lies with the distribution of power, telecommunications and data cables. Speculative offices and older building stock rarely have sufficient capacity to deal with the plethora of wires and the new local area networks. The best solution appears to be the installation of suspended floors fed by extra vertical risers but again the floor-to-ceiling height must be sufficient to allow for this. If it proves impossible then the floor construction should allow for extensive trunking runs in the screed. Fixings to an existing structure can sometimes prove difficult and in some cases it has not been possible to fix the supports for a plasterboard ceiling to a slab. The strength of the slab gives rise to quite a different set of problems. Even with the growth of information technology there still seems to be more and more paper produced and the filing and storage of bulk paper is dependent upon the floor loading. The mandatory minimum of 2.5 kN/m2 is rarely enough - especially when the weight of a suspended floor forms part of the live load. Even normal filing units require 5 kN/m2 and compactors, mechanical and power files, safes, etc. can create serious difficulties. It is often possible to position the storage on beam on lines or directly adjacent to the core walls where maximum strength occurs, but these limitations are detrimental to the functional efficiency of the layout. A value of 4 kN/m 2 live load plus 1 kN/m 2 for partitions, etc. should be achieved if possible. Similar cautions should be voiced concerning the strength of roofs. It is common to site additional service plants in such locations and the strength of the roof construction is of prime importance.

21.19 Structure The design of building structures is an iterative process by which the type, shape, dimensions, materials and location of the various structural elements are initially chosen as a first approximation; loads are then determined and the design developed by a process of adjustment and verification that structural performance will be satisfactory. The structure must also satisfy the functional needs of the building, site factors and the many technical requirements concerned with the safety, health, comfort and convenience of the occupants. 21.19.1 Structural behaviour Assessment of structural behaviour must cover: (1) 'serviceability limit states'; and (2)'ultimate limit states'. 21.19.1.1 'Serviceability limit states' These are concerned with acceptable vibration, horizontal and vertical deflections and structural cracking and the compatibility of these with the secondary elements supported by the structure, such as partitions, cladding, finishes. 21.19.1.2 'Ultimate limit states' These are concerned with the provision of adequate reserves of strength to cater for variations in materials, structural behaviour, loading and consequences of failure. Partial factors are used for this purpose as follows: ym allows for variations in strength and is the product of: y mj to take into account the reduction in strength of materials in the structure as a whole, as compared with the control test specimen; and Ym2 to take account of local variations in strength due to other causes, e.g. the construction process.

yf allows for variability of loads and load effects and is the product of: yf to take account of variability of loads above the characteristic values used in design; Yf2 to allow for the reduced probability of combinations of loads; and Yf to allow for the adverse effects of inaccuracies in design assumptions, constructional tolerances such as dimensions of cross-section, position of steel and eccentricities of loading. Yc takes into account the particular behaviour of the structure and its importance in terms of consequential damage, should failure occur. It is the product of yc and YC where: Yc takes account of the nature of the structure and its behaviour at or near collapse (whether brittle and sudden or ductile and preceded by warning) and the extent of collapse resulting from the failure of a particular member (whether partial or complete); and YC2 takes account of the seriousness of a collapse in terms of its economic consequence and dangers to life and the community. Relevant structural codes do not give values for the subcomponents (ym , Ym , etc.) quoting only global values for Ym and Yr, which vary wiih the circumstances and load combinations being considered. However, the subcomponent definitions are useful reminders of the variables that need to be taken into account. 21.19.1.3 Hazards Building structures may be subjected to such hazards as: (1) impact from aircraft or vehicular traffic; (2) internal or external explosion caused by, for example, gas or petrol vapour or by sabotage; (3) fire; (4) settlement; (5) coarse errors in design, detailing or construction; and (6) special sensitivities, e.g. as to acceptance of movement or differential movement or as to conditions of elastic instability, not appreciated or allowed for in design. Hazards involving risk of collapse or damage may also be introduced during design, construction or service. They derive from mistakes, ignorance or omission, inadequate communication or organizational weakness. These hazards cannot be quantified except in special circumstances. However, for buildings of five or more storeys, the Building Regulations requirements concerning progressive collapse provide a general level of protection whereby the stability of a building is not put excessively at risk as a result of local structural damage arising from whatever cause. In cases of known risk the special requirements should be included in the design brief. Many methods are available for confining the effects of accidental damage to the immediate locality of the incident. These include designing to accept the forces involved, the provision of alternative paths for the loadings, 'fail-safe' and 'back-up' structures. Research has been carried out on partialstability conditions, whereby the remaining components of the building framework are capable of bridging or stringing over an area of total local damage by beam, catenary or membrane action. Statutory requirements as to fire resistance and means of escape are devised to ensure continued stability for sufficient time to permit evacuation of the building and fire-fighting to protect adjoining property. The introduction of new methods and materials requires careful consideration of the structural response to all the events that may occur during manufacture, construction and life of the

structure, not just those idealized in design procedures, codes or standards. It is very important to recognize that hazards exist outside the range of conditions normally considered in design; they must be eliminated or the structure designed so that their consequence is acceptable. The alternative is to accept that a particular hazard is so remote a risk that it can be ignored. This conclusion involves not just the statistical assessment of the hazard risk itself but careful examination of the consequence should it occur, since, though the hazard risk itself may be constant, the consequence in one type of building or structure compared with another may be catastrophically different. This applies not only to the protection of human life but also to particular functions, the continuation of which may be of paramount importance to the building user. Four basic philosophies exist, aimed at reducing hazards or their consequence:

(2) There is provision to eliminate, or reduce to acceptable levels, the risk or consequence of hazards not allowed for in (1) above. (3) The structure is not sensitive to: (a) marginal departures from the design assumptions; (b) local defects or movement; (c) environmental change. (4) The structure does not deflect or vibrate to an extent that alarms the occupants or disturbs intended function. (5) There is an inbuilt ability to cope with remodelling or increased loading to suit changing use. (6) The structure is readily buildable and not unduly dependent upon perfect compliance with the specification for workmanship and materials or future maintenance. (7) The structure is such that early warning would occur before serious defect or collapse.

