 3. Nameplates should be made from noncorrosive material such as bronze, brass, or stainless steel. The nameplate should provide at a minimum the pole weight and moment capacity. 4. Hot-dipped galvanized steel hardware should conform to ASTM A153 (ASTM 2009). 5. Galvanized steel pipe is also used as a sleeve for through bolts where high bearing or shear loads are expected. 6. Tank grounds are for internal and external grounding and are typically brass or copper. See Fig. 3-3 for examples of these materials.

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 3-3. External/internal view of a spun-cast concrete pole Source: Illustration courtesy of Valmont Newmark. Reproduced with permission.

REFERENCES ASTM. (2004). “Standard test method for compressive strength of cylindrical concrete specimens.” C39, West Conshohocken, PA. ASTM. (2007). “Standard specification for steel wire, plain, for concrete reinforcement.” A82/A82M, West Conshohocken, PA. ASTM. (2009). “Standard specification for zinc coating (hot-dip) on iron and steel hardware.” A153, West Conshohocken, PA. ASTM. (2010a). “Standard specification for filled epoxy-coated seven-wire prestressing steel strand.” A882/A882M, West Conshohocken, PA.

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29

ASTM. (2010b). “Standard specification for steel strand, uncoated sevenwire for prestressed concrete.” A416/A416M, West Conshohocken, PA. ASTM. (2010c). “Standard specification for uncoated stress-relieved steel wire for prestressed concrete.” A421/A421M, West Conshohocken, PA. ASTM. (2010d). “Standard specification for zinc and zinc-aluminum alloys in ingot form for foundry and die castings.” B240, West Conshohocken, PA. ASTM. (2011a). “Standard specification for chemical admixtures for concrete.” C494, West Conshohocken, PA. ASTM. (2011b). “Standard specification for concrete aggregates.” C33/ C33M, West Conshohocken, PA. ASTM. (2011c). “Standard specification for portland cement.” C150, West Conshohocken, PA. ASTM. (2011d). “Standard specification for steel sheet, zinc-coated (galvanized) or zinc-iron alloy-coated (galvannealed) by the hot-dip process.” A653 /A653M, West Conshohocken, PA. ASTM. (2012). “Standard specification for deformed and plain carbonsteel bars for concrete reinforcement.” A615, West Conshohocken, PA.

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4 DESIGN

Prestressed concrete poles generally are designed using classical prestressed concrete theory, as described in ASCE/PCI (1997) and PCI (2010). Prestressed concrete poles exhibit both linear and nonlinear behavior, time dependent material behavior, and geometric nonlinear behavior caused by the change in the section properties relative to the applied loads. The section properties of a prestressed concrete pole change when bending forces applied to the cross section (see Fig. 4-1) cause tensile forces on the face of the pole that exceed the compressive forces exerted by the prestressed steel. The bending moment at which this occurs is called the cracking moment. Prior to reaching the cracking moment, the pole section has a relatively constant modulus of elasticity and deflects in a linear manner. Above the cracking moment, the pole section behaves mostly nonlinearly because of the altered properties of the cracked section. Both behaviors can be present at the same time during loading (i.e., the top half of the pole may not yet have exceeded the cracking moment and thus behaves linearly, while the bottom half of the pole has exceeded the cracking moment and thus behaves nonlinearly). Additional time-dependent consideration in the design of a prestressed concrete pole includes shrinkage and creep of the concrete and relaxation of the prestressing steel. These material behaviors should be considered in determining the section behavior. The magnitude and effects are described in ACI 318 (ACI 2008) and PCI (2010). Deflection can sometimes cause a significant additional secondary bending moment because of P-Delta effects. The secondary moments in the structure are a result of the structure deflection causing an applied 31

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 4-1. Spun prestressed concrete pole cross section Source: Illustration courtesy of Valmont Newmark. Reproduced with permission.

eccentric loading of the pole’s center of gravity coupled with the weight of the conductors, insulators, and other applied vertical loads. These secondary moments must be accounted for in the design of a structure. It is important to consider the effects of material and geometric nonlinearity in the structural analysis, not only because of the secondary moments induced but also to reasonably predict structure deflections for insulator, conductor, and other electrical clearance calculations. Prestressed concrete pole designs are best accomplished by suitable nonlinear analytical methods that account for the prestressed concrete material and section behaviors previously described.

DESIGN CONSIDERATIONS Concrete poles and structures designed to meet the same loading and functional requirements may differ significantly in cross-section dimensions and reinforcements because of varying manufacturing capability of the suppliers, quality and regional variability of the raw materials used, the design philosophy of the engineer, and the analytical design methodology used (including recognition that both linear and nonlinear material behaviors can be present within the same analysis). The development of a prestressed concrete pole design requires many considerations including limitations of pole length, diameter, and taper (functions of casting mold variation and availability from producing suppliers). Loading Considerations The required structure strength and behavior requirements are typically defined by the Purchaser and should include the following information:

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33

• specified loading conditions (including load factors and applicable code requirements); • foundation behavior (expected translations and rotations); • height and geometric configuration; • deflection limitations, if applicable; • other limitations, such as diameter restrictions and length and weight restrictions; and • special material requirements affecting pole durability that can be specified (i.e., unique environmental conditions), if applicable. Loading may include wind pressure distributed over the projected area of the pole and arms, or loading may be represented by forces applied to the attachment points or loads translated to the pole via member reactions. When translating forces, it may be important that some accommodation of the geometric displacement of the members be considered. This is especially true when multipart insulators, hinged braces, and arms are specified. It is preferable that when forces are provided at the attachment points, the loads be provided in the form of load trees, using a single orthogonal coordinate system as shown in Fig. 4-2.

Foundation and Boundary Conditions Typically, concrete poles are designed as direct embedded structures. However, socketed piles and base plated foundations are feasible and have been used (see Fig. 4-3). Socketed pile foundations are typically multisided steel or cylinder concrete piles. The socketed joint (the length of the pile that engages the pole) is a relatively short length, typically 8 ft. to 10 ft. to fully develop the prestress strand. It should be noted that cylinder concrete piles are commonly used for bridge supports, wherein they are used in pairs or clusters with pile caps poured in place, thus strengthening the open end that is in contact with the imposed loads. However, when used singly as a socketed pole support, the pile becomes an extension of the pole itself and, as such, must transfer the full reactions coming into the cylinder from the base of the pole. The internal resisting moment couple required will produce extremely high shears that are then imparted to the wall of the concrete cylinder, causing an elevated hoop stress to be developed in the spiral reinforcement within the pile. This stress must be accounted for in the design of the concrete cylinder pile to prevent longitudinal cracks. Variation in soil conditions will influence the type of foundation selected. Those soil variances also influence the degree of fixity

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 4-2. Typical load tree Source: Illustration courtesy of Valmont Newmark. Reproduced with permission.

and movement of the foundation in both rotational and translational directions. The Purchaser should specify how these movements should be considered in the design or allow the engineer to accommodate the movement using any suitable method. Unless the Purchaser specifies the expected foundation movements or point of fixity to be considered in the design, the engineer may assume fixity at groundline and no foundation rotation or translation.

MULTISECTION CONCRETE POLES Prestressed concrete poles can be assembled with several different types of connections, including some proprietary methods of various

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35

Fig. 4-3. Concrete pole in cylinder pile Source: Photograph courtesy of Alabama Power Co./Southern Co. Reproduced with permission.

manufacturers. Multipiece poles are usually required to meet production, handling, and transportation requirements, or to attain additional lengths. Two types of pole connections are illustrated in Figs. 4-4 and 4-5. It is important to note that whatever method of connection is utilized, the typical assembly and installation method is to install the base section and then stack the upper section. The weight of the sections and the loads on the connection may be much different than expected if the sections are assembled on the ground and then the entire pole is erected into position. If sectional assembly on the ground is anticipated, specific assembly and erection instructions should be requested of the Manufacturer. Slip Joint Connection This type of splice connection consists of a steel collar with the same taper as the pole. The upper section is slipped over the top of the lower section. The resulting splice requires a snug fit and proper overlap in accordance with the Manufacturer’s specifications, as shown in Fig. 4-5.

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 4-4. Multisection flanged concrete poles Source: Photograph courtesy of Valmont Newmark. Reproduced with permission. Flange Plate Connection This type of connection consists of steel plates that are affixed to the ends of the pole sections via the prestress strands and/or other suitable reinforcement schemes. The two flanges are then bolted together in the field during erection. HYBRID (STEEL AND CONCRETE) POLES This configuration consists of a tapered tubular steel pole upper section and a direct embedded prestressed concrete pole base section as shown in Fig. 4-6. Common applications include marshy or corrosive

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37

Fig. 4-5. Multisection telescoping splice jointed poles Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

soil conditions, construction sequence requirements (outage constraints), construction preferences of an embedded foundation over a drilled pier construction, or other constructability advantages. Special design considerations should include the transfer of the axial, shear, and moment from section to section, including the potentially high shear effects of the moment transfer through the slip joint connection. REVERSE TAPER POLES This configuration can be a single or multiple piece concrete pole with a reverse change in taper direction. It is usually used in guyed pole

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Fig. 4-6. Hybrid poles: Concrete direct embedded pole section base with tubular steel upper pole sections Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

applications, as seen in Fig. 4-7, where the maximum applied moment is not at the point of fixity, or in applications where the reverse taper may occur below grade to minimize the augered hole dimensions. Special design considerations include the special devices to allow the directional change of the longitudinal prestressing strand and other reinforcements.

CONCRETE POLE DESIGN AS RELATED TO WOOD POLE EQUIVALENCY Some utilities have based at least a part of their standard transmission and distribution pole designs on ANSI 05.1-2008 (ANSI 2008) specifications. It is important to understand that except for basic load carrying capacity, there is no general wood pole equivalent of a concrete pole owing to differences in material properties and behavior. These differences include, but are not limited to, characteristics such as nominal strength and stiffness, which is also true for other engineered pole materials, including steel and fiber reinforced polymer poles. Pole capacity can

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39

Fig. 4-7. Guyed reverse taper concrete poles Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

best be matched to the applied loads when specific height, configuration, and loading have been developed by the Purchaser and evaluated by the engineer. When replacing wood poles with concrete poles, it is recommended that the engineer be involved in the design process and that the pole designs be based on specific applied loads.

DESIGN CRITERIA Distinct design conditions that may be considered in the design of a prestressed concrete pole are outlined as follows. This section deals primarily with the design of the pole shaft. If designing structures with guys or with multiple poles, other design considerations may be necessary. Ultimate Strength The ultimate flexural strength of a pole is the point at which the pole will fail, usually by compressive failure of the concrete. The pole should

40

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

be designed for the ultimate strength at all sections of the pole to exceed the required strength calculated from the appropriate factored loads applied to the structure. Factored loads are to be specified by the Purchaser or specified in applicable codes and other loading guidelines (e.g., ASCE [2010] or other similar documents). Cracking Strength A prestressed concrete pole should typically be designed for the cracking strength to exceed the moments calculated from serviceability requirements. Service loads are those that might occur with sufficient frequency and/or duration such that the structure’s condition could become a concern (i.e., from deflection or visible cracking, general appearance, corrosion, structural behavior, creep, deflection, and others). If the pole remains cracked under frequent or sustained loading, the reinforcement could be exposed and may be susceptible to corrosion. Serviceability requirements should be specified by the Purchaser for structures that are subjected to permanent lateral load—such as unguyed dead-end or angle structures— and for structures controlled by deflection. These types of structures should be designed to have the crack reopen or zero tension strength to exceed the moments calculated from the service loading. Different service load conditions and design philosophies may dictate the specification of different cracking strength levels as discussed in the following sections. The Purchaser should clearly specify the service loads anticipated and the design philosophy to be incorporated. If the design philosophy is left unspecified, the engineer should use good engineering judgment and standard industry practice. First Cracking Strength: This is the pole capacity at which the first circumferential crack will occur. Under this condition, the moment in the pole causes the tensile strength of the concrete to be exceeded on the tension face of the pole. The crack strength is a function of the concrete modulus of rupture and prestress level. Axial load may be a contributing factor in the structure design. These cracks will close upon release of the load. Crack Reopen Strength: This is the pole capacity at which a crack that was previously created (by exceeding the first cracking strength) will open again. Under this condition, an applied moment will not cause any tensile stress in the surfaces of the crack. The frictional effects of the rupture surfaces can contribute to additional capacity. This strength will always be less than the first cracking strength. Zero Tension Strength: This is the calculated capacity at which the extreme fiber of the concrete surface is subjected to zero stress due to the

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41

applied loads, including prestressing forces. The zero tension capacity is controlled by the physical dimensions of the pole section and the prestress level, but it is not a function of concrete strength. Zero tension strength is similar to the crack reopening moment except that it ignores the frictional effects of the rupture surfaces within the concrete. Shear and Torsion Shear and torsion loads seldom control concrete pole designs. However, for concrete poles with very short embedments (i.e., socket pile foundations or short rock embedments), the concentrated shear forces from the moment transfer can be significant and should be accounted for in the pole design. High shear and torsion loads can be developed from conductor or guy wire attachments or under broken wire conditions. Column (Critical Buckling) Loads When unusually large vertical loads are encountered, such as those that will be present in guyed structures and H-frames, the buckling capacity should be checked. A useful reference on the subject is ASCE (1997). Deflection The Purchaser should specify the deflection limit and the associated load cases to be considered in the pole design. In locations where appearance or clearance is important, the Purchaser should specify any additional required deflection criteria. Prestress Losses The magnitude of the prestressing force in the pole is not constant but decreases with time. This decrease in the prestressing force is referred to as the prestress loss. Some prestress losses are instantaneous and some are time-dependent. Instantaneous losses are due to elastic shortening, anchorage slippage, and friction in the case of post-tensioning. Timedependent losses are mainly attributable to shrinkage and creep of concrete and steel relaxation. Experience has shown a detailed analysis of losses for prestressed concrete poles may not be necessary except in unusual circumstances. Lump sum estimates of losses are commonly used. Depending on the materials used, 15 to 25% for total losses are common design assumptions. A good source for information on the calculation of prestressing losses is PCI (2010).

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Development Length It is necessary to examine the bond development length of the reinforcements when high bending stress is experienced near the end of a concrete pole section. Examples of these conditions are at slip joints, flange connections, long davit arms at the tip of a pole, or short embedment conditions. Bond development length should be checked to ensure adequate bond capacity when reinforcing steel is cut, such as by drilling holes. ACI 318 (ACI 2008) covers the calculation for development length.

