Stability of buildings

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The Institution of Structural Engineers

Stability of buildings

DECEMBER 1988 Abu-haikal 2006

The Institution of Structural Engineers 11 UPPER BELGRAVE STREET, LONDON SW1X 8BH

Constitution of ad hoc Committee

C. White, BSc(Eng), CEng, FIStructE, FICE. FlHT. Chairman (Director. Travers Morgan Ltd.) R. Bartle, CEng. FIStructE Davison, BSc. CEng. MIStructE, MICE (London Borough of Wandsworth) H. Fisher, BSc. CEng, FlStructE. FlCE. (Chairman, Editorial Panel) (Cooper Macdonald & Partners) H. B. Gould, CEng. FlStructE. FICE. (Consultant. G. Maunsell & Partners) R. Hankin. BSc(Hons). CEng, MIStructE, MICE (G. Maunsell & Partners) T. W. Hill, CEng. MIStructE. MICE. FlAS (previously District Surveyor) M. W. Manning, MA(Cantab). CEng. MIStructE (Ove Arup & Partners) J. F. A. Moore, MA(Cantab), BSc(Eng), ARSM. DIe. PhD, CEng, MIStructE K. P. G. B.

(Building Research Establishment) BE. MSc. ore. PhD, CEng. FIStructE, FICE, MASCE (The Steel Construction Institute, previously with University College. Cardiff) F. H. Needham, BSc. FIStructE, FlCE (The Institution of Structural Engineers. previously with CONSTRADO) J. B. Price, BSc(Eng), CEng. FIStructE (FC Precast Concrete Ltd.) D. W. Quinion, BSc(Eng), FEng, FIStructE. FlCE (Tarmac Construction Ltd.) J. G. Sunley, BSc. MSc. CEng. FIStructE. FIWSc (TRADA) R. J. M. Sutherland. BA. FEng. FIStructE, FlCE, FIHT (Consultant. Harris & Sutherland) D. J. Wilson, BSc(Eng), CEng, MIStructE (Travers Morgan Ltd.) R. Narayanan,

R.

J. W. Milne, BSc. Secretary (The Institution of Structural Engineers)

© 1988: The Institution of Structural Engineers The Institution of Structural Engineers, as a body is not responsible for the statements made or the opinions expressed in the following pages. This publication is copyright under the Berne Convention and the International Copyright Convention. All rights reserved. Apart from any copying under the UK Copyright Act 1956, part 1, section 7, whereby a single copy of an article may be supplied, under certain conditions. for the purposes of research or private study by a library or a class prescribed by the UK Board of Trade Regulations (Statutory Instruments, 1957 no. 868), no part ofthis publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without prior permission of the Institution of Structural Engineers. Permission is not, however, required to copy extracts on condition that a full reference to the source is shown. Multiple copying of the contents of the publication without permission contravenes the aforementioned Act. 2

IStructE Stability

Contents Foreword

5

1 Introduction 1.1 Aims of the report 1.2 Definition f stability 1.3 Actions 1.4 Scope

7 7 7 7 7

2 General considerations 2.1 Responsibility for building structure 2.2 Structural planning 2.3 Quality of materials and workmanship 2.4 Movements 2.5 Tolerances 2.6 Deterioration and fire 2.7 Alteration and change of use 2.8 Standard designs and factory production of structural components 2.9 Construction on site

9 9 9 9 9 9 9 9

11 11

3 Actions - design stage 3.1 Permanent actions 3.2 Variable actions 3.3 Accidental actions

11 11 11

4 Stability - design stage 4.1 Permanent and variable sections 4.2 Accidental actions 4.3 Other actions

12 12 13 17

5 Stability during alteration or change of use

19

6 Actions - construction stage 6.1 Works below ground 6.2 Works above ground 6.3 Partially or completed buildings

20 20 20

7 Stability - construction stage 7.1 General 7.2 Exchange of information 7.3 Other considerations

21 21 21 21

8 Summary

22

Appendix Further guidance for stability during construction 1 Single-storey steel structures 2 Multistorey steel structures 3 Heavy industrial buildings with cranes 4 Other steel structures 5 Wind loading on steel skeletons 6 Effect of cladding on steel structures 7 Concrete cast in situ framed construction 8 Precast concrete framed and panel construction 9 Other concrete structures 10 Structures of timber. laminated plywood. aluminium and composite materials such as GRP and GRC 11 Masonry References IStructE

Stability

20

23 23 23 23 23 23 23 23 24 24

24 24 24 3

4

IStructE Stabilitv

Foreword

In 1971 the Institution of Structural Engineers published a report on the Stability of modern buildings as a result of its conference on industrialized buildings, the collapse of prefabricated buildings under erection at Aldershot and the aftermath of the tragic accident at Ronan Point. That report was largely concerned with high-rise construction, and it was in 1984 that the Institution formed a Committee to prepare a successor to deal with stability of all types of building whether high or low rise. The new report was to describe the disturbing forces, to advise on stability for most forms of construction, and to consider stability during construction. One ofthe difficulties facing the Committee was to arrive at a definition of stability for buildings and how to separate considerations of stability from those of strength. Expressions such as 'sensitivity', 'structural integrity', 'robustness' and 'lack of obvious or hidden wobbliness' were put before the Committee but rejected in favour of 'stability' with a definition in the text. The report sets out the many aspects of design and construction that have to be taken into account so that structures remain in a stable state at all times. It is hoped that it will make a positive contribution to good practice in the office and on site. During the preparation, many people have commented, and the Institution would be grateful if any further comment could be forwarded to it. Lastly, I would express my thanks to the members of the Committee and their organizations and also to the Committee Secretary, Mr. R. J. W. Milne , for all their efforts, carried out in an enthusiastic and harmonious manner which characterized the work.