21.19.2 Robustness Recent experience has demonstrated the vital importance of the quality of robustness in determining the long-term performance and adaptability of buildings and their structures. These qualities can often be incorporated in the building structure with little, if any, extra cost if appropriate consideration is given in the early stages of design. These desirable qualities include:

A very important aspect of robustness is the ability of the structure to cope with change. Very high demand and limited resources in the 1960s coupled with the Modern movement approach to functional design led to 'tailor-made' buildings with very little thought or scope for future change. It is now realized that functional requirements are changing rapidly in many forms of building. Ample spaces for services and the facility to change them are, for the building owner and user, welcome additions to the general robustness of the structure, as is the ability to accept higher loading and some cutting or rearrangement to accommodate additional lifts or service runs. Loading is particularly significant. In many buildings the cost of the structure is small in relation to the value of the finished building. The relative cost of providing for additional loading may be slight and a sound investment for an uncertain future. In the same way, ample plant room space and extra storey height can prove invaluable for the incorporation of additional building services. A robust structure, however well designed, also needs a robust management and communication system for its production. Analysis of past failures confirms that they result primarily from lack of perception and poor communication rather than from insufficient knowledge of behaviour or circumstance. There are many links in the tortuous chain required to produce a robust building. One of the most significant is that between the designer and builder including those responsible for site inspection and supervision. Two particular aspects emerge from these considerations: (1) buildability; and (2) quality assurance. A readily buildable structure is an essential start to ensuring good quality. Designers should bear in mind the problems of construction and seek advice from a contractor during the early stages of design whenever possible. Conversely, supervisory staff should understand the structural behaviour assumed in design, not only of the structure as a totality on completion but also of parts of the structure during construction of the whole. They should direct particular effort to the more important aspects of quality control and seek to create the right climate on site - to do a good job even when not supervised and to get it right first time. All this is helped by the production of an 'inspection brief prepared by the design team to ensure the effective deployment of the supervisory staff. Such a document should consolidate all the work leading up to the site start, define responsibilities, confirm and clarify relevant documentation and lines of communication, alert people to critical aspects and co-ordinate specialist inputs, all as a formal handover from those responsible for design to those responsible for construction.

(1) An ability to cope with hazards in an acceptable way, i.e. the building and its structure have been consciously designed so that damage would not be disproportionate to the cause.

21.19.3 Wind effects on buildings The airflow around a building is affected by the adjacent land

(1) (2) (3) (4)

The probabilistic approach. Discernment of proneness to hazard. Alternative paths and partial stability considerations. The hazard-consequence relationship.

These philosophies can be applied individually or in combination as a means of risk control or as a tool for risk comparison between alternative courses of action. The probabilistic approach (1) recognizes the statistical hazard of adverse combinations of high load and low strength and provides a method of comparative measurement of safety. It is the basis of current limit-state codes. Proneness to hazard (2) recognizes the 'climate' in which errors, misjudgements or accidents occur and is an aid to discerning potential hazard, e.g. a new form of construction being hurriedly adopted under political and/or commercial pressure. Considerations of alternative path/partial stability aspects (3) of a proposed structure, encourages recognition and, hence, elimination of excessive sensitivity to local damage from unforeseen hazards. The hazard-consequence approach (4) seeks to identify the potentially serious consequence and directs resources and attention to the most vulnerable aspects determining future structural performance. 21.19.1.4 Structural tests The behaviour of the structure is normally assessed by analytical methods but may also be estimated by tests on prototypes or models or by a combination of analysis and experiment. Prototype testing is sometimes used, for example, in precast concrete construction where the accuracy of the design assumptions may be in question or, in cases of repetitive application, where a better or more reliable understanding of structural behaviour may lead to economy. Model testing may be used to determine internal forces or stresses, and in special cases photoelastic analysis may be used to check complex local stress conditions, e.g. around service openings in major structural members.

and building complex and by the shape and size of the building itself, the roof type, the position and size of overhangs, the area and location of openings and the direction of the wind. Account needs to be taken of the loading effect of wind turbulence on the building as a whole (and perhaps during construction) and on components, the local wind environment in the vicinity of buildings, the general weather-tightness, natural ventilation and the air pollution around buildings. Code of Practice 3:1967. Part 2, Chapter V gives the method to be used for assessing wind loads on individual buildings and takes account of the location of the building, the topography, the ground roughness, the building size, shape and height and a statistical factor related to the probability of given wind speeds occurring during the specified life of the building. For groups of buildings, particularly those including tall buildings, the environmental effects are frequently studied by means of model tests in wind tunnels. 21.19.4 Movement The problem of movement in buildings is not so much the determination of its absolute value, but more that of achieving a compatibility of movement between parts. Without this, cracking and other disturbances are inevitable and are likely to recur even after repair; in such cases, the accurate assessment of movement serves little purpose. On the other hand, the simple recognition that relative movement will occur, coupled with a broad assessment of its significance, are essential first steps in establishing a compatible design in which the actual amount of movement is relatively unimportant. When significant parts of a building tend to move appreciably relative to each other, they can be separated into independent blocks. Within each block differential movement will occur between elements, e.g. between the structure and partitions, and also between different parts or different materials forming a particular element. Factors affecting the division of a building into blocks include: (1) differential foundation settlement due to load or soil variations, changes in foundation type or major intervals in construction; (2) longitudinal movements due to changes in temperature, shrinkage or prestress (immediate and long-term); (3) abrupt changes in building plan or floor or roof level; and (4) abrupt differences in structural stress. Within each block, the most common movement problems are: (1) partitions damaged by floor deflection or relative longitudinal movement; (2) crushing or buckling of cladding and partitions due to relative vertical movement between them and the structure; and (3) separation of surfacings due to movement relative to the backing material. Structural joints include: (1) hinge details, to permit rotation; (2) expansion and contraction joints, commonly used with flexible seals; and (3) complete separation, e.g. where double columns are introduced. Joints between nonstructural elements allow for expansion or contraction or lozenging by providing appropriate horizontal and vertical gaps between the elements which are sealed with a flexible material or by cover strips secured to one side of the joint. Joints exposed to the weather require special attention in detailing and manufacture and in the choice of sealing materials. The open drain joint coupled with an effective air and water back-seal has proved a successful and reliable joint. Wherever possible, restraints to longitudinal movement should be avoided by appropriate design and location of wind stiffening cores or bracing walls. For example, in rectangular buildings it is frequently appropriate to locate a service core at one end, providing rigidity in two directions, and a cross-wall at the other which provides the necessary torsional stiffness but permits free movement in the longitudinal direction. Temperature movements are minimized by keeping the structure within the insulation envelope.