DESIGN METHODOLOGY The Principal Assumptions of Ultimate Capacity The ultimate moment capacity of a pole at any given cross section is a function of the strains in the prestressing steel and concrete. The factored design moment should not exceed the ultimate moment capacity. The following assumptions are made in computing the ultimate moment capacity of poles: • Plane sections remain plane. • The steel and concrete are adequately bonded. • The steel and concrete are considered in the elastic and plastic ranges, respectively. • The concrete compressive stress at failure is 0.85f c′ (as taken from equivalent stress block methodology). • The tensile concrete strength is neglected in flexural computations. • The ultimate concrete strain is 0.003 in. • The conditions of compatibility and equilibrium are met. While the first two assumptions become somewhat less valid after the section has cracked, the overall behavior of the member can still be predicted adequately. Determination of Ultimate Capacity Equilibrium of Section Determination of the Compressive Stress Area: Based on the aforementioned assumptions and the provisions in ASCE/PCI (1997) and ACI 318 (ACI 2008), the assumed rectangular compressive stress distribution in the concrete is used herein for simplification and is represented by a statically equivalent concentrated force, defined by the cylinder compressive strength f c′, the parameter β1, the distance from the extreme

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compressive fiber to the neutral axis c, the parameter K such that the distance Kc locates the centroid of the stress block from the extreme compressive fiber (see Fig. 4-8). Equilibrium: Equilibrium of the section requires equal forces in the prestressing steel and concrete. The equation of equilibrium is (without axial loads) Cc = Ts

(4-1)

where Cc is the concrete compression and Ts is the strand tension force. Concrete Compression: The compression in the concrete is then computed from Cc = 0.85 f c′Aa

(4-2)

where Aa is the area of the annulus of the concrete in compression as defined by a rectangular stress block of depth β1c. The parameter β1c is defined as 0.85 for a concrete strength of 4,000 psi and less and is reduced by 0.05 for each 1,000 psi in excess of 4,000 psi with a minimum value of 0.65. Steel Tension:

The steel tension is expressed as n

Ts =

∑A

f

psi sei

(4-3)

i =1

Fig. 4-8. Concrete stress area and assumed stress distribution in a pole section Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

where Apsi and fsei are the area and stress of the ith strand, respectively. Trial and error iteration of the location of the neutral axis c is used to solve for the depth of the stress block, such that equilibrium between tension and compression is satisfied. Ultimate Moment Capacity Equation: The ultimate moment capacity of a pole section is given as the sum of the moments of tensile and compressive forces with respect to the neutral axis: n

φMn =

∑e A i

f + cCc (1 − K )

psi sei

(4-4)

i =1

where e1 = d1 − c and φ is the capacity reduction factor. Note Apsi, fsei, and c are previously defined, c(1 − K) is the distance of the centroid of the reduced compressive concrete area (pressure line) from the neutral axis, di is the distance of the ith strand from the extreme compressive fiber, and ei is the distance of the ith strand to the neutral axis. The quantity ei Apsi fsei is positive when the ith strand is located below the neutral axis (tension zone) and negative when it is located above (compression zone). In the case of braced H-frames and guyed structures, the formula for the ultimate moment capacity should incorporate the effect of the applied axial loads. The capacity reduction factor (which is also known as the strength factor φ) is typically taken as 0.9. However, if a Manufacturer can demonstrate consistency in the quality and predictability of the manufactured pole, a φ of up to 1.0 can be used with consent from the Purchaser. Cracking and Zero Tension Moment Capacity Cracking Moment: Cracking starts when the tensile stress in the extreme fiber of the concrete reaches its modulus of rupture. The cracking moment can be computed by elastic theory to predict the behavior of poles. For a symmetrically reinforced prestressed concrete pole section, a uniform stress P/Ag results from the effective prestress P acting on the gross sectional area Ag. Because of the external moment M, the section area is subject to the extreme tensile stress Myt/Ig, where yt is the distance from the centroidal axis to the extreme tensile fiber and Ig is the gross moment of inertia of the section. The cracking moment may be calculated using the following relationship: Mcr =

f r I g PI g + yt Ag yt

(4-5)

DESIGN

45

where frIg/yt is the resisting moment attributable to the modulus of rupture of concrete fr, and PIg/Agyt is the moment attributable to the direct compression of the prestress. Modulus of Rupture: ACI 318 (ACI 2008) defines the modulus of rupture for normal weight concrete as 7.5 f c′ . However, ACI recognizes a transition zone where the extreme concrete fiber tensile stress ranges from 7.5 f c′ to 12 f c′ . Currently, values of up to 10 f c′ are used by Manufacturers based on individual testing. f c′ is the concrete compressive strength (in psi). Zero-Tension Moment: The zero tension moment M0 may be calculated from the relationship Mo =

PI g Ag yt

(4-6)

Stress Distribution: The stress distribution in a pole section at cracking and zero tension is shown in Fig. 4-9. Shear and Torsion Shear: The design of concrete pole cross sections subject to shear shall be based on Vu ≤ φVn

(4-7)

where Vu is the factored shear force at the section considered, φ is taken as 0.75, and Vn is the nominal shear strength computed by

Fig. 4-9. Stress distribution in pole section at cracking and zero tension Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Vn = Vc + Vs

(4-8)

where Vc is the nominal shear strength provided by the concrete and Vs is the nominal shear strength provided by the shear reinforcement. Square or Rectangular Prestressed Concrete Poles: For square or rectangular prestressed concrete members with an effective prestress force not less than 40% of the tensile strength of the flexural reinforcement, Vc may be computed as ⎡ ⎛ V d⎞ ⎤ Vc = ⎢0.6 f c′ + 700 ⎜ u ⎟ ⎥ bw d ⎝ Mu ⎠ ⎦ ⎣

(4-9)

but Vc need not be less than 2bw d f c′ nor shall it be greater than 5bw d f c′. The quantity Vud/Mu shall not be greater than 1.0, where Mu is the factored moment occurring simultaneously with Vu at the section considered. The variable d shall be the distance from the extreme compressive fiber to the centroid of the prestressing reinforcement, and bw shall be the width of the web. Circular Prestressed Concrete Poles: For circular prestressed concrete members Vc =

ft′ 2 + ft′f pc Q 2It

(4-10)

where f t′ = tensile strength of concrete taken as 4 f c′, fpc = effective compressive stress of concrete due to prestress, Q = moment of area above centroid, I = moment of inertia of cross section, and t = wall thickness. For the shear strength Vs contributed by the steel Vs =

Av f y d s

(4-11)

where Av is the area of the shear reinforcement within a distance s, fy is the yield strength of the steel, and d is the distance from the compression force to the centroid of the prestressing steel, or 0.8 times the outside diameter of the section, whichever is greater.

DESIGN

47

Torsion: The design of concrete pole cross sections subjected to torsion shall be based on Tu ≤ φTc

(4-12)

where Tu is the factored torsional force at the section considered, φ is taken as 0.75, and Tc is the torsional resistance of the prestressed concrete member. Square or Rectangular Prestressed Concrete Poles: For square or rectangular cross sections (Lin and Burns 1981) Tc = 6 f c′ 1 +

10 f pc f ’c

∑ ηx y 2

(4-13)

where 0.35

η=

0.75 +

b d

(4-14)

and x is the shorter overall dimension of the rectangular part of the cross section, y is the longer overall dimension of the rectangular part of the cross section, and b is the width of the compression face of the member. Circular Prestressed Concrete Poles: Tc =

J ro

For circular cross sections ft′ 2 + ft′f pc

(4-15)

where J is the polar moment of inertia and r0 is the outside radius of the section. Combined Shear and Torsion: For members subject to simultaneous flexural shear and torsion, the following interaction equation may be used to represent the strength of the member: 2

2

⎛ Vu ⎞ ⎛ Tu ⎞ ⎜⎝ φV ⎟⎠ + ⎜⎝ φT ⎟⎠ = 1.0 n c

(4-16)

Critical Buckling Loads The best estimate of buckling loads for nonprismatic members can be obtained using numerical methods to solve the differential equations

48

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

obtained from classical elastic stability theory or using a nonlinear finite element formulation. Such numerical or finite element techniques are not practical without computers. These methods are described in advanced analysis textbooks. Deflections For loads less than those causing the first crack, elastic deflections can be determined using classical structural analysis methods. For loads in excess of cracking, an inelastic pole design method should be used. Concrete Creep Deflection: Additional structural deflection can occur because of concrete creep. This is the plastic deformation of the concrete resulting from an application of loads over an extended time period. This could result in increased deflections for poles used as strain poles, self-supporting dead ends, or guyed structures. For most pole applications, creep is not a major design consideration. However, it can be of significance for nonuniform stress distribution resulting from the combined effect of sustained load and prestress. Determination of Elastic Deflection: For loading conditions that do not exceed the cracking capacity of the pole, an elastic method may be used. This could include virtual work, the conjugate beam method, slope deflection, or a finite element computer analysis. Determination of Inelastic Deflection: Up to the point of cracking, the deflection may be computed using elastic methods previously described. After cracking, the modulus of elasticity E becomes both stress and time dependent, and the moment of inertia I becomes crack dependent. Because the product EI varies with stress, time, and pole geometry, computing inelastic deflections is sufficiently complicated to warrant using iterative computer computations. The inelastic deflection can be approximated using reduced values of the elastic product EI. These values may range from EcIg at a level of moment at cracking to EcIg/3 as the member approaches ultimate strength.

REFERENCES American Concrete Institute (ACI). (2008). “Building code requirements for structural concrete.” ACI 318, Farmington Hills, MI. American National Standards Institute (ANSI). (2008). “Wood poles– specifications and dimensions.” 05.1-2008, New York.

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49

ASCE. (1997). “Design of guyed electrical transmission structures.” ASCE Manual of Practice No. 91, Reston, VA. ASCE. (2010). “Guidelines for electrical transmission line structural loading.” ASCE Manual of Practice No. 74, Reston, VA. ASCE and Precast/Prestressed Concrete Institute (PCI). (1997). “Guide for the design of prestressed concrete poles.” Journal of the Precast/Prestressed Concrete Institute, 42(6), 93–104. Lin, T. Y., and Burns, N. H. (1981). Design of prestressed concrete structures, 3rd Ed.,Wiley, New York. PCI. (2010). Design handbook: Precast and prestressed concrete, 7th Ed., Chicago.

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5 CONNECTIONS

The physical attachment of arms, brackets, insulators, climbing provisions, and other appurtenances to concrete poles is a highly important consideration to the overall structure design. These attachments, or “connections,” serve the explicit purpose of transferring load to the supporting pole or frame structure (see example in Fig. 5-1). Pole line hardware, attachment brackets, and additional appurtenances, such as in Figs. 5-2 and 5-3, are typically attached to the concrete pole using bolts placed through holes defined in the pole cross section during manufacture. However, external bands wrapping the pole—or some forms of threaded inserts embedded into the pole cross section—have also been used, depending on the type and magnitude of load. Factors to consider in connection design include the load transfer mechanism, ductility, durability, required pole and bracket fabrication tolerances, aesthetics, constructability issues, maintenance, and general economics. Because of the thin shell nature of spun-cast concrete poles and the important composite material interaction of concrete and steel in concrete poles in general, connections for concrete poles should be developed by an engineer experienced in this type of connection details. In some instances, the pole line hardware supplier may also be able to provide valuable assistance with typical connection design details, such as those shown in Figs. 5-4 and 5-5. In either event, the Purchaser’s specifications should be clear about who is responsible for design and coordination of the adequacy of the connections being made to the concrete pole. The structural adequacy of connections can be determined empirically through full-scale component testing or by using accepted engineering modeling or other rational design methods. The connections should be designed such that the allowable stresses in both the connecting part and 51

Fig. 5-1 Post insulator assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Fig. 5-2. Davit arm assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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53

Fig. 5-3. Braced post insulator support assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission. the concrete pole are not exceeded and that excessive deformation or connection rotation is not experienced. Typical structure appurtenances connected to the pole include items similar to Figs. 5-6 and 5-7.

METHODS OF CONNECTION Bolted Connections Through Bolts: It is typical for most pole line hardware to be connected to concrete poles using galvanized through bolts (see Fig. 5-8).

Fig. 5-4. H-frame crossarm assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Fig. 5-5. X-bracing assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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55

Fig. 5-6. Guy wire attachment assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission. Commonly used fastener standards include ANSI C135.1 (ANSI 1999), ASTM A307 (ASTM 2010a), ASTM A193M (ASTM 2011), and ASTM A325 (ASTM 2010b). The standard selected and the bolt tightening procedure should be determined by the engineer responsible for the connection design. Inserts: The load applied to a threaded insert should not exceed the rated capacity of the insert or that which is determined through testing. Inserts should be made of materials that will not deteriorate in the environment in which they are placed (see Fig. 5-9). Materials should be selected to ensure that the concrete, the insert, and the bolt are not chemically reactive with each other. Care should be taken to ensure that placement of the inserts does not reduce the load carrying capacity of the pole

Fig. 5-7. Shield wire attachment assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Fig. 5-8. Dead-end insulator assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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57

Fig. 5-9. Typical external groundwire and clip assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission.

(e.g., interfere with strands or other reinforcements). The length of the attachment bolts used in threaded inserts should be sized such that the bolt will not bottom out in the insert as this could result in damage to the pole or reduce the load capacity of the insert. Climbing Attachments: Various types of climbing attachments can be used on concrete poles, including ladder clips, step bolts, “ladder rung clips,” and bale steps, or hand holds (see Figs. 5-10, 5-11, and 5-12). For climbing attachments, all components, including the inserts to which they are attached, must be capable of supporting the loads specified by the user and any applicable codes. IEEE (2004) has specific requirements for worker safety. External and Internal Grounding Attachments: Various types of external and internal grounding attachments, including tank grounds, can be used on concrete poles. The Purchaser should clearly coordinate with the pole Manufacturer regarding what grounding provisions will be needed, where the attachments will be located, and how they will be identified.

Fig. 5-10. Detail of step bolt assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Fig. 5-11. Detail of climbing ladder assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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59

Fig. 5-12. Detail of climbing ladder assembly Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Banded Attachments Concrete poles are particularly well suited for banding on appurtenances because a concrete pole is dimensionally stable with no shrinkage and concrete poles generally provide a uniform surface and a consistent pole taper. Common types of banding systems include steel strapping, cast steel links, and fabricated steel bands. Banding systems must be designed for the loads they will support (see Figs. 5-13, 5-14, and 5-15).

CONNECTION DESIGN CONSIDERATIONS During the design of connections, the engineer should consider eccentric and torsional moments, combined stresses, localized stress concentrations, or other loads resulting from the configuration of the connection.

Fig. 5-13. Dead end formed banding Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Fig. 5-14. Linkbanding Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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61

Fig. 5-15. Strap banding Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Location Considerations Careful attention must be given when locating connections within the top 8 to 10 ft of the pole (i.e., guy/conductor attachments or davit arms). This location on the pole does not have fully developed strength capacity because of reinforcement bond development length constraints. Congestion Considerations Careful attention must be given to avoiding excessive congestion of attachment details in a localized area of the pole section. This congestion may prevent proper concrete distribution during the manufacturing process. Sleeved Through Holes As a guide for through bolts, PVC pipes typically are used during manufacturing to define “holes” in concrete poles. Their purpose is to

62

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

define the hole through the pole during concrete placement, but these pipes are also helpful as a guide sleeve during the installation of bolts in larger diameter poles. In special situations, such as instances with high bearing loads, steel pipes may be used in lieu of PVC for the guide. If loading conditions impose high tension and even bending on the bolt, the steel guide pipes may help to redistribute the bearing stress placed on the concrete by the bolt and reduce surface spalling of concrete around a pipe at the surface of the pole. Galvanized steel pipes with an inside diameter slightly larger than the bolt diameter may minimize bolt bending in some connections. Careful attention to bolt group fitting tolerances during manufacturing must be considered. Connections requiring greater projection of the through bolt beyond the surface of the concrete pole to hold the appurtenance are one example where steel through pipes may reduce bolt bending and distribute any concentrated bearing that may result from bolt bending. Drilled Through Holes Drilling a hole through the pole cross section is possible to accommodate changes in pole framing or the addition of holes after manufacturing has been completed. This is best done with a carbide tipped roto-hammer drill bit or a diamond tipped core bit. Special care should be taken to minimize any concrete spalling in the pole wall, as the bit exits the concrete as it is being drilled. Use of the proper carbide or diamond tooth drill bit will allow for drilling through the steel spiral and strand that may be present in the location being drilled. The possibility of cutting strands and spiral reinforcements during field drilling should be discussed with the Manufacturer to obtain additional instructions and recommendations. Bearing or Punching Failure Considerations This consideration includes careful attention to pole surface bearing stress, which refers to the load distribution at the pole surface-attachment interface. Examples include the concentrated load underneath the washer of a through bolt or the contact surface on the compression end of a pole eye plate. A curved washer should be placed between the pole and the head or nut of the bolt to distribute the concentrated load effectively. The thickness of the washer must be sized adequately for the load and the bearing area being considered. The use of cast washers is not recommended. The maximum bolt bearing load is determined by multiplying the diameter of the bolt (or, if used, the diameter of the steel pipe sleeve) by the wall thickness and the effective bearing stress of the concrete (see Fig. 5-16).