June 1988

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Stability

K. C. White, Chairman, ad hoc Committee

5

6

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Introduction

1

1.1 Aims of the report Structural engineering is the science and art of designing and making with economy and elegance buildings, bridges, frameworks and other similar structures so that they can safely resist the forces to which they may be subjected. This report confines itself to buildings. .' Structural safety depends on both material strength and stability. It is vital that designers should distinguish between the two. This report deals with stability not strength, and describes the actions to be considered at the design and construction stages and subsequently so that the building remains stable throughout its life. Stability is essentially an inherent property, very largely independent of strength. Yet lack of stability has probably caused more structural problems than have shortcomings in material strength. Stability is susceptible to numerical analysis only to a limited extent. Fig.1 aims to convey at a glance the essence of stability. A house of cards is unstable, but the material is not seriously stressed. Those masonry buildings that fail do so mostly by instability rather than by overstressing. Slender beams, or columns may become unstable or buckle at loads well below those which the material itself can support 'Lack of bracing can allow instability to occur leading to collapse. Overturning of a complete structure is an extreme example of instability.

1.2 Definition of stability Provided that displacements induced by normal loads are acceptable, then a building may be said to be stable if: • a minor change in its form, condition, normal loading or equipment would not cause partial or complete collapse and • it is not unduly sensitive to change resulting from accidental or other actions Normal loads include the permanent and variable actions for which the building has been designed. The phrase 'is not unduly sensitive to change' should be broadly interpreted to mean that the building should be so designed that it will not be damaged by accidental or other actions to an extent disproportionate to the magnitudes of the original causes of damage.

1.3 Actions In the preparation of Eurocodes, permanent and imposed loads, events such as explosions, impact, fires, etc. and natural agents (e .g. climate, geographical and environmental) are regarded as agents causing actions to be imposed on the structure. It has therefore been decided to use this nomenclature in this report and to define the actions to be taken into account when considering the stability of buildings as follows:

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Stability

1. permanent actions, i.e. dead and permanently imposed loads 2. variable actions, i.e. imposed, wind, snow, dynamic loads and predictable impacts 3. accidental actions, e.g. unpredictable impact, explosion and seismic 4. other actions, i.e. ternperatue , moisture, deterioration, creep, fire and foundation movements 5. actions resulting from alterations or change of use. Permanent and variable actions may both be classed as normal loads because they occur commonly, and are defined in Codes of Practice, as opposed to accidental actions where the likelihood of occurrence is small and the magnitude is largely unknown. Temperature, moisture, deterioration, creep, and fire actions can affect all buildings, and an assessment of their magnitude can be made. Foundation movement may induce an action on a building or occur as a result of the action of a building. Stability should not be jeopardized by combinations of the actions.

1.4

Scope

In this report reference is made to British practice and standards. However, this should not be taken as limiting its scope or its range of applications to the UK only. The overall appreciation of stability and the strategy adopted to achieve stability will be the same wherever the buildings are required, but some actions will assume greater significance in different parts of the world. Considerations that affect the stability of a structure or building may change or assume greater' importance as its size increases. Greater attention may have to be paid to stability in their design and construction. However, any attempt to place structures or buildings into classes in relation to stability would be arbitrary and therefore cause anomalies, whether they are based on type of use, extent of occupancy or materials of construction. A form of construction should be used so that the structure is not unduly sensitive to a particular use, and so that the form of the structure is not being extended beyond the range where experience has shown it to be stable, unless special care is taken during its design. Building regulations place requirements on certain buildings. These requirements do not aim to achieve greater stability than for other buildings but are considered necessary to provide an adequate standard for all buildings. Other considerations, economic or political, may impose more stringent requirements for stability for certain buildings, but these should not detract from the requirement that all buildings should have a minimum level of stability. This report is therefore intended to apply to all buildings whether permanent or temporary and to encompass those where alterations or changes of use are being considered. ""

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2

2.1 Responsibility for building structure One engineer should be responsible for the overall design including stability of any building structure and should have the duty to see that the designs and details of all structural parts and components comply with the stability requirements, even where some or all of the structural designs and details of some parts or components are developed by others. When the working drawings have been completed. it should be the responsibility of one engineer to appraise independently the whole design to see that requirements for stability have been incorporated in all elements. and can be met during the construction stage. A statement accompanied by sketches showing stability requirements should be prepared when necessary. e.g. for any unusual design or for a structure having particular vulnerability. This should be made available to the contractor who will be responsible for the stability of the building s~ructure and for any temporary works during its construenon.

2.2 Structural planning The main loadbearing elements of a building should be positioned and sized to provide a building that is stable against the effects of normal loads. There should be defined paths that transmit these loads to the ground. The location and sizes of the main loadbearing elements should be so arranged that the structure is not unduly sensitive to the effects of accidental and other actions. If a check reveals that the building is likely to be sensitive to these effects then the structure should either be replanned or modified as described in Section 4. When movement joints are incorporated then each part of the structure between joints should be considered as a separate building for stability purposes. Consideration should be given to the arrangements of main loadbearing elements so that they are accessible for inspection and maintenance, and so that non-loadbearing elements can be replaced. Finally, the effects of the construction process on the overall stability of the structure during erection and on completion should be assessed. ~

2.3 Quality of materials and workmanship Designs of buildings are based on specified qualities for the materials and workmanship to be used. Weaknesses or deficiencies by comparison with specified requirements may cause instability. Specifications for materials and workmanship should be unambiguous. precise and appropriate to the particular project. The application of quality-management schemes by suppliers and contractors should provide assurance that the materials and workmanship, respectively, conform to specified standards. It is nevertheless desirable that designers ascertain that such schemes apply to the particular components and workmanship required, and that the schemes are being implemented.