21.19.5 Structural arrangement A great variety of structural arrangement is used in practice depending upon the planning, functional, aesthetic and economic requirements of the building and site. Even for similar buildings, the relative priorities attached to the individual factors affecting structural decisions will vary depending upon the particular circumstances and the views of the client. General rules governing structural arrangement cannot be given, but in most cases the following principles are valid: (1) vertical loads should be transmitted along the shortest and most direct path to the supporting ground; (2) when minimum structural sections are dictated by nonstructural requirements, e.g. sound insulation, they should be deployed to gather extra load; (3) vertical load-bearing elements should be stacked directly over each other; (4) transfer structures, including vertical ties, should be used only when justified; and (5) structural layout should be regular to increase repetition of identical building components and improve construction rhythm. The structural arrangement, construction method and material may be chosen on the basis of some overriding consideration, such as the provision for future alteration or extension, passage for services, speed of construction, availability of labour and materials or difficulty of access. Where alternatives are equally appropriate, the choice is generally made by cost comparison, but this must take into account all aspects affecting the total cost of the building including, where appropriate, running and maintenance costs. The primary structural systems available for spanning vertical loads across space are; (1) the catenary, acting in tension; (2) the arch, in compression; and (3) the beam, in bending, item (3) being of most importance in buildings. Columns and walls are the commonly used members for vertical load support, and on occasion tension members are used to suspend lower work from high-level beam or cantilever construction. Frame structures are of two basic types, those in which horizontal forces are taken by shear walls or bracing and those in which the frames, comprising columns rigidly jointed to beams or slabs, are designed to accept horizontal as well as vertical loading. Structures relying solely on frame action for stability become increasingly inefficient with height and reach a normal practical limit of about 15 to 18 storeys. For tall framed buildings, sway limitations (which must take account of increased lateral displacement due to the action of vertical loads on frames deflected by wind loading) are such that much larger and stiffer members than necessary for vertical loads alone would need to be used. 21.19.6 Resistance to vertical load 21.19.6.1 Columns and walls Column and load-bearing wall positions are determined mainly by the building use. Where large clear spans are not necessary and regular and permanent space division is required as, for example, in multistorey flats, load-bearing walls are commonly adopted. Column spacing, in conjunction with the floor construction, will be affected by the available structural depth and the necessary provisions for the passage of mechanical and electrical services. Concrete columns may be of any reasonable shape; in tall buildings the section may be kept constant for construction convenience or reduced at upper levels to save usable floor area. When floor space is particularly valuable, spiral reinforcement or solid steel sections may be used or tension supports may be provided; the latter may be prestressed, in stages, to minimize the required sectional area and eliminate cracking.

21.19.6.2 Floors In addition to their structural function, floors may need to provide impact and airborne sound insulation, thermal insulation and appropriate fire resistance depending upon their location in the building,and the building type. For longer items, deflection and vibration will also be important considerations. In situ concrete floors. Reinforced and prestressed concrete flat-slab construction has the advantages of: (1) minimum structural depth; (2) adaptability to irregular arrangements of columns or walls; and (3) not requiring a suspended ceiling (but if one is provided gives complete freedom for the passage of services). Solid reinforced concrete construction, 150 to 250 mm thick, may be used for spans up to about 7 m, or greater if posttensioned. Punching shear at the columns, midspan deflection and the size and location of openings require special attention. When larger spans or weight reductions are required, the slab may be coffered to provide one- or two-way spanning: standard or purpose-made plastic, steel or fibreglass moulds are available and can produce a visually attractive self-finished ceiling. Alternatively, permanent cavity formers may be left in position. Beam and slab construction, at the expense of greater floor depth and slower construction, has the advantages of: (1) longer spans; (2) ready provision for large openings, e.g. for stairs and lifts; (3) adaptability to varying size and building shape; and (4) relatively light weight. It is most economic when large repetitive areas, or a heavy loading is required. In situ beam and slab construction may be the only valid method of construction for complex shapes and areas. Precast floors. These have the advantages of speed of erection and quality and accuracy of manufacture; they are economic for medium to large spans particularly where layouts are straightforward and repetitive. They may be used in conjunction with steel or concrete frames in addition to load-bearing wall construction. The largest use of precast flooring is in the form of hollow or solid slabs, reinforced or prestressed, with widths varying normally from 300 mm to 2.7 m and up to 7 or 8 m in the case of large-panel construction where such slabs may incorporate openings, ducting and floating screeds. Precast slabs may be designed to act compositely with the supporting concrete or steel beams. For longer spans, single or double Tbeams, which combine floor slab and beam are available; they are connected by welding, bolting or in situ jointing to provide secondary load distribution and equalize deflection. Composite floors. These rely on the composite action between an in situ concrete topping and precast concrete soffit elements, which may take the form of precast concrete ribs, planks or slabs incorporating the tension element of the composite slab and having projecting reinforcement or other appropriate interface to ensure composite action. Alternatively, permanent steel shuttering may form the soffit. These floors are easily erected, do not need shuttering and provide the shallow depth of in situ slabs. A further example of composite action is that between concrete floor slabs, in situ or precast, and steel beam or frame construction. 21.19.6.3 Transfer structures It is frequently found, in multistorey construction, that the special functions of the lower storeys require an arrangement of columns or bearing walls very different from that required for the efficient support of the superstructure above. A transfer structure is then required to transmit the typical floor column loads to the fewer but larger supports beneath. Often a very substantial structure involving storey height beams is required this should be taken into account in the early stages of design