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63

Fig. 5-16. Bolt bearing on concrete pole wall Source: Illustration courtesy of American Electric Power. Reproduced with permission. The effective bearing stress of the concrete can be calculated as φ(0.85f c′A1 )

(5-1)

where φ, is a capacity reduction factor, 0.85f c′ is the effective compressive strength of the concrete, and A1 is the bolt or sleeve bearing area. In the absence of confirming tests, it is assumed that the bolt-to-concrete interface carries the entire load and none is carried through friction. The maximum effective wall thickness for calculating the bearing load is the lesser of 3 in. (76 mm), four bolt diameters, or the actual wall thickness. Banding Systems The band wraps around the pole and is designed based on the tension load it will experience after the load is applied. The Purchaser should be knowledgeable in selecting the appropriate banding system to withstand the load. The strength of these systems should be verified with the supplier to be adequate for the design load they will support. CONNECTION FAILURE MODES Concrete Spalling Concrete spalling is defined as surface loss of concrete at various depths caused by the bearing force of the bolt on the edges of a cast hole.

64

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

Concrete Bearing Failure Concrete bearing failure occurs when the bearing capacity of the pole wall is exceeded, resulting in a collapsing or crushing of the pole wall. Bolt Shear or Tension Failure Bolt failure occurs when the allowable shear or tensile capacity of the bolt is exceeded. Bolt selection should be based on the applied shear and tension loads and any combination thereof. Allowable loads are to be determined from the latest edition of the AISC Steel Construction Manual, where applicable. In most cases, bolts will be subjected to a combination of shear and tension loads. AISC has specific methods to design for this condition. Careful attention should be given to whether or not bolt threads can be located within the shear plane. Bolt Bearing Failure Bearing failure occurs when the load applied to the bolt by the concrete pole or appurtenance exceeds the bearing capacity of the bolt. This results in deformation of the threads and possibly the shaft of the bolt itself. The bearing stress can be calculated as the force on the bolt divided by the product of the bolt diameter times the thickness of the connected part. This calculated stress should not exceed 1.5 times the specified minimum tensile strength Fu of the connected part or bolt. Generally, for concrete poles, other failures likely will occur before a bolt bearing failure (see Figs. 5-17 and 5-18). Bolts Subject to Combined Shear and Tension For bolts subject to combined shear and tension, refer to the appropriate sections of AISC (2005). Bolt Bending or Prying Action When designing a connection, special consideration should be given to the possibility of bending of the bolt and/or prying action, which can result in bolt failure. Appurtenance Connection Failures The engineer should be aware of failures that can occur in the base of the appurtenance where it connects to the pole. Shear or tear-out failures as shown are the most common forms of failure (see Figs. 5-19 and 5-20).

Fig. 5-17. Tension failure of bolt Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Fig. 5-18. Bearing failure of bolt Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Fig. 5-19. Appurtenance connection bearing failure Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Fig. 5-20. Appurtenance connection tension failure Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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67

Attachments should be engineered and sized appropriately to resist the applied loads.

INSTALLATION CONSIDERATIONS The Manufacturer’s instructions regarding bolt tightening should be followed for all attachment connections. The Purchaser should be aware that over-tightening of the bolts may lead to ovalization of the pole cross section, which can possibly initiate longitudinal cracking along the pole length.

REFERENCES American Institute of Steel Construction (AISC). (2005). Steel construction manual, 13th Ed., Chicago. American National Standards Institute (ANSI). (1999). “Standard for zinccoated steel bolts and nuts for overhead line construction.” C135.1, New York. ASTM. (2010a). “Standard specification for carbon steel bolts and studs, 60 000 psi tensile strength.” A307, West Conshohocken, PA. ASTM. (2010b). “Standard specification for structural bolts, steel, heat treated, 120/105 ksi minimum tensile strength.” A325, West Conshohocken, PA. ASTM. (2011). “Standard specification for alloy-steel and stainless steel bolting for high temperature or high pressure service and other special purpose applications.” A193/A193M, West Conshohocken, PA. Institute of Electrical and Electronics Engineers (IEEE). (2004). “Fall protection for utility work.” IEEE 1307, New York.

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6 FOUNDATIONS

Foundation requirements for a concrete pole structure depend on the loads that will be transmitted to the foundation and the surrounding soil conditions.

DESIGN CONSIDERATIONS Loads The foundation must be adequately designed to resist structure reactions, which include shears, overturning moments, torsion, uplift, or compression imposed by the factored loads. If the foundation is to be designed stronger than the structure that it supports, reduced strength factors can be used together with the foundation strength formula as described in ASCE (2010). Soil Exploration and Determination of Burial Depths To design a foundation properly, the engineer must have information pertaining to the existing soil conditions. The amount and type of information required will depend on the engineer’s experience with similar structures and soil types. Conditions may dictate soil testing. These tests are usually performed by geotechnical or soils engineers. The test results generally include the bearing capacity, lateral soil strength, cohesion values (clays and silts), unit weights, angle of internal friction (sands), depth of water table, and a detailed soil description. 69

70

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

There are several classic methods that are employed to estimate the burial depth required to resist the lateral forces and overturning of a concrete pole. Foundation design is not an exact science because geotechnical conditions are so variable. Once the soil design parameters have been determined, the engineer may approach the design by using formulas available from ASCE, IEEE, or other available publications from similar organizations. There are also commercially available computer programs that use the load-deflection curves of the soil properties obtained from actual fullscale testing of different materials. These programs can provide reasonable values of lateral groundline movement of the foundation and can thus be highly useful in estimating the angle of the pole’s resulting rotation. Engineers are cautioned not to readily accept “rules of thumb” burial depth estimates, such as the classic wood pole formula of 10% of pole length plus 2 ft, because these values will seldom satisfy the burial requirements of heavily loaded transmission poles. Soil conditions are too variable and line reliability is too much at stake to rely on burial depth determinations made without some actual soil investigation. Special Pole Considerations for Shallow Embedments When a concrete pole is installed in a cylinder pile, in other precast foundations, or in rock, it is imperative that allowances be made for transferring the shear and developing the bond of the prestressing strand. Minimum embedments of 8 to 10 ft are commonly used to provide adequate bond lengths and shear resistance to transfer the groundline moment. Additional spiral reinforcing or structural rings also may be employed to resist the shear. See Chapter 4 for further information about this requirement. Performance and Reliability Foundation deflection and rotation should be considered in the design. Excessive deflection or rotation of the foundation will create an undesirable appearance and may cause unfavorable load redistribution and an increase in P-Delta effects. Foundation design should incorporate adequate design factors to minimize future plumbing or adjustment of the structure. In areas containing soils with high plasticity, consideration should be given to the deflections and reactions created by normal loading. Pole weight, as well as configuration eccentricities, may result in more soil creep than in structures made of other materials. FOUNDATION TYPES Foundations for concrete pole structures typically can be classified as one of the following three types:

FOUNDATIONS

71

• direct embedment foundation; • cast-in-place foundation; and • precast foundation. Direct Embedment Foundation This is the most common type of foundation used on concrete pole structures. It consists of placing the pole directly in the ground into an augered hole or by using a jetting device (see Fig. 6-1). Poles embedded in granular or loose soils may be jetted in place with hydrostatic pressure nozzles that will hydraulically displace the soil as the crane supporting the shaft allows the pole’s own weight to force itself down. This method is useful in areas with high water tables that tend to cause the walls of an augered hole to collapse before the pole can be set. The augered hole method involves placing the base of the pole in an excavated hole and backfilling the hole with good quality native material, compacted sand, gravel, or concrete. The backfill material type and compaction should be specified depending on the anticipated performance of the foundation. Cast-in-Place Foundation This term refers to the excavation of soil at the structure location and the placement of steel reinforcement and ready-mixed concrete. Common types of cast-in-place foundations are the reinforced concrete drilled pier, spread footing, and light pole bases.

Fig. 6-1. Direct embedment foundation Source: Illustration courtesy of Valmont Newmark. Reproduced with permission.

72

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

The reinforced concrete drilled pier is an excavated shaft with longitudinal reinforcing bars and circumferential spiral ties. The concrete pole can be placed inside and cast with the reinforcement or may be attached to the top of the drilled pier through the use of anchor bolts and a base plate cast to the base of the pole. Spread footings may be either square or rectangular in shape. They consist of shallow excavations with mats of reinforcing bars. Spread footings are typically specified because of poor subsurface soils or where there may be underlying obstructions that would preclude the use of an excavated augered hole. The most common method for attaching the pole to the spread footing is the use of anchor bolts and a base plate cast to the base of the pole. Precast Foundation Precast foundations are those that are cast in a manufacturing facility or fabricated near the site to be erected at the final structure location. The two most common types are prestressed concrete cylinder piles and square piles. Prestressed concrete cylinder piles are fabricated by spinning concrete in segmental molds and posttensioning the segments together to the length specified. These piles are usually available in diameters ranging from 36 to 66 in. Cylinder piles may be driven, jetted, or excavated and placed to the desired embedment depth. The concrete pole is placed inside the cylinder pile and held in place by specified backfill (note special considerations previously discussed about shallow embedments). The concrete cylinder pile may be used to obtain additional structure height without increasing the pole length. This is accomplished by specifying a pile length that includes the embedment depth plus additional above-ground length for the required structure height. Precast, prestressed concrete piles may also be installed by driving, jetting, or excavating, and placed to the desired embedment length. Lightly loaded structures may require a single pile foundation. The concrete pole may be attached to the pile by plates anchored to the end of the pile and the base of the pole. These plates may be either bolted or welded together. The concrete pole may also be attached to the single pile by bands or through bolts. Heavily loaded structures may require groups of piles connected together by a pile cap to resist the loads. The concrete pole may be cast with the pile cap or attached to the pile cap by a base plate and anchor bolts. GUY WIRES AND ANCHORS Because guyed structures must be designed as a system, guy wires and anchors become an integral part of the design. The engineer

FOUNDATIONS

73

should determine, based on the soil characteristics encountered, the pullout resistance that can be used with screw anchors, or calculate the volume of concrete and soil required for uplift with dead-man anchors. Additional information about the selection, installation, and testing of guy anchors can be obtained from the manufacturers and by referring to ASCE (1997). In addition, guy wires can be terminated into precast stub anchors, which are actually smaller poles or piles that are embedded (using one of the methods already described). These stub anchors are designed to take concentrated loads (shear, axial, and moment) at the tip end, which extends only a few feet out of the ground.

REFERENCES ASCE. (1997). “Design of guyed electrical transmission structures.” ASCE Manual of Practice No. 91, Reston, VA. ASCE. (2010). “Guidelines for electrical transmission line structural loading.” ASCE Manual of Practice No. 74, Reston, VA.

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7 MANUFACTURING AND QUALITY ASSURANCE

Prior to approving bids from concrete pole Manufacturers, the Purchaser should be satisfied that each bidder has procedures in place to ensure that every pole supplied will be in compliance with the specifications. The Manufacturer should provide either a full copy or a summary of the quality assurance program if requested. The Purchaser may inspect the Manufacturer’s equipment and process facility to ensure that the procedures are in accordance with the quality assurance program. The exact contents and procedures in the quality assurance program will vary depending on the production process (e.g., spun cast or statically cast), the types of poles being manufactured (e.g., mass-produced street lighting poles or custom manufactured transmission line poles), and the general quality control philosophy of the Manufacturer. There are, however, several considerations that should be covered by all quality assurance programs. The following guidelines may serve in preparing specifications that include a quality assurance program.

DESIGN AND DRAWINGS The quality assurance specification should indicate the degree of involvement by the Purchaser and the procedure for review of the design concept, detailed calculations, stress analysis, and the Manufacturer’s drawings. Stress analysis of the main structure and its component parts, including all attachments and connections, should be considered. The hole sizes and locations need to be clearly defined on the concrete pole drawings to meet the Purchaser’s requirements. In addition, holes specifically for pole pickup 75

76

PRESTRESSED CONCRETE TRANSMISSION POLE STRUCTURES

points need to be identified on the drawings as well as horizontal support points. The Manufacturer’s drawings should be checked to ensure that they contain proper and sufficient information for manufacturing and erection in accordance with the requirements of the Purchaser.

MANUFACTURING PROCESS Prestressed concrete poles can be spun cast or statically cast. Spun-Cast Poles To manufacture spun-cast concrete poles, concrete is pumped or placed into a steel form consisting of two separable halves equipped with rolling rings. These rings rest on the wheels of a spinning machine that rotates the form at high speeds (see Fig. 7-1). The spinning provides centrifugal compaction to the concrete mixture and creates an inner core void. The high consolidation forces and low water-cement ratios produce exceptionally dense concrete with a high compressive strength (dry unit weight of approximately 155 to 165 pcf for spun-cast poles). The spinning process also results in improved bond between steel and concrete, greater shrinkage reduction, and a smoother, denser surface finish. Concrete cover requirements over reinforcement steel can be reduced to 3/4 in. as a result of the density and compaction of the concrete. Statically Cast Poles Statically cast poles are typically made in tapered configurations that are square, polygon (multisided), or H-shaped in cross section. The square

Fig. 7-1. Mold spinning a prestressed concrete pole Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

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77

or polygon shaped sections can be either solid or made hollow by the use of retractable mandrels or fiber tube voids. Solid poles can have a wire raceway provided by a plastic tube extending through the center. Although not as dense as spun concrete (dry unit weight of approximately 145 to 150 pcf), statically cast poles also can achieve high compressive cylinder strengths. Considering the more porous quality of the concrete, the concrete cover requirements of static-cast poles should be greater than for spun-cast concrete poles. The requirements of PCI (1999) should apply. Materials The specification should include the requirement for review and agreement on the Manufacturer’s materials specifications, sources of supply, material identification, storage, traceability procedures, and acceptance of certified material test reports. The Manufacturer should maintain records of mill certifications and test reports from material suppliers to show that all materials used conform to applicable ASTM specifications. Tests on the concrete mix should be maintained, whether these tests are performed by independent laboratories or by the Manufacturer. In either case, these tests should be conducted in accordance with ASTM procedures. Manufacturing Tolerances Spun-Cast Pole Tolerances: limited to the following:

Product tolerances (see Fig. 7-2) are to be

Fig. 7-2. Pole fabrication inspection Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

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Overall length: +3 in. / −2 in. Pole diameter: ±1/4 in. Wall thickness: Allowable variation along the pole is to be not greater than +20%, with a maximum reduction of 1/4 in. provided that coverage over steel is maintained. Each pole is to be inspected for uniformity of inside appearance and wall thickness variation. If irregularities are encountered, then actual thickness measurements are to be taken by drilling pilot holes through the wall at 10 ft intervals on the longitudinal axis of the pole. These holes are to be alternated 90° at each interval. End squareness: ±1/2 in. per ft of pole diameter. Pole sweep: Sweep is the deviation of a pole from straightness. Sweep will be allowed in one plane and one direction only. A straight line joining the edges of the structure at both the top and the butt shall not be distant from the pole surface at any point more than 3/8 in. for each 10 ft of length between these two points. Weight: ±10% of computed value. Location of longitudinal reinforcement at stressing header: ±1/4 in. for individual strands, ±1/8 in. for the centroid of a group of strands. Spiral reinforcement: ±25% spacing variance, with total quantity per ft maintained. Location of a group of bolt holes from pole tip: ±2 in. Location of centerline between groups of bolt holes: ±1 in. Location of bolt holes within a group of bolts: ±1/8 in. Bolt hole diameter: +1/8 in. of specified hole diameter or +1/4 in. greater than actual bolt diameter. Bolt hole alignment within a group of bolts: within 1/2 of the hole diameter from the bolt plane that is longitudinal to the pole’s crosssectional centerline in a group. Statically Cast Pole Tolerances: to the following:

Product tolerances are to be limited

Overall length: +3 in. / −2 in. Pole diameter: ±1/4 in. Wall thickness: Allowable variation along the pole is to be not greater than +20%, with a maximum reduction of 1/4 in. provided that coverage over steel is maintained. Each pole is to be inspected for uniformity of inside appearance and wall thickness variation. If irregularities are encountered, then actual thickness measurements are to be taken by drilling pilot holes through the wall at 10 ft intervals on the longitudinal axis of the pole. These holes are to be alternated 90° at each interval. End squareness: ±1/2 in. per ft of pole diameter.