2.4 Movements All structures and buildings are subject to movements and deformations during construction and in service. Some arising from various actions are listed on Table 1 in Section IStructE Stability

General considerations

S. Although it should be recognized that some local damage cannot be avoided. movements may be accommodated by: • limiting the extent of movement • the provision of joints to accommodate movements • detailing of connections and bearings so as to accommodate movements and to minimize and control their deterioration.

2.5 Tolerances Implications of tolerances adopted in design should be considered in assessing stability. Problems can occur if standards of accuracv are assumed during design that are not achievable in 'construction. Designs should therefore be based on tolerances that can be reasonablv attained. The effects of unrealistic tolerances specified for ~construction can particularly affect the stability of structures and buildings employing prefabricated structural components or employing prefabricated building components in in situ construction. The assessment of tolerances to be specified should be based on clearly stated assumptions on such matters as material properties, manufacturing and erection tolerances and erection procedures. This will be of particular importance where the design of components is undertaken by other engineers who may, in the absence of specific guidance from the engineer with overall responsibility, adopt differing philosophies in their approach.

2.6 Deterioration and fire Deterioration of structural and building components and the effects of fire can lead to loss of stability. Generally, deterioration will take place over an extended period of time, but an exception to this is the effect of fire. When deterioration becomes evident it is essential that a means of monitoring it is established as soon as possible so that warnings are given should the deterioration continue to the point where stability may be impaired. With regard to fire, guidance in the relevant British Standards and Codes of Practice should be observed, or if the design is based on fire-engineering principles, the assumptions made in the design should be validated.

2.7

Alterations and change of use

Alteration to a structure or building may reduce stability to such an extent that in the extreme (e.g. by the removal or alteration of an element) collapse of the structure or of an adjacent building may result. During the design of the alteration particular attention should be paid to stability considerations and to temporary weaknesses created in the structure arising from the proposed construction sequence. Change of use of a structure or building may result in changes of loading that, in the extreme, may overload an element to such an extent as to give rise to instability. To allow assessments to be made of the effects of change of use or alterations, it is highly desirable to have access to the original structural design documents. To serve this purpose among others, it is recommended that these documents are retained by building owners, occupiers and designers. 9

2.8 Standard designs and factory production

of structural components Modern methods of construction have relied increasingly on factory production of major structural components for some types of building. As a result. buildings are constructed based on common designs and using common components. Consideration of stability of such designs and components should be commensurate with the total quantity of buildings to which they will be applied. Each individual building constructed of such designs and components should be separately checked for stability.

10

2.9 Construction on site Most instances of instability in structures and buildings occur on site during construction. The actions and measures that should be considered so that structures remain stable during construction are described in Sections 6 and 7, respectively. Construction work should be undertaken only by those who will exercise their duty to supervise the work so that the requirements of the design and specification for materials and workmanship are achieved, and so that the stability of the temporary and permanent works is maintained at all times.

IStructE Stability

Actions - design stage

3

The permanent, variable and accidental actions to be taken into account are described in this Section. For the details of other actions see subsection 4.3.

applied to the structure in order to accommodate the effects of inaccuracy in construction of stability. These should be calculated, and their effect assessed in accordance with the requirements of these British standards.

3.1 Permanent actions

Dynamic loads

Dead loads

Where it can be foreseen that normal operating conditions may induce dynamic loading on structures, suitable precautions should be adopted in the design. For simple cases, this may be achieved by a straightforward enhancement of the static load to cater for the dynamic effects. More complex situations mav require a dynamic analysis to determine the response of the structure to the applied load. In cases where significant dynamic loadings are being introduced into a building, structural damping may need to be considered to minimize resonance effects in floors, or in the structure as a whole. These measures may increase the dead loads and thus affect the stability of the structure. Dynamic loads arising from the operation of machinery and gantry cranes are defined briefly in BS 6399: Part 11.

Dead loads as defined in BS 6399: Part 11 are generally regarded as being calculable to a reasonable degree of accuracy. It should be recognized that variations between calculated and the actual load ofthe structure, finishes, cladding and other permanent fixtures may occur as a result of the following: • • • •

deviations deviations changes in alterations,

from design dimensions during construction from design densities moisture content additions and demolition.

For Some buildings

these variations

may be significant.

3.2 Variable actions Imposed loads Imposed loads as defined in BS 6399: Part 11 should normally be used for calculating the loads on all structural members. The maximum and minimum imposed loads on all members should be assessed to determine the adverse or beneficial load combinations to be used in the design for stability.

Predictable impacts The impact loads stated in BS 6399: Part 11 and other British Standards to be used in design are derived from predictable impacts. Their effects on the stability of structures should be taken into account in the design for normal loads.

3.3 Accidental actions Impact

Wind loads Wind loads as defined in CP3: Chapter V: Part 22 (or BS 6399: Part 2, in course of preparation) should be used for. calculating wind loads on all buildings. This Code deals with wind loads on buildings and their components, and treats these as static loads since the dynamic response of most building structures is not significant. However with the present trend towards lighter and taller structures, there is likely to be a need to determine the dynamic response of a wider range of buildings. Special investigations, including model studies, may be necessary In such cases.