since it may provide a suitable location for plant. An alternative is to place the transfer structure at roof or intermediate upperfloor level and to suspend the lower structure by means of steel or prestressed concrete hangers. 21.19.7 Resistance to horizontal load In low- to medium-rise buildings, the structural system is designed primarily to resist the vertical loads and is then checked for lateral forces which may be taken by momentresisting frame action, braced frames or shear walls conveniently located around lift shafts or stair wells. Simple shear walls provide the necessary horizontal restraint for 'pin-jointed' frameworks which are often convenient and economic. A minimum of three bracing walls are required so disposed as to provide: (1) resistance in each of two directions at right angles; (2) an overall torsional resistance; and (3) minimum restraint to thermal or similar movement. 21.19.8 Multistorey construction Steel and concrete are the primary structural materials and both can be used in a variety of forms. The choice of material and form will be dependent upon many factors particularly relevant to the building project, such as: (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)

Required speed of construction. Integration of services. Adaptability for future change of use. Lead times for delivery or construction. Fire protection. Impact on foundation feasibility and cost. Buildability and dependence upon workmanship and materials. Stability during construction. Road access for delivery and erection. Off site/on site labour availability. Dependence upon supplier. Impact on other trades. Securing early watertightness. Cost.

A key aspect is the provision for services either by complete separation from, or integration within, the structural elements. Integration involves deep, perforated structural elements and requires very careful planning and co-ordination. Separation requires zoning outside the structural elements, the configuration of primary and secondary beams to provide these obstruction-free zones is then a very important part of the structural concept. The common forms of concrete and steel multistorey framed construction are given in Figures 21.10 and 21.11. Structural continuity is easily effected and commonly adopted in in situ reinforced concrete construction, increasing both stiffness and economy. In precast or steel construction, continuity is more difficult to achieve and the theoretical savings in structural material are offset by the complications in construction and reduced future adaptability. Repetition, simplicity and continuity of erection are more important in achieving economy than marginal savings in material obtained by a too-tailored approach in design. However, with modern methods of fabrication, 'specials' can be introduced economically provided the needs of quantity, repetition and simplicity of erection can still be met. The optimum structure will also take into account its impact on other building elements and on the construction process as a whole. A number of buildings require a special structural response outside the commonly used systems referred to above. Architec-

Limited spans. Deflections need watching. Uninterrupted service space above suspended ceiling or exposed flat soffit for uplighting and heat sink As for (a) but increased spans. Dead load deflection can be compensated by cable uplift

Increased spans Uninterrupted duct space in direction of beams

Increased spans. Increased overall floor depth unless services integrated. Generally stiffer construction

Increased spans with minimum depth and weight. Decorative soffit if left exposed. Some services can be incorporated in waffle depth

As for (d) but generally faster construction

Very long spans possible. Clear service runs in direction of span

Figure 21.10 Concrete construction, (a) In situ reinforced concrete slab; (b) in situ prestressed concrete slab; (c) flat slab with shallow beams, reinforced or prestressed concrete; (d) in situ reinforced or prestressed concrete beam or slab; (e) waffle reinforced or prestressed concrete slab; (f) precast beam and slab; (g) precast T on double T

Floor slab Duct

Optimizes floor depth structurally. Fitting-in of services is limited and can be difficult. Detailed co-ordination essential

Suspended ceiling Floor slab Stub column Ducts between primary beams

Composite action with floor slab effected through stub columns increases structural depth while providing space for transverse services at the expense of heavier steel

Suspended ceiling Floor slab Secondary beams Ducts parallel to main beams

Main air ducts run parallel to primary beams. Increased floor zone depth but easier to accommodate and later modify services

Suspended ceiling Floor slab

Easiest for services but spans reduced or steel cost increased. Ideal if floor zone depth not critical

Ducts under main beams Suspended ceiling Floor slab

Large spans possible utilizing full beam depths at span centre. Transverse service runs accommodated at beam ends or through holes in beam webs near centre of span

Suspended ceiling Figure 21.11 Floor slabs may be in situ or precast concrete and are frequently designed to act compositely with the beams. Profiled steel decking with through-welded stud shear connectors supporting an in situ lightweight or normal concrete topping can provide a fast and practical composite floor construction, (a) Castellated deep beams; (b) shallow beams with stub columns; (c) deep primary beams with shallow secondaries supporting slab; (d) shallow primary beams at close centres; (e) tapered beams tural form, large spans or great height, or special physical criteria, e.g. seismic, may dictate a totally different approach.

21.20 Tall buildings With increasing height, resistance to horizontal forces begins to dominate the design and may add substantially to the total cost.

In addition to structural safety, sway limitations must be satisfied in terms of horizontal accelerations as well as actual movement. In principle, the lateral resistance may be provided by frames in bending, braced frames or by shear walls, as in lower structures, but the greater magnitude of the forces and movements necessitates a more sophisticated approach. The various systems of resistance to horizontal forces in tall buildings are illustrated in Figure 21.12.