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Pole sweep: Sweep is the deviation of a pole from straightness. Sweep will be allowed in one plane and one direction only. A straight line joining the edges of the structure at both the top and the butt should not be distant from the pole surface at any point more than 3/8 in. for each 10 ft of length between these two points. Weight: ±10% of computed value. Location of longitudinal reinforcement at stressing header: ±1/4 in. for individual strands, ±1/8 in. for the centroid of a group of strands. Spiral reinforcement: ±25% spacing variance, with total quantity per ft maintained. Location of a group of bolt holes from pole tip: ±2 in. Location of centerline between groups of bolt holes: ±1 in. Location of bolt holes within a group of bolts: ±1/8 in. Bolt hole diameter: +1/8 in. of specified hole diameter or +1/4 in. greater than actual bolt diameter. Bolt hole alignment within a group of bolts: within 1/2 of the hole diameter from the bolt plane that is longitudinal to the pole’s crosssectional centerline in a group.

Sealing Strand Ends The ends of strands must be properly sealed against water intrusion. It has been demonstrated that the helical prestressed strand reinforcement can act as capillary tubes and draw water up into the member. Hence, it is extremely advantageous to burn back the strand into the member approximately 1 in. and then seal the area with an epoxy grout or similar impervious material. This is especially desirable when pole installations are in areas with high water tables or for those placed directly in sea water. Wherever strands are terminated in a pole, care should be taken to ensure that the strands are protected against weathering and corrosion. Strands that are attached to bolted flange ends should be similarly sealed. Concrete Mix Concrete mixes for prestressed concrete should be established by methods in accordance with ACI 318 (ACI 2008), Chapter 4. Mixes may be designed either by a commercial laboratory or by qualified plant personnel. All mix designs should be developed using the specified proportions and type of water, cement, aggregates, and admixtures proposed for use in plant mixes. Each mix design needs to be validated by adequate test data. If any of the variables are changed, the mix should be reevaluated. Unless specified, the producer shall have the choice of the type of cement to use for achieving the specified physical properties. Acceptance

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tests for concrete mixes shall include compressive strength (ASTM 2007, 2010c), slump (ASTM 2010e), unit weight (ASTM 2005, 2010d), and air content (ASTM 2010a,b). Curing and Finishing Once the concrete has been placed in the form (see Fig. 7-3), the concrete is compressed either by spinning (spun cast) or vibrating (statically cast). The form filled with freshly placed concrete is then put in a curing cell that elevates the ambient temperature to accelerate the curing process. After the required curing time has passed, the pole is removed from the form, inspected, and placed in the final production stage for completion for its intended end use. At this point, the pole is inspected and compared with the design requirements to ensure the product is complete.

QUALITY ASSURANCE To ensure that proper methods for all phases of production are being followed and the finished product complies with the specified requirements, a regular inspection program that inspects all aspects of production are to be provided. Each pole is to be uniquely marked and inspected per the procedure and a detailed written inspection report on record.

Fig. 7-3. Placing concrete Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

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To establish evidence of proper manufacture and quality of precast concrete products, a system of records is to be used in each plant. The system shall provide full information regarding the testing of materials, tensioning, concrete proportioning, placement, curing, sweep, member dimensions, concrete strength, and the finished product. Generally, pole manufacturing inspections occur before, during, and after concrete placement into the form, after form release, as a completed product, and prior to shipment.

Concrete Cylinder Tests Testing is to be an integral part of the total quality control (QC) program. Testing for quality control of the precast unit is to follow the Manufacturer’s standards, unless otherwise specified. If the plant has contracted for quality control to be performed by an outside independent laboratory, the lab is to be accredited by the Cement and Concrete Reference Laboratory of the National Institute of Standards and Technology. The lab is to conform to the requirements of ASTM E329 (ASTM 2011a), and the plant or independent lab is to meet the concrete inspection testing section requirements of ASTM C1077 (ASTM 2011b). For the control of concrete, testing, and mixes, each plant is to be adequately equipped with the certified testing equipment and staffed with personnel trained in its proper use. A system of records is to be maintained to provide full information on material tests, mix designs, concrete tests, and any other necessary information (see Fig. 7-4).

Inspection of Poles During the Manufacturing Process Certain QC inspections and records of inspections are important to the process review and manufacturing traceability for all poles produced. A review should be made and agreement reached on all quality control procedures. Rejection criteria should be established and agreed upon prior to the start of any fabrication. All structures ready for shipment should have complete and proper identification to avoid confusion at the delivery point. The markings should coincide with the type, length, strength, weight, and identification number required by the customer and approved on the shop drawings. The following inspections and records are required as a minimum: QC Library, Files, and Equipment • manual of standard QC practices; • QC records; and • equipment calibrations.

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Fig. 7-4. QC cylinder break Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

QC Inspections and Records • pre-pour inspections; • post pour inspections; and • final inspection. Materials-Aggregates • cement; • sand; • stone; • mixing water; • admixtures; • visual inspections; • moisture content; and • gradations. Materials-Steel • steel reinforcement; • prestressing strand; • spiral wire; • steel reinforcing bars; and • miscellaneous steel.

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Concrete Mix • mix design; • water-cement ratio; • mixing procedures; • moisture compensation; • temperature; and • hot and cold weather adjustments. Equipment Calibrations • strand stressing jacks; • batch plant weight and flow rate measuring devices; • comprehensive strength cylinder testing machine; and • product weigh scales. Pole Curing Concrete • duration and moisture control. Cylinder Tests • concrete production sampling size; • concrete cylinder compression test results; • strength comparison to standard deviations; and • strength comparison to correlation factor (spun concrete).

REFERENCES American Concrete Institute (ACI). (2008). “Building code requirements for structural concrete.” ACI 318, Farmington Hills, MI. ASTM. (2005). “Standard test method for determining density of structural lightweight concrete.” C567, West Conshohocken, PA. ASTM. (2007). “Standard practice for making and curing concrete test specimens in the laboratory.” C192/C192M, West Conshohocken, PA. ASTM. (2010a). “Standard test method for air content of freshly mixed concrete by the pressure method.” C231/C231M, West Conshohocken, PA. ASTM. (2010b). “Standard test method for air content of freshly mixed concrete by the volumetric method.” C173, West Conshohocken, PA. ASTM. (2010c). “Standard test method for compressive strength of cylindrical concrete specimens.” C39, West Conshohocken, PA. ASTM. (2010d). “Standard test method for density (unit weight), yield, and air content (gravimetric) of concrete,” C138/C138M, West Conshohocken, PA. ASTM. (2010e). “Standard test method for slump of hydraulic cement concrete.” C143, West Conshohocken, PA.

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ASTM. (2011a). “Standard specification for agencies engaged in construction inspection, testing, or special inspection.” E329, West Conshohocken, PA. ASTM. (2011b). “Standard practice for agencies testing concrete and concrete aggregates for use in construction and criteria for testing agency evaluation.” C1077, West Conshohocken, PA. Precast/Prestressed Concrete Institute (PCI). (1999). “Quality control for plants and production of structural precast and prestressed concrete products.” MNL-116, Chicago.

8 ASSEMBLY AND ERECTION

The Purchaser of concrete utility structures should consider the method of transport, access ways and road construction, culvert strength, handling, erection equipment, erection method, and sequence of construction as a part of early project planning (see Fig. 8-1). The specifications for the Manufacturer and the construction crew should clearly define structure and construction requirements to ensure an efficient and successful project. HAULING AND ACCESS Pole Delivery Hauling poles to a structure site may be a one- or two-phase process. Phase one is the transport of the pole from the Manufacturer to the jobsite. It is desirable that this phase be continued directly to the structure site. When this is not possible, a second hauling activity will be required. The Purchaser should clearly define the responsibilities for getting the poles to the individual structure sites or staging yard. To the extent practical, concrete poles should be shipped directly to the structure site to prevent double handling of the poles. Concrete pole Manufacturers generally deliver poles to the individual structure sites provided they are reasonably accessible. Typically, poles are hauled in either a horizontal position with the tip to the rear of the truck, or in an inclined position with the tip over the tractor (see Fig. 8-2). Terrain Considerations Common sense is important in determining good hauling practices. A particular setup that may be acceptable for hauling over a smooth paved 85

Fig. 8-1. Pole delivery considerations Source: Photograph courtesy of Georgia Transmission Corp. Reproduced with permission.

Fig. 8-2. Flatbed and over-the-cab hauling Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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highway may be entirely inappropriate for hauling the same load over the variety of terrain types encountered at the jobsite. Long poles require a large turning radius and can also present problems at sudden changes in elevation along access ways. An access plan to each structure site is an important step in the use of concrete poles. A 75 ft turning radius is a good estimate for planning purposes. Particular attention should be given to transportation of poles over soft or wet ground (see Fig. 8-3). In general, no more than one-third of the length of the pole should be cantilevered and if the terrain conditions indicate that the pole will be handled roughly, the length should be less than that value. Dragging Concrete Poles Dragging concrete poles is poor practice and should be avoided. If dragging is unavoidable, the Contractor and Purchaser should reach agreement on how to proceed without damaging the pole. Spliced poles should not be dragged when connected. It is preferable not to drag a pole with hardware or insulators attached.

Fig. 8-3. Terrain considerations Source: Photograph courtesy of Georgia Transmission Corp. Reproduced with permission.

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HANDLING General Handling a concrete pole at the structure site is one of the most critical phases in the life of the structure. When lifting a pole clear of all supports, attention must be paid to the manner in which it is lifted, because the stress generated by a pole’s own weight may be significant. Poles should be hauled, handled, and erected without incurring flexural cracks. Pickup or Lifting Concrete Poles Concrete poles can be designed to be lifted or erected with a one-point pickup at the center of gravity or may require multiple-point pickups. Designing for a one-point pickup without cracking may not be economical. The Manufacturer should mark the center of gravity and locations for multipoint pickup. A pickup is defined as lifting the pole from a horizontal position at rest with a sling per the Manufacturer’s instructions. The pickup may be a total lift of the pole or may be a lift with the butt of the pole still resting on the ground. The Manufacturer should provide the Purchaser with lifting instructions for particular poles, and the Purchaser should transmit these instructions to the construction personnel. The proper placement of slings will allow either two-point handling or single-point erection without any cracking. The construction crew is responsible for the proper handling of poles per the Manufacturer’s instructions. Efficient handling of the poles will include unloading the pole (usually with a two-point pickup) at the structure site. The structure will be framed, and then using a one-point pickup with the butt on the ground, the pole will be lifted to a vertical position for placement over the hole for setting purposes (see Fig. 8-4).

FRAMING General Concrete poles generally are framed using through bolts (see Fig. 8-5). Bolt lengths should be selected such that no less than 1/4 in. extends beyond the nut or locknut. Bolts should be tightened in accordance with the assembly drawings and Purchaser specifications. Care should be taken to avoid over-tightening the bolts as this can cause longitudinal cracks in a hollow concrete pole. The strength of the pole is sufficient to withstand a reasonable degree of bolt tightness, except near the ends of a hollow

Fig. 8-4. Field handling a concrete pole Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

Fig. 8-5. Assembled shield wire and conductor arm, grounding and climbing attachments Source: Photograph courtesy of Georgia Transmission Corp. Reproduced with permission.

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pole that has not been plugged. If longitudinal cracks occur, the bolts should be loosened until the crack closes. If the cracks do not close, the Purchaser should notify the Manufacturer. Tightening Through Bolts Specific instructions should be supplied by the pole Manufacturer and connection designer for tightening bolts. The construction crew should be held responsible for the correct application of connection specifications. Inserts Ladder clips, step bolts, or other climbing devices, and groundwire connections usually are attached to inserts cast into the pole. Care must be used when making connections to inserts to avoid pulling the inserts out of the pole. Banding Attachments can be made to concrete poles using stainless or galvanized steel bands. Bands should be of sufficient strength to transfer the load from the attachment to the concrete pole. Over-tightening of the band is usually not a problem because of the inherent strength of the concrete pole in compression (see Fig. 8-6).

Fig. 8-6. Concrete pole with band-on assemblies Source: Photograph courtesy of Georgia Transmission Corp. Reproduced with permission.

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FIELD DRILLING General The Manufacturer should always be consulted prior to field drilling holes. Most concrete poles will be sent from the Manufacturer with the necessary holes already in place. Occasionally, it may be necessary to field-drill holes. This can be accomplished with a rotary hammer drill and a carbide tipped bit or a diamond tipped core bit. Field drilling should be performed from both sides of the pole to prevent spalling of the concrete on the outside face of the pole (see Fig. 8-7). Mold marks, which are

Fig. 8-7. Field drilling pole in air Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

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usually visible on the pole, make ideal reference points from which to locate the hole on the opposite face. Full Length Reinforcing Steel Most Manufacturers determine the amount of steel required for the groundline design moment capacity and provide that quantity of prestressing steel throughout the entire length of the pole, even though less steel is needed in the upper portions. Because holes are normally drilled in the upper portions of a pole where there is a considerable excess of steel, it is usually permissible to cut a limited number of strands in the drilling process. Caution should be exercised when cutting longitudinal reinforcement in any critical section of the pole. Examples of critical sections include near the groundline, near any splice joint in a pole, or near any hardware attachment location. Cutting the steel in these areas may weaken the pole below its design requirement. The Manufacturer’s drawings should be reviewed prior to drilling to determine the planes or areas that should not contain longitudinal prestressing strands. In the event that a strand cannot be cut, a new hole location should be identified and drilled. Abandoned holes should be patched with epoxy grout. Drop Out Reinforcing Steel As the need for steel decreases toward the top of the pole, a Manufacturer sometimes can terminate a portion of the steel by masking out some of the strand with plastic tubing, dropping the tendons out through the side wall, or installing additional steel in critical areas by use of posttensioned strands. When using these methods, there may not be an excess of steel near the pole tops and the steel should not be cut. This situation does not preclude drilling these poles. It means, however, that care should be used to ensure that the steel is not cut. The construction crews should obtain approval from the Purchaser and Manufacturer before cutting any prestressing strand. Because there is less steel in pole tops of this type, there is more space between the prestressing steel. Thus, it is easier to avoid the prestressing steel during the drilling process. However, cutting a strand means that the pole may be weakened below its design strength. The actual drilling of these poles is accomplished in the same manner as already discussed. Circumferential Steel Cutting of circumferential steel (spiral reinforcement) is difficult to avoid and is acceptable unless the pole is to be subjected to severe

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torsional loads. Excessive damage or cutting of the circumferential steels should be avoided and should have prior approval from the Manufacturer’s engineer. FIELD CUTTING There are occasions in which it is desirable to shorten a pole in the field. This can be accomplished without damage to the pole by cutting with a handheld concrete saw and an abrasive cut-off blade. The blade will cut both the concrete and the steel. The Manufacturer should always be consulted before field cutting a pole. Carefully mark a straight line around the circumference and saw along the mark. Preferably, the cutting surface should stay at a minimum of 2 ft away from any attachment location. After cutting, the exposed strands should be burned back and the voids sealed with epoxy grout. Care should be taken to repair damage, if any, to an internal grounding system. (See “Grounding the Installed Pole,” later in this chapter.) ERECTION Concrete poles are usually erected in the same manner as other poles. Assuming that the poles were properly placed before they were framed, a single-point pickup with a choker sling is usually permissible. The choker should be placed well above the center of gravity. This means that as the pole is raised from the horizontal position, much of its weight stays on the ground until the pole is nearly in the vertical position. Once it reaches the vertical position, it will not be damaged by lifting its full weight with a single-point pickup (see Fig. 8-8). This practice protects the pole from being subjected to a construction load that exceeds the moment/ first crack capacity of the pole. For an unguyed angle structure, this is extremely important to the integrity of the structure, because it is designed to be in service in an uncracked condition. Chokers Because the surface of a concrete pole is smooth and hard, care should be taken when using chokers. Improper use of chokers can result in the pole slipping and causing injury or property damage. The use of steel or nylon chokers for handling and erection of concrete poles is acceptable. If steel chokers are used, they should be padded to protect the surface of the pole. Chokers are to be tight around the pole. A positive stop against sliding should be provided. The Manufacturer can provide a positive stop mechanism to secure the choker to the pole for the vertical lift. Another method

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Fig. 8-8. Erecting pole structure Source: Photograph courtesy of Valmont Newmark. Reproduced with permission. is by attaching the choker below a solid piece of pole hardware. A step bolt or ladder clip should not be used to lift the pole to a vertical position. Structure Considerations Concrete pole construction utilizes single pole, multipole, H-frame, and hybrid pole construction. A hybrid structure consists of a concrete base section with the remainder of the structure being tubular steel. Because of the weight of concrete poles, each pole in a structure is usually set separately. For braced structures, such as X-braced H-frames, the bracing is usually attached after each pole has been individually set. This requires close attention during erection to establish proper setting elevations and pole-to-pole spacing. The Manufacturer can provide extra holes and special attachment mechanisms that allow for greater flexibility in fitting bracing to installed poles.