Roof loads Imposed loads as defined in BS 6399: Part 33 should be used for calculating the loads on roof and other externally exposed members. This Standard gives variation of the magnitude of the snow load for different climatic and topographical conditions. Due to its geometry or location of certain roofs, drifting of snow or accumulation of melted water may give rise to greater than normal imposed loads on some parts of the roof. The effects of this, especially those due to asymmetrical snow loading, on the stability of the structure should be assessed. In certain environments or locations the possibility of dust deposits imposing loads greater than those specified in the British Standard for roof loading should be considered.

Notional horizontal loads British Standards for the structural use of some of the structural materials define notional horizontal loads to be IStructE

Stability

Accidental impact loads to be considered are those that could arise from the impact of vehicles on structural members that are vulnerable to such impacts, and which exceed the predictable impacts stated in subsection 3.2.

Explosions The nature of explosions and the factors influencing their magnitude make it difficult to characterize explosive loads satisfactorily. Guidance for some types of explosives (i.e. dust, high-explosive chemicals, etc.) may be obtained from the relevant British Standards. Generally, it will be necessary to adopt an approach that does not require knowledge of either explosive loads or of detailed structural response but which will enable measures to be taken to avoid disproportionate collapse of the building. Some British Standards prescribe arbitrary values for detailed design based on a specified pressure of 34 kN/m2, intended to lead to buildings that will not be unduly sensitive to the effects of explosions.

Seismic loads The UK is not free from earth tremors, although the majority of these are not normally of such significance as to cause more than superficial damage to buildings designed to meet the minimum strength and stability criteria required by British Standards. Where specific design is required because of the particular nature of the building, information on the frequency and magnitude of recorded earth tremors in the UK, and worldwide, can be obtained from the Institute of Geographical Sciences in Edinburgh. Use can then be made of one of the recognized codes in determining the seismic loads to be used. 11

4

Stability - design stage

This Section describes the measures to be considered in design so that structures remain stable. These n:e~sures are listed separately for each ot th~ actions, but, It ISImport~nt that the possibilities of some of these acting In combination should also be considered.

4.1 Permanent and variable actions Structural

members

should be so arranged

that:

• there is at least one clearly defined path through which the effects of the actions are transmitted to the ground • structural members whose stability relies on the assumption that they are restrained in position are connected to a bracing member that is of sufficient strength and stiffness to provide the required restraint.

Braced frames Examples of some typical braced frames are shown in Figs. 2, 3 and 4: • Fig. 2 shows a building that is re~tangular on plan and has bracing members around the !Ift shaft a!1d stairs • Fig. 3 shows a buil.di~g square on plan WIt~ centr~l core • Fig. 4 shows a building square on plan with penmeter bracing members . Note: Not all the bracing members are shown on these Figures. some having been omitted far clarity . In all these examples. the horizontal fo~ces in the x- and y-direction are transmitted to the bracing members by suitably designed floors and roofs acting as hO~Izontal girders. There are other arrangements or locations of

There are two forms of structure to be considered. i,e. those that have braced frames and those that are unbraced and are allowed to sway.

Rest of bracing not shown for clarity

Bracing members around lift shaft

Bracing members around stairs (Walls or Structural members)

Fig. 2 Braced frame: rectangular on plan Note:

12

roof and floo r-, will act a" horizontal

girdc rs pr ovidc d that they

arc designed

and detailed

to do so

IStructE Stability

Central core

Roof and floors act as .horizontal girders' taking wind load from external walls to core, provided they are designed and detailed to do so.

Central core full height (foundation-roof)

Fig. 3 Braced frame:

bracing members that could be used, but emphasis should be on the provision of bracing members in two vertical planes approximately at right-angles to each other. These bracing members may take the form of walls, or structural members in either tension or compression, which should preferably be symmetrically positioned on plan to avoid torsional effects, especially in the case of very tall buildings.

Unbraced frames Examples of single-storey frames unbraced in one direction are shown in Figs. 5 and 6, for single and multiple spans. respectively. While single-storey buildings are shown in the Figures, the same structural arrangements also apply to multistorey buildings. In each case the frames in the x-direction are unbraced, and their stability is provided from within the frame by rigid connections that should be of sufficient size and stiffness to provide restraint against the IStructE Stability

square on plan with centre core

rotation assumed in the analysis. Their deflection in the x-direction should be kept within serviceability limits. In the y-direction the horizontal forces should be transmitted to the bracing members by the root. or floors for multistorey buildings, or by specially designed tie members for single-storey buildings. Such tie members should also be provided to restrain in position the frames in the y-direction, i.e. on their weak axis in bending. Bracing members may take the form of walls or structural members in either tension or compression or members with their strong axis in bending in the y-direction designed as portals in the longitudinal direction. Roof plane bracing shoud also be provided to transmit the horizontal forces on the gable ends to the bracing members in the v-direction. The location and number of braced bays shown in Figs. 5 and 6 are indicative only. 13

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IStructE Stability

Masonry structures (Masonry structures should normally be considered as braced forms of construction, with strategically placed masonry elements providing the bracing. The provision of structural staircases, and lift shafts may contribute to satisfying the bracing requirements.JAn example of a masonry structure is shown in Fig. 7. For overall stability, particular attention should be paid to the following items as ringed in Fig. 7: • bonding or tying together of all intersecting walls • provision of returns where practicable at ends of load bearing walls • provision of bracing walls to external walls • provision of internal bracing walls • provision of strapping of the floors and roof at their bearings to the loadbearing walls.

In addition, the effects of movement joints, uplift due to wind and accidental actions should be considered. Common features

Diaphragms One of the common features of many forms of construction is that the floors and in some cases the roofs act as horizontal diaphragms distributing forces to the 'bracing'

members. Special attention must be paid to these floors and roofs so that they and their connections are capable of providing the load path to the 'bracing' members.