Steel weight Ibf/ft2 of floor

Distribution without shear lag Inefficient system

Actual axial stress distribution Distribution without shear lag

Realistic wind premium Framed tube

Axial stress distribution in columns

Wind direction Plan

Total gravity steel Elevation

Type II

Exterior diagonalized tube

Bundled framed tube

Exterior framed tube

End C framed tube with interior shear trusses

Frame with shear, band and outrigger trusses

Frame with shear truss

Rigid frame

Semi-rigid frame

No of storey

Type I

End C and middle I framed tubes

Effect of shear lag

Number of storeys Gravity steel vs. wind premium

Type IV

Type III

Trusses

Semi-rigid connection

Rigid connection

Shear truss outrigger truss Figure 21.12 Systems of resistance to horizontal forces in tall buildings. (After lyengar (1972) 'Preliminary design and optimization of steel building systems', Conference on planning and design of tall buildings. ASCE/IABLE)

Framed end channels

Framed Exterior middle framed tube I

Bundled tubes

Ext diag tube

21.20.1 Frames in bending Internal frames are comparatively inefficient and flexible as a result of the planning necessity for a wide spacing of internal columns and limited floor beam depth. On the other hand, exterior frames, formed in the plane of the external wall, may have closely spaced columns connected by deep spandrels. In this way, the entire perimeter of the building may be developed as a major lateral load-resisting system referred to as the 'boxed frame' or 'framed tube'. Subject to 'shear lag' considerations, the building walls act respectively as the webs or flanges of a box section cantilevering from the foundation. To allow for 'shear lag', two channel sections may be considered operative in place of the complete box. Deep spandrels, although advantageous, are not essential and apartment buildings of up to 46 storeys have been constructed in the US using part of the adjacent flat slab floor as the beam continuous with the closely spaced external columns. 21.20.2 Braced frames Braced frames usually incorporate single- or double-diagonal braces or K-bracing within the beam and column framework and may be used internally, around service cores or in the external wall. When used externally, intermediate transfer structures may be incorporated to transmit a major proportion of the total vertical load to the corner columns. Such transfer structures may also support and, hence, separate, different configurations of internal supports required when the function varies between different vertical zones of the building. Such arrangements have a major impact on the internal planning and external appearance, and have important relevance to the planning and construction of very tall buildings using steelwork. Similar external truss action may be achieved in concrete by blocking out windows to form solid and continuous diagonal members or, for limited height, by using precast cladding forming a multiple-diagonal system. When they can be accommodated, internal trusses can bring into action lengths of external wall otherwise rendered ineffective by 'shear lag'. By alternating the plan position of such internal trusses, from storey to storey, the structural span of the floors may be reduced to half of the architectural planning bay by providing additional hanging supports from the trusses in the storey above.

requirements are: (1) at least three must be provided of which at least two must be parallel and widely spaced, to provide torsional resistance, with the third at right angles; (2) the centroid of the shear walls should be close to the centre of gravity of the loading; and (3) walls likely to need very large openings should be avoided if alternatives are available, since their stiffness and, hence, load-resisting contribution will be diminished substantially. Walls with openings produce a stiffness intermediate between that of the total combined length acting as a monolithic wall and the sum of the stiffness of the parts acting separately, depending upon the relative size and location of the openings. Normal analysis assumes that all the shear walls or cores act from a completely rigid foundation such that relative rotation or vertical movement does not occur. Since even small relative movements could seriously invalidate the design, it is important that this assumption is realized in the foundation design or, if this is not practical or economic, the shear wall system should be designed to suit. In general, the total horizontal load is distributed between the shear walls in proportion to their relative stiffnesses taking into account any eccentricity of the applied load. The floor system then acts as a horizontal diaphragm equalizing horizontal displacement and rotation at each floor level. Shear walls and cores are almost invariably constructed in concrete and are often slipformed. Precast large panels have been successfully used, particularly in high-rise blocks of flats, with the combined functions of load-bearing walls and vertical wind braces. The joints between the panels and the lintels over openings require particular consideration in the design of such structures. The use of shear walls or cores is an economic and efficient method for resisting large horizontal forces but, in most cases, deflection limits their use to below 30 to 40 storeys. However, if the shear wall, and building, are shaped in plan along their length, a vertical shell or folded plate action could be developed permitting greater heights; otherwise, a 'boxed frame' or one of the combined systems described below is required to control deflection. Another limitation of shear walls, particularly if lightly reinforced, arises from the possibility of brittle failure which could make them unsuitable for seismic structures. However, by suitable framing around the shear wall, the necessary ductile behaviour can be obtained to absorb the considerable strain energy arising from an earthquake.

21.20.3 Shear walls and cores Shear walls may be internal or external or may surround internal service areas to form cores; their location and dimensioning are major design elements since they seriously impinge on internal planning and may affect external appearance. In the early formative stages of design, quick structural appraisal of alternative shear walls will be required followed by careful design and analysis of the final arrangement. In office buildings, the service core - which includes lifts, stairs, ducts and toilets - can occupy 20% or more of the total floor area while fire and sound insulation require this area to be bounded by heavy wall construction. These conditions naturally lead to the use of the service core as a major vertical wind brace. However, away from the core area, open office space is generally required and even if partitions are used they would be demountable to allow for future alteration; internal bracing walls are therefore a planning impediment in offices and are generally avoided. External bracing walls, however, are often used, generally in conjunction with internal cores. In housing or hotels, partitions are normally fixed, need to be heavy for sound insulation and are regularly spaced; they therefore provide many convenient locations for internal bracing walls. When shear walls alone are used, the general structural

21.20.4 Combined systems Internal cores may be used in conjunction with external moment-resisting frames so that the substantial overturning resistance of the fa9ade frame is combined with the excellent shear resistance of the core to form a highly efficient total system known as 'tube in tube'. This arrangement still relies upon closely spaced external columns; when widely spaced external columns are required, a beneficial interaction between the core and the external columns can still be obtained by connecting the two with deep stiff beams rigidly connected to the core and located at convenient levels (roof and service floors). In this arrangement, the core continues to take the shear but the overturning resistance of the full building depth is called into play and deflection is reduced. Another advantage of this system is that it can help control the effects of differential expansion or contraction of the external columns. Figure 21.13 illustrates the application of the bundled tube principle to the 110-storey Sears building in Chicago. The faces of each tube are stiffened by deep beams and columns in vierendeel action. The tubes are bundled to their maximum effect at ground level but are dropped off with increasing height to suit both structural and internal planning requirements. The

internal framing at the junctions between tubes reduces shear lag in the long faces of the multiple tubes as shown in Figure 21.13(b).14 Maximum effect would be achieved with diagonal bracing incorporated in the tube faces.

