Handbook 1999 HVAC - Seismic and wind restraint design

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SEISMIC AND WIND RESTRAINT DESIGN

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SEISMIC RESTRAINT DESIGN ............................................. Tciwinolog~~ ............................................................................

Ctiloiltrrion.~............................................................................

Applying S!crtic Anci1~wi.sUsing I994 UBC ............................. Anclror B o l t s

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W d d Gl/"iciric.s ...........!...........................................

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53. I 53.2 53.2 53.3 53.8 53.8

A

LMOST all inhabited areas of the world are susceptible to tlie tla~nngingeffects of either eaitliquakes or wind. Restraints that are designed to resist one may not be adequate to resist the other. Consequently. when exposure to either earthquake or wind loading is a possibility. strength of equipnient and attachments should be evaluated for both conditions. Earthquake tlamage to inatlequately restrained HVAC&R equipiiieiit can be extensive. Mechanical equipiiieiit that is blown off tlie support structure can become a projectile. tlveatening life and property. The cost of properly restraining the equipment is small comIxiretl to the high costs of replacing or repairing daniagetl equipment, or compared to tlie cost of building down-rim clue to Design n u l installation of seismic antl wind restraints has the following primary objectives: *

Life safety to reduce the tlveat to life Reduce long-term costs due to equipment tlamage antl the resulrant down time

This chapter covers tlie design of restraints to limit the nioveof equipnient and to keep tlie equipment captive tluring an eanhquake or [luring extreme wind loading. Seisinic restraints and seismic isolators do not reduce the forces transmitted to tlie equipnieiit to be restrained. Ingteatl, properly designed and installed seisinic restraints antl seismic isolators have tlie necessary strength to withstand tlie iiiiposetl forces. However, equipment that is to be restrained niust also have the necessary strength to remain attached to the restraint. Ec~uipnieiitiiianufacturers slioultl review structural aspects of tlie design i n tlie areas of atrachinent to ensure the equipment will remain attached to the restraint. For ineclianical systems. analysis of seisiiiic and wind loading conditions is typically a static analysis, and conservative safety facrors Lire applied to reduce the complexity of earthquake antl wind loading response anal rid evnluatioii. T h e e aspects are consitleretl i n ;I properly designed restraint system. ineiii

Seisrnic Snu

E.rtrriiples .................................. Instullotiort WIND RESTRAINT DESIGN ... Terminology ................................................ Calcfhrion

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SEISMIC RESTRAINT DESIGN Most seismic requirements adopted by local jurisdictions in North America are based on model codes developed by the International Conference of Building Officials (ICBO). Building Officials and Code Administrators International (BOCA), the Southern Building Code Conference, Inc. (SBCCI). and the National Building Code of Canada (NBCC); or on the requirements of the National Earthquake Hazards Reduction Program (NEHRP). The model code bodies are working through the International Code Council (ICC) to unify their model codes into the International Building Code (IBC) by the year 2000. Local building officials must be contacted for specific requirements that inay be more stringent than those presented in this chapter and to determine if the unified specification has been invoAetl. Other sources of seismic restraint information include Seisviie Restrciint Mnnuul: Guidelinesfor Mechanical Sysrerns,

published by SMACNA (1998). includes seismic restraht information for mechanical equipment subjected to seismic forces of up to 4.7 m/s2 (0.48~). The National Fire Protection Association (NFPA) has developed staotlartls on restraint design for f i e protection systems. US.Department of Energy DOE f~430.1Aand ASME AG-1 cover restraint design for nuclear facilities. Terlinicol Mwucil TM 5-809-10, published by the United States Army. Navy, and Air Force (1992). also provides guidance for seismic restraint design.

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In seisinicallv active areas where -governmental agencies regulate tlie earthquake-resistive design of buildings (e.g., California), the HVAC engineer usually does not prepare the code-required seismic restraint calculations. The HVAC engineer selects all the heating and cooling equipiiient and, with the assistance of the acoustical engineer (if tlie project has one), selects the required vibration isolation devices. The HVAC engineer specifies these devices and calls for shop drawing submittals from the contractors, but the manufacturer employs a registered engineer to design and detail the iostallation. The HVAC engineer reviews the shop design and details the installation, reviews the shop drawings and calculations, and obtains tlie approval of the architect and structural engineer before issuance to tlie contractors for installation. Anchors for tanks, brackets. and other equipment supports that do not require vibration isolation are designed by the building's structural engineer, or by the supplier of the seismic restraints, based on layout drawings prepared by the HVAC engineer. The building officials maintain the code-required quality control over the tlesigii by requiring that all building design professionals are registered (licensed) engineers. Upon completion of installation, the supplier of the seismic restraints. or a qualified representative, should inspect the installation antl verify that all restraints are installed properly and in compliance with specifications.

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1 . Arr(icliiiwnr of cqiiipti~t~nt ro re,srrtririt. The equipiiieiit must be positively attached to the restraint, and must have sufficient strength to withstand tlie imposed forces, and to transfer tlie

forces to the restraint.