Tie members Another common feature of many forms of construction is that tie members are provided to connect individual structural members to the 'bracing' members. Special attention must be paid to the design of these tie members so that they are capable of affording the restraint assumed in the design, and to the buckling and torsional restraint of the individual tie members.

4.2 Accidental actions Buildings and their structural members should be designed to limit the extent of damage that may be caused by accidental actions. Acceptable limits of damage are defined in building regulations and British Standards for the safety of people. There may be other considerations that may impose further limits, but it should be recognized that some damage arising from accidental actions cannot be avoided. Defence strategies available for limiting damage arising' from accidental actions are:

Fig. 6 Frame un braced in one direction: IStruc,E

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multiple span 15

• • • • •

enhancement of continuitv strengthening of structure" provision of multiple load paths provision of relief control measures. The appropriateness of the choice of strategy to be used for any particular design may be constrained by economic or practical considerations.

Enhancement of continuity This strategy consists of the provision of increases in the resistance of the joints of structural members to enhance the effects of continuity on the structure.

Strengthening of structure This strategy consists of the provision of local or general increases in the resistance of structural members to enhance the overall strength of the structure. The strategy will increase the size of the accidental action needed to precipitate failure and will reduce the likelihood of instabilitv. It will not necessarilv alter anv brittle characteristics of the structure nor the nature of its final failure.

Provision of multiple load paths This strategy requires the design of a structure in such a way that load may be shed into other paths of resistance in the event of a local failure caused bv an accidental action. A practical way of checking whether multiple loadpaths are present is to assess whether the structure will remain

stable following the notional removal of individual elements in turn. Generally, structures should not be wholly dependent for their stability on the structural integrity of a single connection or element. Where this situation is found during design, the structural form should be re-examined. If the situation cannot be avoided, then particular consideration should be given to the design of the connection or element to ensure that it will have ample strength and toughness.

Relief provision This strategy consists of the inclusion of devices to allow building to avoid carrying the load resulting from accidental action. Examples of this approach include introduction of load-shedding devices such as venting explosions or the introduction of weak joints in walls floors to prevent transmission of load.

the an the for and

Control measures This strategy consists of the use of environmental performance monitoring and control systems They may be designed so that:

and

• the building is used as the designer intended by rapid feedback of information to prevent misuse by the occupants • repair and maintenance is instigated following noncritical damage • accidental actions affecting the structure or building are avoided, e.g. by the installation of bollards, fire compartmentation, fire alarms, etc. It is expected that the use of control systems will become increasingly appropriate as their cost reduces and their reliability increases.

Fig. 7 Masonry 16

structure

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4.3 Other actions This subsection describes some of the other actions that may contribute to loss of stability and gives advice on how to limit the damage caused by the effects of such actions on stability. To limit the effects of these other actions: • the recommendations and rules of good practice contained in the relevant British Standards should be followed to provide protection against the possibility of the effects of these other actions reaching extreme values • the measures and defence strategies as described in subsection 4.2 should be considered as applying to these other actions.

movement properties of the materials and the relevant environmental conditions to be expected in the location of the building, or its components.

Deterioration Most structural materials will suffer some form of deterioration of physical properties in certain adverse environmental conditions. Extreme magnitudes of deterioration can cause instability. Table 2 contains a list of the potential causes of deterioration of common structural materials, i.e. metals, concrete, timber and masonry. This list should however not be considered as being exhaustive.

Movement effects All structures and buildings are subject to movements and deformations, which may occur during construction, in service or in both. Some of the actions and resulting movement effects are listed in Table 1, although this list should not be regarded as exhaustive.

When movements are restrained or are non-uniform. actions may arise within the structure or its building elements which mav affect their local or indeed their general stability. Collapse of a structure because of such movements is rare, but serviceabilitv failures could, if unchecked or neglected. eventuallv threaten the stability of the structure or ~its components. ' . Progressive changes in the condition of structural elements and the accumulation of local damage or irreversible movement may lead to instability. The bodily displacement of a wall or beam on its seating or rotational movements at bearings could cause progressive spalling of the supports, as illustrated in Fig. 8. The magnitude of the imposed deformations listed in Table 1 may be estimated from the thermal and physical properties of the materials and the expected range of temperatures in the location of the buildings. and the aspect of the components. Some guidance may be obtained from BRE digest 22Sc) in respect of thermal movements, and other documents should be consulted for the effects of chemical changes.

Moisture The ingress and egress of moisture into structural members and building components can cause movements giving rise to instability. An indication of the magnitude of such moisture movements can be derived from the moistureIStructE

Stabilitv

Signs of deterioration will often emerge in the form of rusting. cracking or spalling, occurring well before any significant signs of instability become noticeable. The consequences of deterioration of structural elements that are not accessible for inspection should be considered. When deterioration becomes evident. it is essential that a means of monitoring it is established as soon as possible to provide warning of the need for action before stability is impaired.

Creep Deformation in structural members and building components caused by creep can cause instability. An estimate of the likely magnitude of these deformations can be made from the time-dependent properties of the materials and the duration and incidence of the sustained loads.