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Multiple-piece poles may be able to satisfy the need for special requirements such as length restrictions, additional height, or weight limitations on shipping. Slip-joints and bolted flanged joints are used on multiplepiece concrete poles. Installation and erection of these types of structures should be done in accordance with the Manufacturer’s recommendations. Unlike steel poles, multiple-piece concrete poles typically are erected vertically one section at a time. Assembling a spliced concrete pole on the ground is not recommended practice. For both two-piece and hybrid pole construction, it is desirable to lift the bottom section in a vertical or plumb position. This allows the embedded section to be plumbed much easier and facilitates the alignment of the complete structure. For hybrid pole bottom sections, the use of two chokers at the Manufacturer supplied lifting point will allow the concrete base section to hang in a vertically plumb position for alignment in the hole. Instructions to the pole supplier should require that a mechanism is provided to keep the chokers in place. The location of the lift point for the hybrid base should be as near to the top of the section as is practical. The Manufacturer should be consulted if a multiple-piece pole is not plumb after assembly. Hole Size and Placement of Backfill Concrete poles are typically direct embedded in earth or rock similar to wood poles. Holes for concrete poles are excavated by augering a hole at least 12 in. larger than the diameter of the pole at the butt or the largest diameter of the embedded section. Setting depths may vary depending on the type of soil encountered at the site. Unless approved by the engineer, the foundation setting depth should not be decreased. After the pole is set in the hole, the annular space around the pole is backfilled with natural earth, graded rock, or selected backfill material (see Fig. 8-9). For poles subjected to high sustained lateral loads, nonreinforced concrete is sometimes used for backfill. The backfill should be placed and compacted in lifts that will allow optimum compaction. Vibrating rock backfill can be more effective than tamping. Failure to compact the backfill adequately may allow the pole to lean after the load is applied. In cohesive soils, holes may be augered in advance of pole erection. In loose sandy soils, poles may be jetted into place or slurry used to advance augered holes. For any excavation, precautions should be taken to protect the public and workers from danger. Erecting Guyed Structures When installing guyed structures, it is important that the installation of guys does not introduce unintended bending moments into the pole

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Fig. 8-9. Select backfill around embedded pole Source: Photograph courtesy of Georgia Transmission Corp. Reproduced with permission.

(see Fig. 8-10). If these stresses are present, they will try to open up cracks in an in-service pole. Raking a pole back with guy wire tension that was installed plumb will result in additional bending moments. Guyed poles should be initially set in the position they will be under normal everyday loads. This means that, regardless of whatever bending and flexing occurs during construction and long-term use, once the conductor installation is complete and the guys adjusted, the top and the alignment along the length of the pole should be in the same position as it was originally set. Additionally, any change in guy configurations should not be made without the Manufacturer’s approval. Use of Temporary Guys Concrete poles may be temporarily guyed when it is determined that construction loads will exceed the original allowable design loads. It is recommended that a factor of safety of 2.0 be used when comparing construction loads to the ultimate capacity of the pole. Temporary guys should be installed in such a manner that they do not damage the pole.

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Fig. 8-10. Guy wires tensioning for plumb pole Source: Illustration courtesy of American Electric Power. Reproduced with permission.

Use of existing holes is preferred for attaching the guy hardware. Where these holes do not exist, the pole can be drilled or the guy hardware banded to the pole. The construction crew should contact the Manufacturer to verify that a structure can be temporarily guyed prior to application of the construction load. Helicopter Erection Helicopters may be used in special applications to erect concrete poles. Helicopter erection is advantageous when terrain or other considerations limit access to the structure site. Concrete poles can be flown from delivery points or erected into position at the site. Lifting limits of helicopters vary depending on the size of the helicopter but are generally limited to 22,000 to 23,000 lbs at sea level conditions. For higher elevations, the lifting limits are reduced. The structure designer, Manufacturer, construction crew, and helicopter company should coordinate design, manufacture, and erection early in the design process. Because concrete poles usually are heavier than other pole materials, special attention should be paid to limiting structure weight. It is also important to select delivery or staging locations as close to the final structure location as possible to avoid excess flying time with the concrete pole.

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Grounding the Installed Pole Installed poles must have a low resistance continuous path to ground. Poles can be grounded using either an external ground wire attached with ground clips on the surface of the pole or with an internal ground wire cast into the pole. In either case, the pole’s steel reinforcement should be bonded to the grounding system. Tank grounds should be provided at each hardware attachment location on internally grounded poles to bond line hardware to the internal ground wire (see Fig. 8-11). An erected pole should be grounded as soon as possible with the installation of a ground rod or equivalent at a minimum. When a pole line is parallel to an existing energized line it is advisable to install the full grounding requirement as part of the pole erection activity. IEEE 524 (IEEE 1992) requirements for stringing and working parallel to energized lines should be implemented. Temporary grounds should not be attached to step bolts. Step bolt inserts are not a bonded path to ground. Caution should be exercised when field drilling or cutting a pole with an internal ground. No field drilling should be permitted in close proximity of an internal ground wire. If there is a possibility that an internal ground may have been damaged by drilling or cutting, the Purchaser should be immediately contacted and an approved ground restoration completed prior to further field work on the pole.

Fig. 8-11. Pole grounding drawings Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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WEIGHT CONSIDERATIONS The range of weights of the poles on a project is important information and should be conveyed in the documentation for the construction contract and for construction specifications. There are expected differences between the calculated weight of a pole, the physical weight at the plant, and weight at time of erection. Each concrete pole has a calculated weight and is physically weighed at the plant. The physical weight should be recorded on the pole name plate. Additionally, jobsite conditions can affect lifting and handling requirements. Collected water inside a spun pole and lifting a pole from a muddy environment can affect the lifting force required. The construction crew should size the crane based on the published physical weight (not the calculated weight) along with jobsite conditions.

CLIMBING Concrete poles are climbed with the use of step bolts and ladders. Construction personnel may be required to install clips and climbing provisions.

STORAGE When possible, concrete poles should be delivered to the site just prior to construction. In the event that the poles are stored for a period of more than two weeks, special storage procedures are required. Concrete poles should be stored in a flat condition. Cribbing for poles should be placed in intervals sufficient to prevent the pole from developing a permanent camber (see Fig. 8-12). For poles shorter than 100 ft in length, at least two crib points should be installed at points approximately 20% of the overall length from each end of the pole. Poles longer than 100 ft should use three crib points at the one-third points of the structure. Cribbing should consist of 4 × 4 or 6 × 6 wood lagging. Cribbing should be placed on firm, level soil. Wood chocks should be installed to prevent the pole from rolling off the cribbing. Stacking of poles may be required when sufficient storage area is not available. The Manufacturer should be consulted to determine how many poles can be stacked and the manner in which they should be stacked. Special care should be taken to ensure that the soil at the storage site would support the heavy loads associated with stacking the poles.

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Fig. 8-12. Stored pole properly cribbed Source: Illustration courtesy of Valmont Newmark. Reproduced with permission.

REFERENCES Institute of Electrical and Electronics Engineers (IEEE). (1992). “Guide to the installation of overhead transmission line conductors.” IEEE 5241992, New York.

9 INSPECTION, MAINTENANCE, AND REPAIR

INSPECTION Poles should be inspected upon receipt by the Purchaser, immediately after construction, and periodically during the life of the structure. Prestressed concrete poles, when properly installed, require little or no maintenance. However, the prestressed concrete poles should be inspected periodically under the terms of agreements or warranties, as required, to ensure that there are no points of deterioration in the product. Isolated damage may occur as a result of storms, high winds, impact damage by vehicles, equipment and flying debris, vandalism, and a host of other reasons. Although the ultimate life of prestressed concrete poles may vary, existing research suggests a 50- to 100-year life is reasonable depending on some key factors, such as the environment, materials, design, and manufacturing methods. Pole Surface The surface of the prestressed concrete pole should appear smooth and uniform. Variations in color and hue are common. Over time the surface appearance may change. Age, environment, and raw materials used in manufacturing the pole all contribute to these changes. Surface appearances and changes in appearance over time are normally acceptable. Small surface shrinkage cracks may exist. These cracks can appear on small or large areas. They are close together, short in length, hairline or less in width and usually have minimal depth. Typically the cause is associated with a thermal transition during the life of the structures. These small surface shrinkage cracks have no negative impact to structural integrity. 101

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Minor marring, scars, scrapes, cracks, abrasions, nicks, voids, chips, and spalls are normal and usually acceptable. Exceptions to this would be the following: • Minimal or no protection remains over the internal steel reinforcement, allowing corrosion to take place. • Size and depth of the damage compromises the structural integrity of the pole. • The damage is a symptom of a structural issue creating an excessive load and resulting in stresses beyond the structural capacity of the pole, regardless of the cause. Cracks that are circumferential (around the pole) or longitudinal (up and down the pole) need to be closely reviewed. If there is only one crack that is short (1 to 6 in.) and has a small width (2 to 4 mils), it is probably acceptable. Larger cracks, especially if there are many, should be reported to the Purchaser to determine the proper course of action. Rust bleeds on the surface of the pole may indicate a metal attachment or hardware may be corroding. Another possible cause of rust bleeds could be a piece of interior metal, usually inside the wall of the concrete pole, which may be at the pole surface. Normally, there is a minimum of 3/4 in. of concrete cover over spiral reinforcement. In most instances, the exposed piece is a scrap or overlooked piece of tie wire that was not properly turned to be below the pole surface during manufacturing of the pole. In this case, the steel should be ground down into the pole at least 1/4 in. and repaired according to the methods to be discussed later in this chapter. Burn marks usually indicate a fire, lightning strike, or some other electrical arc. These burns should always be investigated. The pole may not be severely damaged, but the burn marks indicate possible problems. All metal attachments and hardware should be checked to ensure that they are in good condition and that they are properly secured and grounded. The groundwire, either external or internal to the pole, should be intact and properly grounded. Occasionally, an electrical arc will cause a small area of the concrete wall to spall. This is usually caused by an improperly grounded system or a lightning strike. The grounding method and any related damage should be corrected and the concrete pole repaired similar to the methods described later in this chapter. Straightness The pole should always be reasonably plumb vertically. Major curvature along the length of the pole during everyday loading conditions may suggest the pole is over-stressed for its intended purpose. Guyed, braced,

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or framed structures tend to be structures subject to this type of condition. Poles should be inspected for possible cracks in the area of curvature and the Purchaser should be notified to determine the proper course of action. Guyed poles should be plumb and straight with all wires and attachments installed and the guy wires installed to final tensions. Guy wire tensions should be checked during routine inspections and tension adjustments made as required (see Chapter 8, Fig. 8-10). After construction is complete, the alignment of crossarms, arms, and attachments on a structure should be in accordance with design specifications. Other Items The pole cap (if originally required) should be present. Absence of the cap may be acceptable provided there is little or no evidence of corrosion. Usually the Manufacturer can furnish replacement caps should they be needed. The name plate should be present and visible. All attachments should be secured and aligned properly; all bolts, nuts, and locking devices should be present and properly fastened. The ground near the pole should be checked for evidence of pieces or parts that may have fallen from the structure, and it should be determined whether action is required based on the findings. Some prestressed concrete poles have been treated with a color stain or other coating. This application may have been for a barrier coating or for aesthetics. The coating should be examined for any apparent deterioration. It may be necessary to consult the coating manufacturer should any repairs be required. Guy wires, which are important components of the overall stability of a structure, should be examined and retensioned, if needed. Per applicable codes, climbing devices, if present, should be inspected and replaced as needed. To inhibit climbing by unauthorized persons, no climbing devices should be permanently installed below the required elevation above ground. Grounding systems and attachments should be checked for proper installation to prevent electrical damage to the pole and other related components.

MAINTENANCE AND REPAIR Minor repairs to the concrete pole or galvanized steel components can be addressed by a field crew. An example of minor damage to a concrete pole is a small area of local surface spalling of concrete that does not compromise the pole’s structural integrity.

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Significant repair work should be addressed by the Purchaser and possibly the Manufacturer to ensure the adequacy and durability of the repair. Damage that can affect the structural integrity and longevity of the pole or its components should be reported to the Purchaser immediately. Field crews should not attempt repair of this type of damage without first being advised of the proper repair procedure and obtaining the engineer’s permission. The pole may need to be analyzed to determine its suitability and whether replacement is necessary.

Damage The location, size, and shape of a crack/damage may be critical to the structure’s performance. Cracks or damage of concern should be initially reviewed by the Purchaser’s qualified inspector. Further investigation, when required, should be performed by a licensed professional engineer who is familiar with the requirements of the specific structure. There are two basic types of damage to concrete that normally occur: local spalling and cracks. Local spalling is where a small area of the concrete has been broken away, typically as a result of an isolated high-impact load. When concrete has spalled, a void is made in the concrete wall of the pole. The other type of damage that may occur is cracking. Cracks may be longitudinal (up and down the pole), or circumferentially around or partially around the pole. Cracking during construction typically occurs when the pole has been overstressed during transportation, handling, or installation. Cracks that appear after the pole is in service can be the result of severe storms, very high winds, impact loads, incorrect installation, misuse, or defects, and other causes. Circumferential cracks through the thickness of the concrete may or may not expose a small portion of steel reinforcement. These cracks, when caused by temporary overload, tend to close and naturally seal, minimizing possible corrosion. Circumferential cracks that remain open for a period of time suggest possible permanent overloading of the section and require the Purchaser’s review to correct and repair the crack or replace the pole. Longitudinal cracks should be closely monitored. Should a longitudinal crack open sufficiently and have a significant length, the rate of possible corrosion of the steel reinforcement can be much greater. Cracks may vary in width from being barely visible to easily visible with the naked eye. If the crack is short and barely visible, it probably does not propagate through the thickness of the concrete. Many hairline cracks that are long in length may not require repair. If the cracks appear to close after installation, normally no further repair is necessary. Larger cracks that appear to remain open should be monitored and possibly repaired.