Zone 3

Zone 2

Zone 1 Multiple or bundled tube

Overturning moment

Floors 1-50

Column wind axial loads

Wind Figure 21.13 The Sears building in Chicago. (After Fischer (ed.) (1980) Engineering for architecture. McGraw-Hill, New York) 21.20.5 Vertical movement Another aspect distinguishing tall buildings is the need to consider the possibility of differential vertical movements due to temperature or stress and, in the case of concrete structures, those due to creep and shrinkage. The movement is most marked between the internal structure and the external columns, particularly if the latter are totally or partly outside the external wall and glazing. It affects most the external cladding and partitions located at right angles to the external wall, as well as any linking structural element. The effects are best controlled by attempting to achieve uniformity of stress and exposure (including insulation where necessary) and uniform surface: volume ratios for the concrete elements to reduce differential shrinkage or creep. When the problem is particularly severe, the building can be divided into two or more sections by incorporating intermediate transfer structures, in effect producing horizontal expansion or contraction joints. Alternatively, the movement can be restrained by stiff beams connecting the external columns to the core. Another approach is to freely permit the movement and incorporate appropriate movement details in the structure, partitions and finishings. 21.20.6 Lateral movement and dynamic effects Wind loading on tall buildings is not simply a matter of statics. As with long-span bridges, aerodynamic effects and the dynamic response of the structure under variable wind loading are important considerations. The primary considerations are: (1)

vortex shedding, arising from the plan shape of the building; (2) the extent of horizontal movement and related accelerations that can be tolerated; and (3) the means of dissipating the wind energy imparted to the building, i.e. damping. The damping characteristics of the early tall buildings of traditional construction were good. The exterior walls and internal partitions helped to dissipate the wind energy imparted to the building by friction between their parts as they moved relative to each other under wind action. The fa9ades were generally textured, creating turbulence, and sculptured, thus reducing the risk of transverse oscillation from vortex shedding. With the development of simpler shapes and taller, more flexible buildings, these damping qualities have been diminished. Damping through the structure itself is small since, in this context, building structures are very elastic. Damping through the nonstructural elements is difficult to control and can be expensive in maintenance and repair. This leaves aerodynamic damping, through form and cladding texture, or specially designed mechanical damping as the means of absorbing energy and controlling movement to within acceptable limits. The alternative is to increase mass but this can be very expensive. Acceptable lateral movement is not simply a matter of deflection but more of people's perception of, and psychological response to, movement. Deflection is a product of stiffness and can be controlled, at a cost, through structural quantity and configuration. However, people's reaction to sway is related more to acceleration than actual amount of movement and, more particularly, to the rate of change of acceleration. Increasing the structural stiffness reduces the deflection but does not affect acceleration since the frequency is increased. Indeed, the more critical rate of change of acceleration is also increased. Reducing stiffness increases sway and the relative movement between building elements as well as visual disturbance. Thus, in some cases the only two practical methods of dealing with this problem are to increase the mass or improve the damping characteristics of the building. Increasing the mass is expensive. Aerodynamic damping is possible if suitable building forms are acceptable. Where this is not possible, or in cases of extreme height, additional damping can be provided by incorporating inertia elements strategically located within the building, or energy-absorbing devices at points of movement within the structure. Such damping has the effect of reducing both the amount of movement and the acceleration. An example incorporating inertia elements is the Citycorp Center in New York which has a tuned mass damper to slow down and reduce movement due to wind. The system is housed in the roof structure and consists of a 400-t concrete inertia block mounted on a 'frictionless' bearing which is free to remain still when the building starts to move under wind action. The mass is connected to a system of pneumatic springs and dashpots in which the energy is absorbed by oil. The dynamics of the damping system are adjustable to suit the building's actual dynamic characteristics. Fail-safe devices are incorporated to ensure that the movement of the mass relative to the building is kept within predetermined limits. Analysis and wind-tunnel testing indicated that this device would reduce the acceleration under wind loading by 38%. The other methods of damping were adopted at the World Trade Center, also in New York. The simple geometric shape of the twin 110-storey towers was such that the vibration due to vortex shedding could not be discounted. To combat this danger, the building corners were chamfered, so modifying vortex generation, and a viscoelastic damping system introduced. Some 10000 such dampers were built into each tower, consisting of steel connections incorporating viscoelastic material between the floor trusses and columns. Figure 21.15 illustrates diagrammatically the dampers incorporated in the 110-storey World Trade Center in New York.

Figure 21.14 The Citicorp Center, New York, (a) The tower; (b) open space at the bottom of the tower; (c) braced external wall structures. (After Fischer (ed.) (1980) Engineering for architecture, McGraw-Hill, New York)