Fire Dimensions of structural members are most commonly chosen to give satisfactory performance for prescribed periods of fire exposure using prescriptive design data based on the results of the tests. Elements designed in this way can be expected to retain adequate strength and stiffness for the prescribed period. Particular attention should also be paid to joints and connections and to the maintenance of continuity in the structure in fire conditions. In some circumstances. a more detailed view of behaviour will be required in order to assess stability. For instance, it may be felt necessary to determine the deflections or forcesinduced by heating and their influence on stability. Here, a full fire-engineering approach will be 17

required with the fire action being defined by an ambient temperature/time characteristic and due allowance being taken of the change in properties of the structure with time and temperature. The different strategies described in Section 4 may be considered in these circumstances. The nominal hours of fire resistance for structural members are determined by the uses for which a building or part thereof are to be designed and are usually prescribed by building regulations for the saving of life. The required 'hours' or 'fire loading' for structural members may be obtained from: • data tabulated in the relevant British Standards for the material of which the structural member is to be constructed • data determined from specific fire-resistance tests • fire-engineering calculations.

18

Fire may cause large displacements or rotations of those elements directly exposed to fire and may also affect parts of the structure remote from the vicinity of the fire. The whole structure should be assessed for stability following a fire. The stability of a fire-damaged structure prior to repair should be assessed as described in Section 5, although the assessment of the nature and significance of the damage and of any repair measures may be more complex.

Foundation movements The magnitude of the foundation movements that can cause destabilizing actions (i.e. differential settlement, heave or sliding) may be estimated from geotechnical considerations, together with the effect of trees. The effects of these movements on the structure should be restricted to values that will not cause instability in any part of the structure.

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5

Stability during alteration or change of use

Alteration to a structure or building may reduce stability to such an extent that, in the extreme, collapse may result. Change of use of a structure or building may result in changes of loading that in the extreme may overload an element, to such an extent, as to give rise to instability. The procedures to be observed both before and during carrying out alterations to a structure and before permitting a change of use are: • the careful assessment of the stabilitv of the structure at each stage of any alterations or change of use (see Section 4)

• if available, the assembly of the following original documents: the 'as-built' drawings original design parameters any special features affecting demolition details of any modifications or additions to structure, finishes and se rvices subsequent to the original design or construction records of inspections records of maintenance work • even where these basic structural data are available, the undertaking of a full or partial structural appraisal of the existing structure depending on the extent of the alterations and change of use proposed. Guidance can sometimes to obtained from contemporary design guides and textbooks in respect of likely design loadings and design methods. Records may also sometimes be obtained from the appropriate local building control department or district survevor's office. Caution should however be exercised when using records, as these may not fully reflect the current or indeed the 'as-built' structure • the preparation of a method statement detailing the sequence of the work, the type of construction plant and tools to be used and the precautions to be taken at each stage to guard against instability.

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19

6

Actions - construction stage

During construction of a building, actions can occur that may affect the stability of the temporary works, the partially completed permenent works, or adjacent structures. Actions to be considered at the various stages in the construction process are described under the following headings: works below ground works above ground partially/totally completed

permanent

Variable actions from the environment The environmental

loads to be considered

include:

wind water, and wave action if present snow ice direct sunlight.

works.

6.3 Partially or totally completed buildings The actions to be taken into account:

6.1 Works below ground Actions to be taken into account should be assessed from the results of the site surveys and soil investigations, which should be carried out before the design and construction of any building commences. These are: earth pressure water pressure actions on and actions on and actions on and actions on and

6.2

from from from from

adjacent structures/building adjacent traffic routes groundwater lowering any services adjacent to the site.

• the actions used for the design of the building as defined in Section 3, and their effect on the stability of the partially or totally erected building. In particular, adverse variations in the geometric shape and/or reduced strength arising from incomplete construction should be considered • the actions used for the design of the temporary works as defined in subsections 6.1 and 6.2 and their effect on the stability of the partially or totally erected building.

Works above ground

The magnitudes of the loads to be taken into account for the design of temporary works above ground should be determined from the relevant clauses in BS 5975~. These are:

Permanent actions The self-weights

to be considered,

which include:

the formwork the false work structure any ancillary temporary works connected to the falsework, such as: access ramps and scaffolding hoist and other tower structures loading storage platforms raking and flying shores temporary struts and bracing any permanent works elements forming an integral part of the temporary works

Variable actions The imposed loads to be considered arising from:

should include those

permanent works (i.e. structural steelwork reinforcement and concrete with particular attention to wet concrete) construction operations, including: working areas storage areas pedestrian traffic vehicular traffic static plant mobile plant impact from any of the above 20

IStructE

Stability

7

Stability - construction stage

This Section describes some of the measures to be considered during construction so that partially and fully completed structures remain stable particularly as a partially erected structure may behave in a manner quite different from that of the completed structure. 7.1 General The constructor should appoint one of his engineers to be responsible for the stability of the permanent, partially completed and temporary works during the whole of the construction period. It should be his duty to see that structural members are so constructed that: • there is a clearly defined path through which the actions listed in Section 6 are transmitted safely to the foundations • structural members that are assumed to be restrained in position ate securely connected to a bracing member or members of sufficient size and stiffness to provide the required restraint • the consequences of damage to temporary or permanent works are assessed in a similar manner to those described in subsections 4.2 and 4.3 to take account of accidental and other actions, respectively.

• stacking of building materials on temporary works or on completed permanent works such that stability of the partially completed permanent work could be effected • the necessity of the provision of temporary bracing or propping and when they may be removed • the design of end-connections of temporary steel or timber bracing or strutting to resist forces in the bracing members. It should not be possible for wedges and packs to become displaced under load reversals or vibration. The insertion and securing of bracing should be controlled so that distortion of members and the creation of excessive loads are avoided • checks so that loads applied to foundations can be safely accepted without undue movement • demolition operations and the effect of use of explosives on the stability of the building should be planned with equal care to that used for the design of the permanent works. Where more than one contractor is operating on the site, there is need for close cooperation to avoid one jeopardizing the work of the other. Further guidance for stability during construction applicable to different forms of structure is given in the Appendix.