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Prestressed high-strength concrete poles that have experienced some form of cracking can experience autogenous healing. This phenomenon generally occurs when a crack resulting from a temporary excessive load that has been applied and released appears in the concrete. In the presence of newly introduced moisture, the unhydrated cement in the high-strength concrete creates a paste and seals the crack, protecting the internal steel reinforcements. Once autogenous healing takes place, the resulting bond is normally as strong or stronger than the original concrete in that area. Concrete Repair The repair area should be totally cleaned by removing loose concrete and debris. Any exposed steel should be cleaned of dirt, grease, and rust so that it may be properly coated to protect against corrosion. The concrete patching material should have nonshrink characteristics and approximately the same thermal expansion coefficient and similar color as the pole concrete. The repaired area’s surface should match the surface level and contour of the pole. Care should be taken to follow the product manufacturer’s specifications. The worker or inspector should verify that the material and ambient temperatures are within the allowed range declared in the product specifications. Repair of Surface Voids: Examples of surface voids are small cavities, honeycomb, seam leaks, or other small pits. A small cavity is not larger than 1 in. in diameter nor deeper than 1/4 in. and has no exposed internal steel reinforcing. All loose or unsound material in the area of the void should be removed. Next, the area to be repaired should be cleaned and washed with water to remove all remaining dust and other foreign particles and then allowed to dry. A thin coat of the bonding agent should be applied to the area to be patched and allowed to dry. Next, the fill material should be prepared as specified by the Purchaser. Typical fill materials include mortar mixes, grout mixes, a low viscosity epoxy, or grout/bonding agent compounds. All mixes and compounds should be prepared in accordance with the product manufacturer’s specifications. The fill material should be carefully applied to the prepared patch area. The outside surface of the patch should be leveled, usually with a steel trowel or sponge float, and possibly sanded smooth after curing. The resulting patch should follow the contours of the pole’s appearance prior to the damage. Large cavities (approximately 1 to 6 in. long) should be repaired by opening the cavity sides to approximately a 1 to 1 slope, at a minimum. All unsound material should be removed. Typically this is accomplished with a chipping hammer, chisel, sandblaster, or a mechanical grinder. Next, the area should be thoroughly cleaned and washed with water to

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remove all remaining dust and other foreign particles and then allowed to dry. The repair area should now be clean, open, and textured. If internal steel reinforcement has been exposed, the steel should be cleaned of all contaminates and corrosion and then coated, usually with an epoxy, and allowed to dry. A thin coat of the bonding agent should be applied to the area to be patched and allowed to dry. Next, the fill material should be prepared as specified by the Purchaser. Typical fill materials include mortar mixes, grout mixes, a low viscosity epoxy, or grout/bonding agent compounds. All mixes and compounds should be prepared in accordance with the product manufacturer’s specifications. The fill material should be carefully applied to the prepared patch area. The outside surface of the patch should be leveled, usually with a steel trowel or sponge float, and possibly sanded smooth after curing. The resulting patch should follow the contours of the pole’s appearance prior to the damage. Poles with cavities larger than the foregoing should be brought to the Purchaser’s attention for review and assignment of a satisfactory repair. Major concrete repairs also may include fiberglass resin wraps around the damaged zone of the pole to further protect and reinforce the affected section. Repair of Cracks: Cracks that exist after the pole is installed and propagate through the pole to the interior steel should be monitored and possibly repaired. A tapered grinding cone is commonly used to bevel the crack for the full length. All unsound material should be removed. Next, the area should be thoroughly cleaned and washed with water to remove all remaining dust and other foreign particles and then allowed to dry. If internal steel reinforcement has been exposed, the steel should be cleaned of all contaminates and corrosion and coated, usually with an epoxy, and allowed to dry. The crack is then filled with a structure epoxy that has approximately the same thermal expansion coefficients as the parent concrete. Major cracks should be reported to the Purchaser and the pole Manufacturer to determine the extent of damage to the pole and the type of repair required. Repair Finish: The pole repaired area should have a smooth appearance. Sharp or rough edges should be tooled until they are smooth. The outside surface along the length of the structure should be leveled with a steel trowel or sponge float until all projections, depressions, and irregularities have been removed, and the entire surface has a smooth texture with neat lines. At times, it may be necessary to sand the cured repaired surface to achieve the desired finish. Repair Compounds: Special patching compounds, bonding agents, and repair procedures are normally available from the concrete pole

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suppliers. The patching compound quite often is a two-part component. These components should be thoroughly blended to the correct consistency and color. The repair procedure accompanying the patching material should be followed to ensure a proper repair. Galvanizing Repair for Metal Attachments The repair area should be cleaned to thoroughly remove all rust, grease, and other contaminants. The area should be totally dry and clean prior to coating. The coating Manufacturer’s repair procedure should be followed. For different applications and size of damage, the repair material and methods may be different. Examples may include a zinc-based solder (“hot-sticking”), spray zinc (metalizing), paint containing zinc dust, or a zinc-rich paint (“cold galvanizing”). ASTM A780/A780M (ASTM 2009) should be followed.

REFERENCES ASTM. (2009). “Standard practice for repair of damaged and uncoated areas of hot-dip galvanized coatings.” A780/A780M, West Conshohocken, PA.

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10 STRUCTURE TESTING

A pole structure test may be performed to verify structural design. This test is the ultimate check on the adequacy of the entire design and manufacturing process. Poles may be tested in either a horizontal or an upright position. If only the pole is being tested, a horizontal test may be satisfactory and is easier to carry out than an upright test. In instances where the pole is being tested as a part of an entire structure, the entire assembled structure should be tested in the final design configuration. The overall responsibility for determining the need for a test typically lies with the Purchaser or the Purchaser’s representative. (In this chapter, the Purchaser and Purchaser’s representative are collectively referred to as the Purchaser; the Manufacturer and the Manufacturer’s representative are collectively referred to as the Manufacturer.) Once the need has been determined, a testing team should be formed. This team typically consists of the Purchaser, the Manufacturer, and the Test Facility Responsible Test Engineer. The Purchaser will specify the functional requirements of the test. The testing team will determine the loading and configuration requirements to meet the functional requirements. The Test Facility Responsible Test Engineer will then develop a proposed test procedure to be reviewed and modified, if necessary, by the testing team. Equipment limitations and potential risks and hazards need to be specifically addressed and communicated to the testing team, especially when testing to failure. The test facility has the primary responsibility for the safety of conducting the test. The test facility should designate a Responsible Test Engineer. This person should be familiar with the functionality and the design of the structure, the proposed procedure for structural testing, the test equipment, and the potential risks and hazards. Also, this person should be present at all times during the testing sequence and approve each modification and decision made during the process. 109

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The agreed-upon test procedure should be incorporated into the test specification. The test specification typically includes the party or parties responsible for the cost of testing; party or parties that must witness the test; decision-making matrix during the test; structure assembly details and erection specifications; test sequence, duration, and schedule; specific requirements for the test report; and all other details critical to the test. In a traditional proof test, the test setup conforms to the design conditions (i.e., only static loads are applied); the structure has level, welldesigned foundations; and the restraints at the load points are the same as in the design model. This type of test will verify the adequacy of the main components of the structure and their connections to withstand the static design loads specified for that structure as an individual entity under controlled conditions. Proof tests may provide insight into the actual stress distribution of unique configurations, fit-up verification, the performance of the structure in a deflected position, and other benefits. This test cannot confirm how the structure will react in the transmission line application where the loads will be both static and dynamic, the foundations may be less than ideal, and there is some restraint from intact wires at the load points. This chapter presents guidelines for performing a proof test using a test frame that has facilities to install a single structure in an upright position to load and monitor pulling lines in the vertical, transverse, and longitudinal directions and to measure deflections. Horizontal tests follow many of the same guidelines as an upright test. FOUNDATIONS AND ANCHORS Most prestressed concrete poles are embedded directly into the ground. It is unlikely that soil conditions at the test site will match those at the installation site. However, efforts should be made to reasonably account for differences in the deflection and P-Delta effects on the pole under load in the test foundation as compared with the actual field installation. Single Pole Structures The primary consideration in designing and installing a single-pole foundation is the ability to control the groundline rotation so as not to exceed the allowable design rotation. For test purposes, the actual amount of rotation makes little difference within a wide range except under very heavy vertical loads, where secondary moments can be significant. H-Frame Structures For an H-frame with typical configuration, the critical point in the structure is at the top of the X-brace. The magnitude of the groundline

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rotation has very little effect on the structure at the top of the X-brace. It is important, however, that the uplift, down-thrust, and structural movement be contained adequately so that the structure does not suffer premature failure when subject to unanticipated loads and as a result of twisting the structure (see Fig. 10-1). MATERIAL The test structure should be made of materials that are representative of the materials that will be used in the production structures. Material test results should be available for each important member in the test structure. All test structure material should conform to the requirements of the material specified in design. MANUFACTURE Manufacture of the prototype structure for testing should be done in the same manner and to the same tolerances and quality control as specified for the production structures.

Fig. 10-1. Full-scale vertical test of a concrete pole H-frame with steel crossarm and X-bracing Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

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ASSEMBLY AND ERECTION The test structure should be assembled in accordance with the Manufacturer’s recommendations. It may be desirable to specify detailed methods or sequences for the test structure to prove the acceptability of proposed field erection methods. Pickup points designed into the structure should be used during erection. Using the pickup points may allow observation of structure behavior under these construction loads. The completed structure should be set within the tolerances permitted in the construction specification. After the structure has been assembled, erected, and rigged for testing, the Purchaser should review the testing arrangement for compliance with the contract documents. Safety guy wires or other safety features may be loosely attached to the test structure and used to minimize consequential damage to the structure or to the testing equipment in the event of a premature failure, especially if an overload test to failure is specified.

TEST LOADS The loads to be applied to the test structure should be the loads specified for design and should include all appropriate load factors. Wind-onstructure loads typically are applied in a test as concentrated loads at selected points on the structure in a pattern to make a practical simulation of the in-service uniform loading. The magnitudes and points of application of all design loads should be specified in the test procedure. The loading rate should also be determined and specified in the agreed-upon test procedure.

LOAD APPLICATION Load lines should be attached to the load points on the test structure in a manner that simulates the in-service load application as much as possible. The attachment hardware for the test should have the same degrees of movement as the in-service hardware. V-type insulator strings should be loaded at the point where the insulator strings intersect. If the insulators for the structures in service are to be a style that will not support compression, it is recommended that wire rope be used for simulated insulators in the test. If compressed or cantilever insulators are planned for the structures, members that will simulate those conditions should be used. As the test structure deflects under load, load lines may change their direction of pull. Adjustments must be made in the applied loads so that

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the vertical, transverse, and longitudinal vectors at the load point in the deflected shape are the loads specified in the structure loading schedule. Test rigging should be designed with an adequate safety factor for the specified test loads (generally 1.25 or as otherwise deemed appropriate by the engineer developing the rigging plan).

LOADING PROCEDURE The number and sequence of load cases tested should be specified in the test procedure. It is recommended that those load cases having the least influence on the results of successive tests be tested first. In addition, the sequence should simplify the operations necessary to carry out the test program. Loads are normally incremented to 40, 50, 75, 90, and 100% of the maximum specified load and to the load at which the concrete first cracks (usually in the range of 40 to 60%). If the test facility does not have the capability for continuous recording of loads, an additional increment to 95% may be added. There should be a pause after each load increment application to allow time for recording deflections and to permit the engineers observing the test to check for signs of structural distress. The maximum load for each load case should be held for five minutes. In most cases, loads should be removed between load cases. In some noncritical situations, with the permission of the Responsible Test Engineer, the load may be adjusted as required for the next load case. Unloading should be controlled to avoid inadvertently overstressing any components.

LOAD MEASUREMENT All applied loads should be measured as close to the point of application to the test structure as possible. Loads should be measured through a suitable arrangement of load cells or by predetermined dead weights. The effects of pulley friction should be minimized. Measurement devices should be used in accordance with the device manufacturer’s recommendations and calibrated before and after the conclusion of the testing sequence.

DEFLECTION Structure deflections under load should be measured and recorded (see Fig. 10-2). Locations to be monitored should be selected to verify the

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Fig. 10-2. Horizontal concrete pole test showing deflection under load Source: Photograph courtesy of Valmont Newmark. Reproduced with permission.

deflections predicted by the design analysis. Deflection readings should be made for the before-load and load-off conditions and at all intermediate holds during loading. Deflections should be referenced to common base readings, such as the initial plumb position taken before each test load case is applied. Groundline or support displacements and rotation should be taken into consideration. Upon release of test loads after a critical load case test, a structure will normally not fully return to its un-deflected starting position. The testing specifications should state the acceptable permanent deformation.

FAILURES The provisions of the test procedure must specify whether failure occurs when there is structure collapse, initial cracking, zero tension condition, or permanent deformation. If a premature structural failure occurs, the cause of the failure mechanism should be determined and corrected. Failed and damaged members should be replaced. The load case that caused the failure should be repeated. Load cases previously completed normally are not repeated. After the structure has successfully withstood all load cases and assuming that the structure was not tested to destruction, the structure should be dismantled and all components be inspected. The test specification should state what use, if any, may be made of the test structure after the test is completed.

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REPORT The test facility should furnish a signed and sealed test report within 30 days of test completion and furnish the number of copies required by the test specifications. The report should include the following details: 1. Designation and description of the structure tested; 2. Name of the utility that will use the structure; 3. Name of the organization that specified the loading and test arrangement of the structure; 4. Name of the Responsible Test Engineer; 5. Name of the Manufacturer and Engineer of Record; 6. A brief description and the location of the test facility; 7. Names and affiliations of the test witnesses; 8. Dates of testing each load case; 9. Design and detail drawings of the structure, including any changes made during the testing program; 10. A rigging diagram with a detail of the point of attachments to the structure; 11. Calibration records of the load measuring devices; 12. A loading diagram for each load case tested; 13. A tabulation of deflections for each load case tested; 14. In the case of a failure: a. Photographs of the failure, b. Loads at the time of failure, c. Remedial actions taken, d. Physical dimensions of the failed components, and e. Material test coupon reports of failed components, if required; 15. Photographs of the overall testing arrangement and rigging; 16. Air temperature, wind speed and direction, precipitation, and other pertinent meteorological data; 17. Mill test reports for steel and concrete cylinder compression breaks taken at the time of the test; and 18. Additional information specified by the Purchaser.

SPECIAL CONSIDERATIONS FOR HORIZONTAL TESTING Horizontal testing is primarily used to test a single pole. Most of the previous sections of this chapter also apply to horizontal testing. A fullscale horizontal destruction test should verify the structural integrity of the pole to withstand the maximum design stresses. All critical points along the pole shaft should be tested to maximum design load.

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Test Arrangement The structure is placed in a horizontal position. Locations along the shaft will be selected as the load pulling point for each test load case. The purpose of the load pull will be to duplicate the maximum design stress at all critical sections in the pole shaft based on the cross-sectional geometry of the shaft and yield strength of the materials. Critical sections are those sections on the shaft with the highest stress.

Equipment Used in Test The load is pulled at predetermined points along the shaft by a crane or other suitable pulling device. Loads should be determined with a calibrated load instrument located in the pulling line. A tape or transit should be used to take deflection measurements.

Test Procedure for Pole Test—Horizontal Pull The pole is placed between the reaction blocks and locked in place. One or more wheeled support devices should be used to support the weight of the cantilevered end of the pole (see Fig. 10-3). An initial load of at least 10% of the maximum test load should be applied to “set” the pole into the blocking. When the “setting” load is removed, the zero position is then established from which to measure subsequent deflections. Some concrete poles may require an intermediate, free-rolling vertical support(s) to minimize moment created by the weight of the pole.

Fig. 10-3. Horizontal testing configuration Source: Illustration courtesy of American Electric Power. Reproduced with permission.

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It is critical that the wheeled support device can operate with a minimum of friction to obtain meaningful results. Typically, the setup includes steel wheels with bearings or steel rollers, either of which will roll on the steel plate. All of the rolling surfaces must be kept free of debris. Other types of reduced-friction supports have been proven successful as well.

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APPENDIX I SAMPLE PURCHASER TECHNICAL SPECIFICATIONS FOR SPUN-CAST PRESTRESSED CONCRETE POLES FOR TRANSMISSION AND DISTRIBUTION STRUCTURES 1. SCOPE This specification covers the minimum requirements for the design, materials, fabrication, inspection, delivery, and drawings for spun-cast prestressed concrete poles and all appurtenances including holes, inserts, and hardware. 2. GENERAL REQUIREMENTS 2.1 Manufacturer The Manufacturer must be an established company that has produced poles of a similar type and height within the last two years, built similar structures of like quantity per project, has proven quality and delivery performance, or has been prequalified by the Purchaser prior to bidding. All structural design calculations must be prepared by an engineer experienced in prestressed concrete design, working under the direction of a registered professional engineer. 2.2 Specifications Any specification, code, or document referred to in this specification is to be considered as part of this specification. In the event of conflict between this specification and referenced documents, the requirements of this specification shall take precedence. The latest revision of the following specifications, standards, and codes apply throughout this specification unless otherwise noted: 119

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2.2.1 Precast/Prestressed Concrete Institute (PCI). “Guide Specification for Prestressed Concrete Poles” (JR 257) and “Guide for Design of Prestressed Concrete Poles” (JR 412) 2.2.2 Precast/Prestressed Concrete Institute (PCI). “Manual for Quality Control for Plants and Production of Structural Prestressed Concrete Products” (MNL-116) 2.2.3 ASCE. “Guide for the Design and Use of Concrete Poles” 2.2.4 IEEE. “National Electric Safety Code” (ANSI C-2) 2.2.5 American Society for Testing and Materials (ASTM) as pertaining to the steel, concrete, and admixtures used in pole fabrication: ASTM C150 “Portland Cement” ASTM C33 “Concrete Aggregate” ASTM C494 “Chemical Admixtures for Concrete” ASTM A421 “Uncoated Stress-Relieved Steel Wire for Prestressed Concrete” ASTM A416 “Steel Strand, Uncoated Seven-Wire for Prestressed Concrete” ASTM A615 “Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement” ASTM A82 “Steel Wire, Plain, for Concrete Reinforcement” ASTM C618 “Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete” 2.2.6 American Concrete Institute Standard (ACIS). “Building Code Requirements for Structural Concrete” (ACI 318)

3. DESIGN 3.1 Design Criteria When moment curves, shear, torsion, and axial components are furnished by the Purchaser, those values shall be the final ultimate conditions for structure design purposes, including any secondary moments (P-Delta effect) due to the structure deflection. Otherwise, the pole designs shall be prepared from load trees or wire data, configuration drawings, and design load cases furnished by the Purchaser or the Purchaser’s consulting engineer. The pole’s ultimate capacity shall be capable of withstanding all specified loads, including the secondary moments. The load information provided by the transmission and distribution (T&D) Purchaser will include wind and/or ice criteria on the wires. The Manufacturer will apply the specified load conditions to the structure and appurtenances (platforms, fixtures, antennas, etc.), when applicable.