The damping unit comprises two Ts bonded to a central plate by viscoelastic material. The plate is attached to the bottom chord of the beams and the Ts to the columns. Wind energy is absorbed by shear displacement of the viscoelastic material. Figure 21.16 illustrates the general arrangement of a tuned mass damper while Figure 21.17 illustrates graphically the beneficial impact of damping on building oscillation. Similar principles can be used to control the vibration of light structures. 21.20.7 Additional considerations In addition to the wind loading on buildings, the wind effect between buildings and, particularly, that produced by the presence of a tall building, has proved of considerable concern. Wind-tunnel tests on models are an essential element of design, to check the environmental impact created by a new building within an existing complex. Special arrangements in the lower levels of tall buildings are encouraged in some locations by city planning policy. In New York, for example, a 20% additional floor area is given as a bonus for providing a public open space or plaza in front of the building; this has now been extended to encourage covered and secure pedestrian freeways through the buildings. Incentives are also given to encourage a richer mixed use of tall buildings, e.g. by incorporating a theatre when the building is in a theatre district, and shops within a shopping district. The intention is to produce a lively combination of activities and more demand for transport and services over a longer period of time. These aspects have given rise to another important consideration in the design of both high- and low-rise buildings, i.e. their treatment at ground level. For example, the Citycorp Center in New York incorporates at its base a minicentre for culture and commerce including a church, a theatre, a room for jazz performances and a complex of international food boutiques. To accommodate these, the bottom of the structure consists only of the centre core and massive columns placed not at the corners but at the mid-point of each side. In most cases the bridging requirements to achieve these open spaces at ground level are met by a straightforward transfer structure in steel or concrete. This is sometimes placed at roof or upper level and the building beneath hung from it. In other cases, more subtle bridging means have been employed to make use of the whole superstructure, acting as a total bridging entity within itself. Figures 21.14(a) and (b) illustrate how open space was created at the bottom of the Citicorp Center tower and utilized for religious and cultural purposes. Figure 21.14(c) illustrates the braced external wall structures. Each eight-storey tier is structurally independent with loads from each tier gathered to the four exterior mast columns via the diagonal truss members. Wind shear in each eight-storey tier is taken by a central core structure but the overturning moment is then transferred to the external masts. In the urban context, the placing and form of tall buildings is of great importance. The architect and engineer have learned to solve the technical and aesthetic problems of tall buildings but generally as isolated elements. As one example which recognizes the very great positive or negative impact that a building of exceptional height may have, the City of San Francisco has adopted an urban plan which sets down fundamental principles and critical urban design relationships to govern future major development. This not only protects community interests but also gives reliable guidance to design.

21.21 Special structures Special structures include means of covering or enclosing large areas without internal supports (by means of beam, membrane, tension, skeletal or pneumatic structural action), space frames

and tall buildings, each of which has its own particular field of application and specialized technology. They also include cases where the normal assumptions regarding structural action may not apply. For example, deep beams, in which the span: depth ratio is small (less than 5:1) behave differently from shallower beams and the normal theory of flexure does not apply. In portal frames, in which the beam spans are large in relation to the column lengths, the distribution of bending moments is highly sensitive to the relative stiffnesses of the members, and the normal assumptions of stiffness of reinforced concrete sections (whether cracked or uncracked) or of steel sections at yield stresses are insufficiently accurate. Similarly, the geometry of the system may exaggerate the effects of movement. Membrane structures involve the use of thin surfaces geometrically arranged to support vertical loading mainly by forces parallel to the surface. They may be folded plates or singly curved, as in barrel vaults and arches, or doubly curved as in domes, hyperbolic paraboloids and other special forms of curved surface. While secondary bending effects are present at right angles to the surface, the main internal forces are parallel to it and it is essential that the surface and boundary conditions are geometrically correct for resisting the loading. The simple example is the dome, in which compressive forces occur within the dome, and ring tensions or external restraints are required at the perimeter. Concrete, timber and sometimes steel are used in forming long-span membrane structures; they use materials efficiently, but their economic viability depends mainly on the workmanship and labour required. Folded plates provide functional and aesthetically interesting roofs, in which normal flexural action occurs; additional considerations of edge support, end shear, buckling and distribution of out-of-balance loading also apply. Tension structures involving cable-supported sheeting have been used in exhibition buildings and in sports stadia; the structural system involves steel cables acting in catenary from which decking or tenting is supported. Skeletal space-frame structures are used in the form of plane grids of rectangular, diagonal ('diagrid'), triangular or hexagonal pattern, arches, domes and other structures analogous to membrane structures, in that the geometric shape of the surface is such that the principal resisting forces act parallel to the surface. These structures have been applied to sports stadia, exhibition buildings, aircraft hangars, terminals and other places requiring very large column-free areas. Pneumatic construction uses air pressure in various ways to stabilize the membrane of the building. Such structures are light, economical, easy to erect, dismantle and transport, and have proved to be practically successful in application to housing temporary exhibitions, warehousing and covering sports halls and other specialist buildings. The basic engineering principle employed is that the membrane can accept tensile stresses and will fold when not in tension. Internal air pressure is used to maintain membrane tensions when dead and other loads are imposed. Various types of pneumatic structure have been developed, including the basic air-supported membranes and inflated dual-walled and ribbed structures, and various hybrid types. The larger spans are achieved by the use of arched or domed forms, and cables, cable nets and internal membrane walls are used to control shapes and improve stiffness.

21.22 Foundations Foundations must safely transfer loading from the superstructure to the ground without excessive absolute or differential settlement. The foundation type will depend upon the nature of both the underlying soil and the superstructure since the two must be compatible as far as the settlement characteristics are concerned. In some cases, to economize in foundations, the

Viscoelastic layers

Attachment to column

Attachment to chord

Figure 21.15 Dampers used in the World Trade Center, New York. (After Fischer (ed.) (1980) Engineerng for architecture. McGraw-Hill, New York)

Anti-yaw device

Mass block

Failsafe device

Control system Spring mechanism

Figure 21.16 General arrangement of a tuned mass damper. (After Fischer (ed.) (1980) Engineering for architecture, McGraw-Hill, New York)