7.2 Exchange of information Before construction starts the following exchange of information should take place so that all concerned understand what needs to be done: Information from designer to constructor In addition to the drawings and specifications the following should also be supplied: design loading factual results of site surveys and soil investigations stability criteria, if requested load capacity of members, if requested limits on positions of construction joints lifting positions on members to be erected as single pieces influence of post-tensioning on adjacent members of the supporting falsework Information from constructor to designer A construction method statement should be prepared, which should include the following: construction or erection procedure use, weight and location of plant programme sequence of construction or erection details of temporary works to be used to ensure stability at all stages details of provision and timing of installation and removal of temporary bracing of support members details of holes to be drilled and of fixings to be attached to

permanent construction purposes.

for construction

or erection

7.3 Other considerations The following should also be considered for their effects on the stability of the temporary or permanent works: • partial cladding that could affect the magnitude and distribution of wind load IStructE

Stability

21

8

Summary

This report has been written by practising engineers for the use of their colleagues. It has attempted to assemble in one document the many considerations affecting stability of buildings. In some instances. it may be argued that the report has strayed into strength consideration, but this merely illustrates the difficulties in separating these two concepts. Much detailed advice has been included for the sake of completeness, but it must be emphasized that the relevance of the many matters described to anyone particular building must be left to the judgment of the engineer. The report is intended to describe the 'state of art' as existing in 1988, and it should not be considered as increasing the duties and responsibilities of engineers beyond those accepted as good practice. Finally, it must be accepted that a degree of local damage may occur because of the effects of the actions described in this report. This cannot be avoided, but the implementation of the measures described in this report should enable such damage to be both localized, and limited to an extent proportionate to the cause of the damage.

22

IStructE Stability

Appendix Further guidance for stability during construction

Guidance is given for stability during construction different forms of structure as follows:

applicable

to

Ai Framed single-storey steel structures Single-storey steel structures without cranes consist of either portal frames or column and beams (which may be plain-rolled or cold-formed sections), lattice girders or a combination of both. The first bay to be erected should be that incorporating the permanent bracing, in all three planes. This steelwork should be plumbed, lined and levelled prior to the bracing being installed. and the permanent connections in the whole of this bay, including the grouting of the bases should then be effected. In this way, a stiff box should be created to which subsequent steelwork can be tied. In the event that the design does not provide permanent bracing (i.e. it relies on the subsequent cladding to provide shear stiffness, either vertically or horizontally or both) then temporary bracing should be provided, preferably in the end bays. Should difficulty be experienced, due to the permanent or temporary bracing not fitting as a result of fabrication errors or accidental damage during transport, no attempt to proceed with erection should be made until the problem has been remedied and the bracing installed.

A2 Multistorey steel structures A multistorey steel structure should be divided into sections for the purpose of lining and levelling. Each section should be of such size that lining and levelling, or adjustment. may be carried out without difficulty. The correct checking sequence should be adopted; first, the position and alignment of columns at foundation level; secondly, the plumbing of the columns: and thirdly. the levels, which are best checked at the ends. of the lowest level of beams. The permanent connections in that part of the structure should then be made. The temporary bracing should not be removed until the bases have been grouted. the grout had time to harden, and the permanent connections made. Unclad structures may experience temperature ranges well in excess of those anticipated in service. which may affect the verticality of columns subsequent to the making of the permanent connections. Ideally, the temperature at which the columns are to be within the specified tolerance on verticality should be stated in advance. It may therefore be necessary to carry out the surveying work at night. In any case. this should be carried out at a time when the steelwork is not subject to strong sunlight. and its temperature is reasonably uniform. Some beams in a partly erected frame may be laterallv unrestrained. even though they may have been designed as fully restrained by the floors to be erected later. This needs to be taken into account when crane or other loads. such as members stacked prior to erection, are to be imposed on the steel framework. In addition. although some measure of joint rigidity may have been assumed in design. such rigidity will be almost entirely absent when the steelwork is merely tack-bolted together. This needs to be taken into account when considering wind loads during construction, and the need for temporary bracing. A crane mounted on the steelwork of a structure that is tall relative to its plan dimensions can impose a torque on the steelwork when the crane is slewing under load. The structure should possess adequate resistance to this torque in order to avoid unacceptable movements. Further details may be obtained from reference 6.

A3 Heavy industrial buildings with cranes Most of the guidance above is equally applicable in this type of building. which may incorporate multistorey sections as well as single-storey bays of considerable height and span. The accuracy of the tracking of the gantry rails will be critical to the stability of the building. Inaccuracy in these components could IStructE

Stability

lead to crabbing and possibly jamming of the crane when installed. which may impose large forces on the structure. Trial runs. with the crane under load. should be carried out. and if such troubles occur, the structure should be carefully examined to determine if overstraining has taken place. particularly to bolted connections.

A4 Other steel structures These comprise everything from fire-escape staircases to radiotelescopes, including hoppers and silos. water towers. conveyor frames. electricity transmission towers. and a wide variety of industrial plant structures. As there are few common factors in these. advice can be only in general terms. It is not uncommon for large girders or assembled sections to be lifted by crane into position. This often requires careful selection of the lifting procedure to avoid damage. Members or sections that are weak on one plane must be protected from handling stresses in that plane.