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3.1.1 Fixity: Fixity, unless otherwise specified, will be assumed to be located at the groundline. 3.1.2 Buckling on guyed poles: Guyed structures shall be checked for critical buckling to determine the ability to withstand the combined effect of the vertical guy component plus the induced moment in the pole. The Manufacturer shall assume fixity at the groundline for guyed structures and no preload in the guy wires unless specified. The Purchaser will specify the guy wire properties to be used in the design, including size, strength, area, modulus of elasticity, and any stress reductions to be applied. 3.1.3 Handling requirements: The pole design shall include allowances for loads due to handling, hauling, storage, and erection without failure (including an appropriate impact factor) when handled according to the Manufacturer’s recommendations and instructions and reasonably accepted construction practices. The following handling and erection loads are to be provided for: 3.1.3.1 Lift points: The handling of poles in the horizontal position shall be accommodated by the provision of a two-point pickup, with the center of gravity and two lift points specified by the Manufacturer. 3.1.3.2 Handling instructions: The necessary support criteria for hauling and horizontal storage of poles shall be specified by the Manufacturer. 3.1.3.3 Pole installation: The pole shall be designed for a single-point pickup for erection, located at a distance of approximately onethird the pole length from the pole tip, with the butt of the pole on the ground. 3.1.3.4 Special consideration: Unless specified, the poles are not required to be designed for a single-point pickup at the balance point. If it becomes necessary to require this condition, it will be called out specifically on the drawings or in the stated load requirements furnished by the Purchaser. 3.1.4 Embedment depth: Poles should be designed to an embedment depth (or point of assumed fixity) as specified by the purchaser.

4. STRENGTH REQUIREMENTS 4.1 Ultimate Strength Pole designs shall be based on the Ultimate Strength Method. Ultimate capacity is defined as the point at which the pole fails, usually by crushing of the concrete in compression when ultimate strain is reached. Factored loads resulting from criteria given in loading tables or wire data,

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combined with the structure’s configuration and deflection under load, will generate the resulting applied ultimate moment, ultimate shear, ultimate vertical force, and ultimate torsion at each elevation investigated or at critical points along the pole. The pole’s capacity along the length of the pole shaft shall exceed the combined effect of these values. All poles shall be designed to withstand the ultimate controlling load cases as specified. 4.2 Cracking Strength All poles shall be designed to withstand first crack and crack reopening load cases as specified. 4.3 Below Grade Reinforcement If the pole strength is initially determined by groundline moment, the Manufacturer shall make certain that sufficient reinforcing is included in the portion of the structure that is below grade to resist the increase in both moment and shear due to specified loads. 4.4 Analysis Assumptions 4.4.1 Free standing poles: Free standing pole analyses generally will result in the maximum moment occurring at the groundline, unless a different point of fixity is supplied by the Purchaser. The pole designer must account for the additional P-Delta load when the Purchaser requires design parameters allowing the pole to translate and/or rotate in the foundation. 4.4.2 Guyed poles: In the design of guyed poles, the poles shall be considered as indeterminate structures and shall be designed as such, taking the strength and material properties of the poles and guy wires into account as an integral part of the structure. In making the analysis for guyed structures, the maximum load applied to the guy wires shall not exceed 90% of its rated breaking strength or as specified by the Purchaser. The Manufacturer shall advise the Purchaser if any guy wire sizes initially specified are inadequate. 4.4.2.1 Guyed in one plane: Structures that are guyed in one plane only shall be treated as free-standing cantilevers in the plane 90° to the guyed direction and shall have the P-Delta effect considered in that direction. 4.4.2.2 Guyed on the bisector: For structures guyed on the bisector or in two directions, a three-dimensional stiffness analysis or finite element analysis shall be made to account for the combined effects of the various load applications. 4.4.2.3 Guyed structures are plumb: All guyed structures are assumed to be plumb (no vertical eccentricity in the longitudinal centerline of the pole) when installed and in service.

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5. PHYSICAL CHARACTERISTICS 5.1 Shape and Size All poles shall be circular or polygonal in cross section and centrifugally spun and shall have a uniform outside taper not greater than 0.216 in. per ft. Unless shown otherwise on the drawings or load tables, all structures shall have a minimum of a 2-1/2-in. wall thickness at the pole tip. 5.2 Void and End Treatment The pole shall contain a void designed to be consistent with the strength requirements and weight reduction. Both the top and bottom ends of the pole shall be plugged with an epoxy-grout or nonshrink dry-packed concrete mix. The plug shall be a minimum of 12 in. thick, except that the tip plug need not exceed the distance to the first through hole or 6 in., whichever is the least distance. The prestressed steel strands at both ends of the pole shall be burned back at least 1 in. into the concrete, and the slag and carbon shall be cleaned off prior to the plugging of the resulting hole with an epoxy-grout mix. A metal pole cap, suitably fastened to prevent removal, may be used at the pole tip in lieu of plugging the void and burning the strand. If a pole tip cap is used, the strand shall be cut flush with the tip surface and the ends cleaned and coated with an epoxy. The bottom pole plug will have a 2-in. diameter hole through the entire thickness of the plug to allow ground water to escape from the interior void of the pole. When specified, some Purchasers may require the pole bottom be left open. 5.3 Finish The pole shall have a smooth, uniform natural concrete finish with no cracks. Sharp edges shall be tooled to smooth. The outside surface along the length of the structure shall be troweled until all projections, depressions, and irregularities have been removed and the entire surface has a smooth texture. All small cavities caused by air bubbles, honeycomb, or other small voids shall be cleaned, saturated with water, and then carefully painted with mortar. A small cavity is defined as one not larger than 1/2 in. in diameter nor deeper than 1/4-in. Large cavities not exceeding 2 in. long shall be repaired by opening the cavity sides on a 1 to 1 slope with a mechanical grinder, cleaning thoroughly, and patching with an epoxygrout mix in accordance with the Manufacturer’s product specifications. Damaged poles with cavities larger than the foregoing shall be reviewed

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by the Manufacturer’s engineer for disposition, and the engineer will notify the Purchaser for further instruction, if the poles are deemed repairable with no reduction in required strength or durability. The Manufacturer shall take necessary measures to prevent mold seam leaks that may occur during the spinning process. If excessive seam leaks are detected, the pole will be further inspected to ascertain whether sufficient quantity of cement paste has escaped to cause honeycombing or other damage to the wall. Poles exhibiting excessive signs of honeycombing shall be rejected. 5.4 Holes The Manufacturer shall drill or cast holes in the pole as specified on the drawings. All cast-in through holes shall contain a PVC sleeve extending through the full diameter of the pole. The diameter of all holes and the tolerances between holes shall be as specified as in Section 9.0 of this Appendix. 5.5 Step Inserts Unless otherwise specified on the drawings, all poles shall have 5/8-in. noncorrosive step bolt inserts cast in, spaced at a minimum distance of 15 in., but not more than 18 in. on center of 90° planes. The inserts shall be placed along the longitudinal axis of the pole starting 8 ft above groundline to 4 ft from the pole tip. Step working locations, if required, will be specified by the Purchaser. 5.6 Grounding Unless otherwise specified, the Manufacturer shall provide 1/2-in. noncorrosive inserts cast into the pole, spaced at a distance of 5 ft, beginning at the pole butt and continuing to the top of the pole, or at distances specified by the Purchaser. A brass insert (tank ground) shall be cast in at a point 6 in. from the pole top and at a point near the groundline and connected internally to the steel reinforcing by a #6 copper wire. Each insert shall be equipped with a 1/2-in. bolt and galvanized wire clip for the attachment of the Purchaser’s groundwire. 5.7 Marking All poles shall have cast in, on one face, a noncorrosive metallic identification plate containing the Manufacturer’s name, the name of the Purchaser, date of manufacture, pole length, actual scaled weight, a unique fabrication or serial number, and the ultimate moment capacity at the groundline for strength identification. The name plate should be approximately 5 ft above the groundline.

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5.8 Concrete Cover The vertical or prestressing steel shall have a cover of concrete to the outside face or to the inside void not smaller than 1 in. The minimum concrete cover on spirals shall be at least than 3/4 in.

6. MATERIALS 6.1 Concrete 6.1.1 Mix: The wet concrete mix used shall contain no more than 0.40 pounds per cubic yard (lbs/cy) of total chloride ions and shall have a maximum water-cement ratio of 0.40 by weight. 6.1.2 Concrete cylinder strength: Concrete used shall have a static cylinder compressive strength (known as f c′) at 28 days not less than 10,000 psi. The cement, water, aggregates, and admixtures used shall conform to the applicable ASTM standards for those materials and be of such quality as to prevent pyrite staining or damage due to sulfates or adverse aggregate alkali reaction. 6.1.3 Materials: All concrete material shall conform to the latest revision of ACI 318. The chemical properties of all concrete materials shall be such that neither pyrite staining nor efflorescence due to sulfates and/or chlorides will occur. Portland cement shall conform to the latest revision of ASTM C150. Ready-mix concrete shall conform to the latest revision of ASTM C94. Aggregates shall conform to the latest revision of ASTM C33 or C330. Course aggregate shall be crushed stone that is clean, tough, and well graded. The maximum size shall be 3/4 in. or threefourths of the clear spacing between the reinforcing steel or of the clear spacing between the reinforcing steel and the surface of the pole. The resistance to abrasion shall not exceed a percentage of wear of 40% per the latest revision of ASTM C131. Fine aggregate shall be natural sand consisting of clean, strong, hard, durable, uncoated, and well-graded particles. Water used in mixing concrete shall be free of oils, acids, alkalis, salts, organic matter, or other substances in amounts that may be harmful to concrete or steel. Chemical admixtures shall conform to the latest revision of ASTM C494. Air entraining agents, if any, shall conform to the latest revision of ASTM C260; the fly ash or other pozzolanic admixtures shall conform to the latest revision of ASTM C618.

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6.2 Prestressing Steel Tendons 6.2.1 Specifications: The prestressing steel tendons shall be stress relieved seven-wire strand conforming to the latest revision of ASTM A-416, “Specification for Uncoated Seven-Wire Stress-Relieved Strand for Prestressed Concrete,” for both normal and low relaxation strand. 6.2.2 Welding: Welding of the prestressed steel is not allowed. 6.3 Inserts All inserts shall be manufactured of a noncorrosive material. 6.4 Spiral Wire All poles shall contain spiral steel reinforcement throughout the entire pole length. The minimum diameter of the spiral wire shall be 3/16 in. and conform to ASTM A-82. The spacing shall conform to the following: 6.4.1 Pitch spacing: For a distance of 3 ft from the tip and butt of the pole, spirals shall be spaced a maximum of 1-3/4 in. on center but no less than 1-1/4 in. 6.4.2 Torsional loads: If torsional loads are encountered, the spiral spacing shall be adjusted, if necessary, to provide sufficient reinforcement in accordance with the latest version of ACI 318. 6.4.3 Groundline and embedded portion of the pole: Closer spiral spacing may be required in the region between 3 ft above groundline to the pole butt to adequately resist the increased shear encountered in developing the resisting soil pressure and near guy wire or dead-end attachments where high shear concentration occurs. 6.4.4 Maximum spacing: The maximum center to center spacing (pitch) throughout the remainder of the pole shall be such that the following condition is satisfied: Av(Fv/s) ≥ 0.48 where Av is the area of one spiral leg in square inches, Fv is the design yield stress of the grade of steel used in ksi (≤60 ksi), and s is the center to center spacing of the spirals in inches. In no case shall the spacing s exceed 4 in. 7. TESTING FOR CONCRETE STRENGTH The Manufacturer shall, at company expense, take at least eight representative cylinders of each day’s pour and test as follows: one at release, two at seven days, two at 14 days, and three at 28 days. A copy of the test reports shall be furnished to the Purchaser upon request.

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7.1 Strength Correlation Factor The Manufacturer shall show a correlation factor obtained by documented test results between the value of the ultimate concrete strength (f c′) used in the design of spun poles and the value of the static-cast cylinders. If no correlation between static-cast and spun concrete is furnished, then the maximum spun concrete design strength allowed shall not exceed the static-cast cylinder test by more than 5%. 7.2 Minimum Shipping Strength Poles may not be shipped until the static concrete strength test has achieved the 28-day value used in the design. An exception to this may be taken only with prior approval of the Purchaser and only if the Manufacturer can produce statistical records showing a consistent gain in concrete strength over a predictable time period that would indicate that an earlier shipping date would still result in the design strength being achieved within the 28-day period. 8. INSPECTION AND QUALITY ASSURANCE 8.1 Inspection by Purchaser The Purchaser or a company-authorized representative shall have access to the work at all times for inspection wherever the poles are in preparation or progress. The Manufacturer shall provide proper facilities for such access and inspection without additional cost to the Purchaser. All materials will be subject to jobsite inspection. To avoid delays and complications, the Purchaser will make every effort to inspect the poles upon delivery and notify the Manufacturer of any problems immediately. Material may be rejected at the time of the first inspection or at any time defects are found during the progress of the erection or installation. Inspection by the Purchaser or waiving of inspection shall not relieve the Manufacturer of the responsibility for furnishing products that conform to the requirements of this specification. 8.2 Manufacturer’s Quality Assurance Program The Manufacturer shall have an active in-plant quality assurance program and perform daily checks and tests on the products made. The program shall cover the entire production process, including the delivery of the product. A written record of these tests and inspections shall be made at the Manufacturer’s plant facility for the Purchaser’s review. Each pole manufactured shall have a unique identification number that correlates to a specific quality control inspection traveler and report.

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9. MANUFACTURING TOLERANCES 9.1 Product Tolerances Product tolerances shall be limited to the following: 9.1.1 Overall length: +3 in. / −2 in. 9.1.2 Pole diameter: + / −1/4 in. 9.1.3 Wall thickness: Allowable variation along the pole shall not be greater than +20%, with a maximum reduction of 1/4 in., provided that coverage over steel is maintained. Each pole shall be inspected for uniformity of inside appearance and wall thickness variation. If irregularities are encountered, then actual thickness measurements shall be taken by drilling pilot holes through the wall at 10-ft. intervals on the longitudinal axis of the pole. These holes shall be alternated 90° at each interval. 9.1.4 End squareness: + / −1/2 in. per ft. of pole diameter. 9.1.5 Pole sweep: Sweep is the deviation of a pole from straightness. Sweep will be allowed in one plane and one direction only. A straight line joining the edges of the structure at both the top and the butt shall not be distant from the pole surface at any point more than 3/8 in. for each 10 ft of length between these two points. 9.1.6 Weight: + / −10% of computed value. 9.1.7 Location of longitudinal reinforcement at stressing header: + / −1/4 in. for individual strands, + / −1/8 in. for the centroid of a group of strands. 9.1.8 Spiral reinforcement: + / −25% spacing variance, with total quantity per ft. maintained. 9.1.9 Location of a group of bolt holes from pole tip: + / −2 in. 9.1.10 Location of centerline between groups of bolt holes: + / −1 in. 9.1.11 Location of bolt holes within a group of bolts: + / −1/8 in. 9.1.12 Bolt hole diameter: +1/8 in. of specified hole diameter or +1/4 in. greater than actual bolt diameter. 9.1.13 Bolt hole alignment within a group of bolts: within 1/2 of the hole diameter from the bolt plane that is longitudinal to the pole’s crosssectional centerline in a group.