Zero damping

Amplitude

Critical damping

Exponential Inherent decay curve damping Time Figure 21.17 The impact of damping on building oscillation. (After Fischer (ed.) (1980) Engineering for architecture, McGraw-Hill, New York) superstructure will be designed to allow for differential settlement by the incorporation of, for example, jointed construction in the structure, cladding and finishings. In other cases, a type of foundation will be adopted which limits settlements to amounts acceptable to the previously determined superstructure. In special instances, the superstructure may be designed to act compositely with the foundation. In all cases the designs of the superstructure and of the foundations are interdependent; knowledge of the soil conditions is thus essential at all stages of the design process, beginning with at least broad local knowledge prior to land acquisition. Such information may also influence the location of particular buildings on the site. The risk of mining subsidence must be assessed at an early stage and the appropriate course of action decided, e.g. whether to seal and grout the workings, pile through the workings, design the superstructure to accommodate subsidence, or to accept the risk without special action. The main types of building foundation are individual pad footings, strip footings, rafts, piers and deep footings, deepspread foundations and piles. Basement construction is useful in reducing settlements and in controlling differential settlements between parts of buildings of different height. Ground floor slabs may be suspended, but normally bear directly on the soil; they are required to span local weaknesses or voids due to settlement and to transfer local loading to an appropriate area of the ground. Their design is largely an empirical process; specification, workmanship, joints and surface treatments are important. Basement construction and ground floor slabs must resist penetration of water or water vapour to a degree determined by the building use; provisions may include: (1) land drainage; (2) good-quality concrete of suitable thickness and with appropriate workmanship and details particularly at the joints as to be adequately waterproof; and (3) the provision of internal or external waterproof membranes. In some buildings it may be more economic to accept some leakage and provide for discharging it than to attempt completely watertight construction. The presence of water may give rise to serious uplift problems in basements during or after construction unless adequately provided for in design. The construction of deep basements through ground material other than rock is bound to cause ground movement which could affect both the new construction and adjacent property. The magnitude and extent of such movement will be affected by the method of excavation, the sequence of basement construction and any dewatering that may be required. Top-down methods of basement construction incorporating diaphragm walls or secant piling can be effective in controlling

movement. Where speed of construction is important, such methods also allow simultaneous erection of the superstructure. While mathematical modelling exists to predict ground movements, experience has shown that the most reliable indications are obtained by a combination of modelling and reference to basement construction through similar ground conditions. Frequent monitoring of ground movements during construction is necessary to ensure that no excessive movements are developing and to check the reliability of the mathematical predictions. The extent of movement that can be tolerated by existing adjacent property will depend upon its construction and foundation system.

References 1 2 3 4 5 6 7 8 9

10 11 12 13 14

Royal Institute of British Architects (1980) Handbook of architectural practice and management, 4th rev. edn. RIBA, London. American Institute of Architects (1976) Current techniques in architectural practice. AIA, Washington. Royal Institute of British Architects (1973) Plan of work. RIBA, London. Telling, A. E. (1986) Planning law and procedure, 7th edn. Buttervvorth, London. Building Research Establishment (1979) Thermal, visual and acoustic requirements in buildings. Building Research Establishment Digest No. 91. BRE, Watford. Chartered Institute of Building Services Engineers (1984) Code for interior lighting. CIBSE, London. Neufort, E. (1980) Architects' data, 2nd edn. Crosby Lockwood, London. Fire Officers' Committee Rules, 29th edn. FOC, London. Building Design Partnership, Aarens, Burton and Karalec, and GifTord and Partners (1981/82) Low-energy hospital study. Report compiled on behalf of the Department of Health and Social Security. HMSO, London. Her Majesty's Stationery Office (1985) The Building Regulations: mandatory rules for means of escape in case of fire. HMSO, London. Her Majesty's Stationery Office (1982) Guidelines for the construction of fire-resisting structural elements. HMSO, London. Association of Fire Protection Contractors/Constrado Fire protection of structural steel in buildings. AFPC/Constrado, London. Simpson, J. W. and Horrobin, P. J. (1970) Weathering and performance of building materials. Manchester University Press, Manchester. Fischer, E. (ed.) Engineering for architecture - architectural record 1980. McGraw-Hill, New York.

Bibliography General design planning and management Eldridge, H. J. (1976) Common defects in building. HMSO, London. Harper, D. R. (1979) Building: the process and the product. Construction Press, London. Martin, D. (ed.) (1985) Specification building methods and products. Architectural Press, London. Mills, D. (ed.) (1985) House's guide to the construction industry, 9th edn. Van Nostrand Reinhold, London. Mills, E. D. (1985) Planning: the architect's handbook, 10th edn. Butterworth Scientific, Guildford. Osbourn, D. (1986) Introduction to building. Batsford Academic and Educational, London. Ransom, W. H. (1981) Building failures. Spon, London. Reid, E. (1984) Understanding buildings. Construction Press, London. Rich, P. (1982) Principles of element design, 2nd edn. Longman, Harlow. Saxon, R. G. (1986) Atrium buildings development and design, 2nd edn. Architectural Press, London.

Cost planning and control

Energy

Bathurst, P. E. and Butler, D. A. (1980) Building cost control techniques and economics. Heinemann, London. Cartlidge, D. P. and Mehrtens, I. N. (1982) Practical cost planning. Hutchinson, London. Seeley, I. H. (1983) Building economics. Macmillan, Basingstoke. Stone, P. A. (1983) Building economy. Pergamon Press, Oxford.

Kasabov, G. (ed.) (1979) Buildings: the key to energy conservation. RIBA Energy Group, London.

Internal environment British Standards Institution (1975) Code of basic data for the design of buildings: the control of condensation in buildings. BS 5250. BSI, Milton Keynes. British Standards Institution (19??) Sound insulation and noise reduction in buildings. BS CP3, Part 2. BSI, Milton Keynes. Chartered Institute of Building Services Engineers (1984) Guide to current practice. CIBSE, London. Faber, O. and KeIl, J. R. (1979) Heating and air-conditioning of buildings, 6th edn. Architectural Press, London. Lord, P. and Templeton, D. (1986) The architecture of sound. Architectural Press, London. Parkins, P. H., Humphreys, H. R. and Cowell, J. R. (1979) Acoustics, noise and building, 4th edn. Faber and Faber, London.

Regulations Her Majesty's Stationery Office (1985) Manual to the Building Regulations. HMSO, London. Her Majesty's Stationery Office (1985) The Building Regulations: Approved Documents. HMSO, London.

Fire building security and control Hopf, P. S. (1979) Handbook of building security planning and design. McGraw-Hill, Maidenhead. Vincent, G. S. and Peacock, J. (1985) The automatic building. Architectural Press.

Structure and tall buildings American Society of Civil Engineers (1985) The engineering aesthetics of tall buildings. ASCE, New York. Institution of Engineers (1984) Proceedings, international conference on tall buildings. IE, Singapore.