A5 Wind loading on steel skeletons Assessment of the wind forces on the unclad or partly clad structure should be carried out. It is possible that in a complex structure the resistance of it to the passage of wind may be greater than when fully clad, and thus total wind forces may be greater than on the completed building. In addition. certain individual members. such as gantry girders. may well experience wind loads of considerable magnitude for which they may not have been designed.

A6 Effect of cladding on steel structures When a structure is to be clad to form a building it is common practice that the cladding is commenced as soon as. but not before. a section of the framework has been lined. levelled and the connections finalized. Although this practice is undesirable. clear instructions should be given regarding the stacking of sheeting or decking members on the framework during erection to avoid excessive local loads or torsional instability. A partially clad structure will be subjected to different magnitudes and distributions of wind loads than the completed structure. In some cases. the sequence of cladding operations should be defined to control such loads and reduce the risk damage to the cladding and its fixings.

A7 Concrete cast in situ framed construction This type of construction proceeds in a progressive manner using formwork to shape the reinforced concrete members and falsework to provide temporary support until the structure (and not just individual members) becomes self-supporting. The Joint Report of the Institution and the Concrete Society on Formwork7 and the Code of Practice on Falsework BS 5975" provide guidance to good working practices. For multistorey construction. it is necessary for the designer and constructor to agree how beams and floors are to be cast and supported to avoid the application of unacceptable loads on to construction already completed beneath them. To control the contraction stresses that develop as concrete matures. it may be necessary to incorporate construction joints and/or shrinkage bays between adjacent bays or members. and any necessary requirements should be indicated by the designer. It is important that the expectations of alignment and accuracy are compatible with achievable standards of workmanship. When concrete members are cast on formwork they will deflect. and precambering may be needed. Post-tensioned members will move when stressed and will redistribute loads among supporting falsework members or between any points of restraint. The effect of inclement weather on in situ construction should be assessed and protection provided if considered necessary. 23

A8 Precast concrete framed and panel construction

sections as the concrete is placed. It may be necessary to provide propping to control deflections during casting.

This consists of an assembly of concrete members previously cast at ground level in a factory or on the site. The design of the members and the accuracy expected in their construction should recognize the standards achievable and methods available in site conditions. Concrete members will generally be heavier than comparable structural steel members and be more demanding on crane capacity and handling methods. The detailing of connections, between members should recognize the possible difficulties in making the connections, which are usually above ground level. When panel members are assembled on surfaces, allowance should be made for the probable unevenness of the matching surfaces and the use of packings and/or grouting. When prestressed units are used and assembled alongside each other they should have similar shapes before and after loads are applied to them. When precast floor beams or units are designed to work in conjunction with a cast-in-place concrete topping, they may need to be supported temporarily until that additional concrete has matured and the complete section can act compositelv under its dead load. . Instability may arise when insufficient care has been given to the specification and achievement of tolerable deviations in dimensions of members, particularly when they are supported on limited bearing areas. The consequences of minimal support being obtained and of impact when members are placed on their supports should be considered.

A10 Structures of timber, laminated plywood, aluminium and composite materials such as GRP and GRC

A9 Other concrete structures The guidance in subsection A8 applies to other concrete structures. which frequently contain a mixture of precast and cast in situ construction. The appropriate use of precast members can speed up erection time for members that otherwise could not sustain loads until the in situ concrete has matured. The stabilitv of precast concrete components of members. which will finally be composite, should always be checked when receiving loads and moments during construction. Concrete floors are frequently cast on permanent metal soffit forms. and care should be taken to avoid overloading the metal

These require attention to the guidance outlined above so that the members remain stable during and after erection. Structures are increasingly being conceived not as frames but as membranes or vaults often assisted into and maintained in position by tension members or internal air pressure. In such cases. the elements of the structure should be handled without damage or detriment to their subsequent performance. These structures. being comparatively light, should be handled with great care when exposed to wind loads until erection is complete.

All Masonry When masonry is incorporated into the facade of tall buildings the consequences of movements in the frame affecting the stability of the masonry should be considered. A concrete frame will creep under load and the resulting movements transmitted into the masonry should be absorbed by soft joints usually formed at storey heights. Racking movements of structural frames may also pass into the masonry. These movements should be resisted by the masonry or voids should be provided to accommodate them. When the masonry is to provide restraint to lateral movements, the connections should be designed for the purpose. External masonry has also to resist external inward and suction wind loads and is subject to thermal movements. The consequences of these loads on stability during the erection stage should be considered and temporary supports or protection provided if necessary. When masonry is to be supported on concrete nibs to beams, the nibs should be detailed to such tolerances that the necessary bearing is provided. They should always be reinforced. During the construction of masonry. due attention should be paid to it; stability under wind and other imposed loads, including accidental impacts. The sequence and rate of laying of masonry should be adjusted so that the masonry can develop adequate strength to resist these loads.

References 1. BS 6399: Loading for buildings, Part 1: Code of practice for dead and imposed loads, British Standards Institution. London, 1984 2. CP 3: Code of basic data for the design of buildings, Chapter V: Loading, Part 2: Wind loads, British Standards Institution, London 1972 3. BS 6399: Loading for buildings, Part 3: Code of practice for snow loads, British Standards Institution, London. 1987 24 Abu-haikal

4. Estimation

of thermal

and moisture

movements

and stresses:

Part 2. Digest 228, BRE Watford, Aug. 1979 5. BS 5975: Code of practice for [alsework, British Standards Institution. London. 1982 6. Needharn, F. H.: 'Site inspection of structural steelwork', P roc. ICE, Part L 70, 1981. p. 395 7. F ormwork, Joint report of Concrete Society and IStructE, Concrete Society, London. 1987 IStructE Stability