10. POLE TESTS 10.1 Qualifying Tests If the successful bidder to this specification has not previously supplied concrete poles of similar types and is unable to submit sufficient strength data from previous structural tests, the bidder may be required to provide

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pole tests. The design and test procedures shall be submitted for approval before the test is conducted. All tests shall be witnessed by the Purchaser or authorized representative. Samples shall be selected randomly by the Purchaser and be tested to failure. 10.2 Random Test Option At any time during the life of this contract, the Purchaser, at the Purchaser’s expense, may request that certain poles, selected randomly from the production, be tested to their full design loads. If the test shows a complete satisfaction without significant permanent deflection, that tested pole and the cost of performing the test may be applied to the Purchaser’s purchase order. In case of failure prior to achieving full design load, the Manufacturer shall be responsible for the total cost of the initial test and shall then undertake corrective measures or redesign of the structure, as deemed necessary, and shall retest the structure, at the Manufacturer’s expense, in the presence of the Purchaser or the Purchaser’s representative. 10.2.1 Test procedure: The test shall be performed in accordance with a method mutually approved by the Purchaser or authorized representative and the Manufacturer. The Manufacturer shall furnish a test report for each structure tested that is signed and sealed by a licensed professional engineer within four weeks of each structure test. The test report shall include the method of application of the loads, the positions of hairline and/or failure cracks, and the deflections under various loading conditions.

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APPENDIX II SAMPLE PURCHASER TECHNICAL SPECIFICATIONS FOR STATIC-CAST PRESTRESSED CONCRETE POLES FOR TRANSMISSION AND DISTRIBUTION STRUCTURES 1. SCOPE This specification covers the minimum requirements for the design, materials, fabrication, inspection, delivery and drawings for static-cast prestressed concrete poles and all appurtenances including holes, inserts, and hardware. 2. GENERAL REQUIREMENTS 2.1 Manufacturer The Manufacturer must be an established company that has produced poles of a similar type and height within the last two years, built similar structures of like quantity per project, has proven quality and delivery performance, or has been prequalified by the Purchaser prior to bidding. All structural design calculations must be prepared by an engineer experienced in prestressed concrete design, working under the direction of a registered professional engineer. 2.2 Specifications Any specification, code, or document referred to in this specification is to be considered as part of this specification. In the event of conflict between this specification and referenced documents, the requirements of this specification shall take precedence. The latest revision of the following specifications, standards, and codes apply throughout this specification unless otherwise noted: 131

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2.2.1 Precast/Prestressed Concrete Institute (PCI). “Guide Specification for Prestressed Concrete Poles” (JR 257) and “Guide for Design of Prestressed Concrete Poles” (JR 275) 2.2.2 Precast/Prestressed Concrete Institute (PCI). “Manual for Quality Control for Plants and Production of Structural Prestressed Concrete Products” (MNL-116) 2.2.3 ASCE. “Guide for the Design and Use of Concrete Poles” 2.2.4 IEEE. “National Electric Safety Code” (ANSI C-2) 2.2.5 American Society for Testing and Materials (ASTM) as pertaining to the steel, concrete, and admixtures used in pole fabrication: ASTM C150 “Portland Cement” ASTM C33 “Concrete Aggregate” ASTM C494 “Chemical Admixtures for Concrete” ASTM A421 “Uncoated Stress-Relieved Steel Wire for Prestressed Concrete” ASTM A416 “Steel Strand, Uncoated Seven-Wire for Prestressed Concrete” ASTM A615 “Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement” ASTM A82 “Steel Wire, Plain, for Concrete Reinforcement” ASTM C618 “Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete” 2.2.6 American Concrete Institute (ACI) “Building Code Requirements for Structural Concrete” (ACI 318)

3. DESIGN 3.1 Design Criteria When moment curves, shear, torsion, and axial components are furnished by the Purchaser, those values shall be the final ultimate conditions for structure design purposes, including any secondary moments (P-Delta effect) due to the structure deflection. Otherwise, the pole designs shall be prepared from load trees or wire data, configuration drawings, and design load cases furnished by the Purchaser or the Purchaser’s consulting engineer. The pole’s ultimate capacity shall be capable of withstanding all specified loads, including the secondary moments. The load information provided by the transportation and distribution (T&D) Purchaser will include wind and/or ice criteria on the wires. The Manufacturer will apply the specified load conditions to the structure and appurtenances (platforms, fixtures, antennas, etc.), when applicable.

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3.1.1 Fixity: Fixity, unless otherwise specified, will be assumed to be located at the groundline. 3.1.2 Buckling on guyed poles: Guyed structures shall be checked for critical buckling to determine the ability to withstand the combined effect of the vertical guy component plus the induced moment in the pole. The Manufacturer shall assume fixity at the groundline for guyed structures and no preload in the guy wires unless specified. The Purchaser will specify the guy wire properties to be used in the design including size, strength, area, modulus of elasticity, and any stress reductions to be applied. 3.1.3 Handling requirements: The pole design shall include allowances for loads due to handling, hauling, storage, and erection without failure (including an appropriate impact factor) when handled according to the Manufacturer’s recommendations and instructions and reasonably accepted construction practices. The following handling and erection loads are to be provided for: 3.1.3.1 Lift points: The handling of poles in the horizontal position shall be accommodated by the provision of a two-point pickup, with the center of gravity and two lift points specified by the Manufacturer. 3.1.3.2 Handling instructions: The necessary support criteria for hauling and horizontal storage of poles shall be specified by the Manufacturer. 3.1.3.3 Pole installation: The pole shall be designed for a single-point pickup for erection, located at a distance of approximately onethird the pole length from the pole tip, with the butt of the pole on the ground. 3.1.3.4 Special consideration: Unless specified, the poles are not required to be designed for a single-point pickup at the balance point. If it becomes necessary to require this condition, it will be called out specifically on the drawings or in the stated load requirements furnished by the Purchaser. 3.1.4 Embedment depth: Poles should be designed to an embedment depth (or point of assumed fixity) as specified by the purchaser.

4. STRENGTH REQUIREMENTS 4.1 Ultimate Strength Poles designs shall be based on the Ultimate Strength Method. Ultimate capacity is defined as the point at which the pole fails, usually by crushing of the concrete in compression when ultimate strain is reached. Factored loads resulting from criteria given in loading tables or wire data,

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combined with the structure’s configuration and deflection under load will generate the resulting applied ultimate moment, ultimate shear, ultimate vertical force, and ultimate torsion at each elevation investigated or at critical points along the pole. The pole’s capacity along the length of the pole shaft shall exceed the combined effect of these values. All poles shall be designed to withstand the ultimate controlling load cases as specified. 4.2 Cracking Strength All poles shall be designed to withstand first crack and crack reopening load cases as specified. 4.3 Below Grade Reinforcement If the pole strength is initially determined by groundline moment, the Manufacturer shall make certain that sufficient reinforcement is included in the portion of the structure that is below grade to resist the increase in both moment and shear due to specified loads. 4.4 Analysis Assumptions 4.4.1 Free standing poles: Free standing pole analyses generally will result in the maximum moment occurring at the groundline unless a different point of fixity is supplied by the Purchaser. The pole designer must account for the additional P-Delta load when the Purchaser requires design parameters allowing the pole to translate and/or rotate in the foundation. 4.4.2 Guyed poles: In the design of guyed poles, the poles shall be considered as indeterminate structures and shall be designed as such, taking the strength and material properties of the poles and guy wires into account as an integral part of the structure. In making the analysis for guyed structures, the maximum load applied to the guy wires shall not exceed 90% of its rated breaking strength or as specified by the Purchaser. The Manufacturer shall advise the Purchaser if any guy wire sizes initially specified are inadequate. 4.4.2.1 Guyed in one plane: Structures that are guyed in one plane only shall be treated as free-standing cantilevers in the plane 90° to the guyed direction and shall have the P-Delta effect considered in that direction. 4.4.2.2 Guyed on the bisector: For structures guyed on the bisector or in two directions, a three-dimensional stiffness analysis (or finite element analysis) shall be made to account for the combined effects of the various load applications.

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4.4.2.3 Guyed structures are plumb: All guyed structures are assumed to be plumb (no vertical eccentricity in the longitudinal centerline of the pole) when installed and in service. 5. PHYSICAL CHARACTERISTICS 5.1 Shape and Size All poles shall be circular or polygonal in cross section and shall have a uniform outside taper not greater than 0.216 in. per ft. Unless shown otherwise on the drawings or load tables, all structures shall have a minimum of a 2-1/2 in. wall thickness at the pole tip. 5.2 Void and End Treatment The pole shall contain a void designed to be consistent with the strength requirements and weight reduction. Both the top and bottom ends of the pole shall be plugged with an epoxy-grout or nonshrink dry-packed concrete mix. The plug shall be a minimum of 12 in. thick except that the tip plug need not exceed the distance to the first through hole or 6 in., whichever is the least distance. The prestressed steel strands at both ends of the pole shall be burned back at least 1 in. into the concrete, and the slag and carbon shall be cleaned off prior to the plugging of the resulting hole with an epoxy-grout mix. A metal pole cap, suitably fastened to prevent removal, may be used at the pole tip in lieu of plugging the void and burning the strand. If a pole tip cap is used, the strand shall be cut flush with the tip surface and the ends cleaned and coated with an epoxy. The bottom pole plug will have a 2-in. diameter hole through the entire thickness of the plug to allow ground water to escape from the interior void of the pole. When specified, some Purchasers may require the pole bottom to be left open. 5.3 Finish The pole shall have a smooth, uniform natural concrete finish with no cracks. Sharp edges shall be tooled to smooth. The outside surface along the length of the structure shall be troweled until all projections, depressions, and irregularities have been removed and the entire surface has a smooth texture. All small cavities caused by air bubbles, honeycomb, or other small voids shall be cleaned, saturated with water, and then carefully painted with mortar. A small cavity is defined as one not larger than 1/2 in. in diameter nor deeper than 1/4 in. Large cavities not exceeding 2 in. long shall be repaired by opening the cavity sides on a 1 to 1 slope with a

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mechanical grinder, cleaning thoroughly, and patching with an epoxygrout mix in accordance with the Manufacturer’s product specifications. Damaged poles with cavities larger than the foregoing shall be reviewed by the Manufacturer’s engineer for disposition, and the engineer will notify the Purchaser for further instruction, if the poles are deemed repairable with no reduction in required strength or durability. The Manufacturer shall take necessary measures to prevent mold seam leaks that may occur during the spinning process. If excessive seam leaks are detected, the pole will be further inspected to ascertain whether sufficient quantity of cement paste has escaped to cause honeycombing or other damage to the wall. Poles exhibiting excessive signs of honeycombing shall be rejected. 5.4 Holes The Manufacturer shall drill or cast holes in the pole as specified on the drawings. All cast-in through holes shall contain a PVC sleeve extending through the full diameter of the pole. The diameter of all holes and the tolerances between holes shall be as specified as in Section 9.0 of this Appendix. 5.5 Step Inserts Unless otherwise specified on the drawings, all poles shall have 5/8-in. noncorrosive step bolt inserts cast in, spaced at a minimum distance of 15 in., but not more than 18 in. on center of 90° planes. The inserts shall be placed along the longitudinal axis of the pole starting 8 ft above groundline to 4 ft from the pole tip. Step working locations, if required, will be specified by the Purchaser. 5.6 Grounding Unless otherwise specified, the Manufacturer shall provide 1/2-in. noncorrosive inserts cast into the pole, spaced at a distance of 5 ft, beginning at the pole butt and continuing to the top of the pole, or at distances specified by the Purchaser. A brass insert (tank ground) shall be cast in at a point 6 in. from the pole top and at a point near the groundline and connected internally to the steel reinforcing by a #6 copper wire. Each insert shall be equipped with a 1/2-in. bolt and galvanized wire clip, for the attachment of the Purchaser’s groundwire. 5.7 Marking All poles shall have cast in, on one face, a noncorrosive metallic identification plate containing the Manufacturer’s name, the name of the

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Purchaser, date of manufacture, pole length, actual scaled weight, a unique fabrication or serial number, and the ultimate moment capacity at the groundline for strength identification. The name plate should be approximately 5 ft above the groundline. 5.8 Concrete Cover The vertical or prestressing steel shall have a cover of concrete to the outside face or to the inside void not smaller than 1 in. The minimum concrete cover on spirals shall be at least than 3/4 in.

6. MATERIALS 6.1 Concrete 6.1.1 Mix: The wet concrete mix used shall contain no more than 0.40 pounds per cubic yard (lbs/cy) of total chloride ions and shall have a maximum water-cement ratio of 0.40 by weight. 6.1.2 Concrete cylinder strength: Concrete used shall have a static cylinder compressive strength (known as f c′ ) at 28 days not less than 8,500 psi. The cement, water, aggregates, and admixtures used shall conform to the applicable ASTM standards for those materials and be of such quality as to prevent pyrite staining or damage due to sulfates or adverse aggregate alkali reaction. 6.1.3 Materials: All concrete material shall conform to the latest revision of ACI 318. The chemical properties of all concrete materials shall be such that neither pyrite staining nor efflorescence due to sulfates and/or chlorides will occur. Portland cement shall conform to the latest revision of ASTM C150. Ready-mix concrete shall conform to the latest revision of ASTM C94. Aggregates shall conform to the latest revision of ASTM C33 or C330. Course aggregate shall be crushed stone that is clean, tough, and well graded. The maximum size shall be 3/4 in. or threefourths of the clear spacing between the reinforcing steel or of the clear spacing between the reinforcing steel and the surface of the pole. The resistance to abrasion shall not exceed a percentage of wear of 40% per the latest revision of ASTM C131. Fine aggregate shall be natural sand consisting of clean, strong, hard, durable, uncoated, and well-graded particles. Water used in mixing concrete shall be free of oils, acids, alkalis, salts, organic matter, or other substances in amounts that may be harmful to concrete or steel.

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Chemical admixtures shall conform to the latest revision of ASTM C494. Air entraining agents, if any, shall conform to ASTM C260; the fly ash or other pozzolanic admixtures shall conform to the latest revision of ASTM C618. 6.2 Prestressing Steel Tendons 6.2.1 Specifications: The prestressing steel tendons shall be stress relieved seven-wire strand conforming to the latest revision of ASTM A-416, “Specification for Uncoated Seven-Wire Stress-Relieved Strand for Prestressed Concrete,” for both normal and low relaxation strand. 6.2.2 Welding: Welding of the prestressed steel is not allowed. 6.3 Inserts All inserts shall be manufactured of a noncorrosive material. 6.4

Spiral Wire

All poles shall contain spiral steel reinforcement throughout the entire pole length. The minimum diameter of the spiral wire shall be 3/16 in. and conform to the latest revision of ASTM A-82. The spacing shall conform to the following: 6.4.1 Pitch spacing: For a distance of 3 ft from the tip and butt of the pole, spirals shall be spaced a maximum of 1-3/4 in. on center but no less than 1-1/4 in. 6.4.2 Torsional loads: If torsional loading is a factor, the spiral spacing shall be adjusted, if necessary, to provide sufficient reinforcing in accordance with the latest revision of ACI 318. 6.4.3 Groundline and embedded portion of the pole: Closer spiral spacing may be required in the region between 3 ft above groundline to the pole butt to adequately resist the increased shear encountered in developing the resisting soil pressure and near guy wire or dead-end attachments where high shear concentration occurs. 6.4.4 Maximum spacing: The maximum center to center spacing (pitch) throughout the remainder of the pole shall be such that the following condition is satisfied: Av(Fv/s) ≥ 0.48 where Av is the area of one spiral leg in square inches, Fv is the design yield stress of the grade of steel used in ksi (=