3.

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R m r r i i n r tksign. Strength of

the restraint must also be sufficient to withstand tlie imposed forces. This should be tleterinioed by the ni:inufacturer by tests a n t h analyses. 3 . Att(ic1iiiwnt of rcsrrciinr to .siihstrir(:fnrt,.Attachment inay be by n i e m of bolts, welds or concrete anchors. The sub structure wust be capable of surviving tlie iniposetl forces. The prepnl-nrion of rhis chnprer is nssignetl Design.

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TC 2.7. Seismic Restraint

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zyxwvutsrqp zyxwvuts zyxwvutsrqpo 1999 ASHRAE Applications Handbook (SI)

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TERMINOLOGY

Base plate thickness. Thickness of the equipment bracket fastened to the floor. Effective shear force (Ve& Maximum shear force of one seismic restraint or tie-down bolt. Effective tension force (T,II). Maximum tension force or pullout force on one seismic restraint or tie-down bolt. Equipment. Any HVAC&R component that must be restrained from movement during an earthquake. Fragility level. Maximum lateral acceleration force that the equipment is able to withstand. This data may be available from the equipment manufacturer and is generally on the order of four times the acceleration of gravity ( 4 ~ for ) mechanical equipment. Resilient support. An active seismic device (such as a spring with a bumper) to prevent equipment from moving more than :I specified amount. Response spectra. Relationship between the acceleration response of the ground and the peak acceleration of the earthquake in a damped single degree of freedom at various frequencies. The ground motion response spectrum varies with soil conditions. Rigid support. Passive seismic device used to restrict any movement. Shear force (V). Force generated at the plane of the seismic restraints, acting to cut the restraint at the base. Seismic restraint. Device designed to withstand an earthquake. Seismic zones. The geographical location of a facility tletermines its seismic zone, as given in the Uniform Building Code or International Building Code. Snubber. Device made of steel-housed resilient bushings ananged to prevent equipment from moving beyond an established gap. Tension force (0.Force generated by overturning moments at the plane of the seismic restraints, acting to pull out the bolt.

CALCULATIONS The calculations presented here assume that the equipment support is an integrated resilient support and restraint device. When the two functions of resilient support and motion restraint are separate or act separately, additional spring loads may need to be added to the anchor load calculation for the restraint device. Internal loads within integrated devices are not addressed in this chapter. Such devices must be designed to withstand the full anchorage loads plus any internal spring loads. Both static and dynamic analyses reduce the force generated by an earthquake to an equivalent static force, which acts in a horizontal direction at the component's center of mass. The resulting overturning moment is resisted by shear and tension (pullout) forces on the tie-down bolts. Static analysis is used for both rigid-mounted and resilient-mounted equipment.

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A dynamic analysis is based on site-specific ground motions developed by a geotechnical or soils engineer. A coiniiio~iapproach assumes an elastic response spectrum. The results of the tlynainic analysis are then scaled up or down :IS a percentage of the total lateral force obtained from the static analysis performed on the builtling. The scaling coefficient is established by the UBC or by the governing building official. The scaled acceleration calculntetl by the structural engineer at any level in the structure c:in be tletermined and compared to the force calculated in Equation ( I ) . The greater of the two should be used in the anchor design. The horizontal force factor CIJshould be iiiultiplietl by a factor of two in either case, as shown in Table 5 .

Static Analysis as Defined in the International Building Code

The final tlrafi of the International Building Code (ICC 19YB) specifies a design lateral force Fl) for iioiistructur:iI coniponents ;IS

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but FI, need not be greater than F], = l.6SD,yfI,WI,

( 2)

nor less than FI' = 0.3SS,,sII,W,

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A vertical force is specified as Fpv

= ".2SDSwp

R,

.

up = component amplification f i 'Lt or i n accordance with Table I SD,s = design spectral response acceleration at short periods as determined by SD,s = 2FUS,,/3.S,$is the mapped spectral acceleratiuiis from Fignre I and 0 . X C F , 5 2 . 5 . F,, is a fiinction of the site soil characteristics and tiiiisl be determined i n consultntion wirh either

K = 'I = I + 22Jh = W =

'

the project geotechnical (soils) or strnctiirnl engineer. Valnes for F, are given i n Table 2. component response motlilication factor in accordance wirh Table I . Nore: If expansive anchors, chemical aiichon. ur s h a l l u ~ eiiihedtled cast-in-place anchors are used. then K,,= 1.25. component importance factor in accordance with Table 3. height amplification factor where :is the height of attachment in the structnre and h is the average height of the roof ahove grade. The value of: should not be taken as less than 0. mass of eqnipmeiit. which incliides all items attached or contained in the equipment

Table 2 Values of Site Coefficient F, as Function of Site Class and Mapped Spectral Response Acceleration at 1 s Period (S,) Soil Profie Name

Mapped Spectral Response Acceleration At Short Periods" S,yC 0.25 S, = 0.50 S, = 0.75 S,