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The Stability of Tall Bldgs_RH Wood

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The Stability of Tall Bldgs_RH Wood
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  Paper No. 6280 THE STABILITY OF TALL BUILDINGS* bY Randal Herbert Wood, D.Sc., Ph.D., A.M.I.C.E. Principal ScientSc Officer, Building Research Station, D.S.T.R. For discussion at an Ordinary Meeting on Tuesday, 21 October, 1958, at 5.30 p.m. and for subsequent written discussion SYNOPSIS The benefits arising from plasticity in structures are well established, but it is not so well known that the beneficial effects may be curtailed in multi-storey frames because of simultaneous deterioration of elastic stability. This study of frame instability is perhaps he most perplexing and intriguing research subject of the moment in frame design, and an attempt is here made to clarify what is involved. Part of the present difficulty arises from a neglect n the past to study the stiffening effects of the cladding of tall buildings. Within this setting an account of the work of the Building Research Station is given, and in relating this work to that of other research schools it is convenient to present the subject in the form of a brief historical account f the development of research. The emphasis throughout is on the necessity of producing rapid design methods. The great distinction between “no-sway” designs and designs involving side-sway is brought out, together with the effects which frame in- stability and composite action have on this issue. In spite of the tremendous numerical work obviously involved a mathematical reatise is purposely avoided. It appears that, whereas it is imperative to make provisions for probable loss of carrying capacity of frames due to instability, the general behaviour of practical frames favours “collapse” design, particularly if some simple, and in the meantime modest, contribution from composite action can be devised. NOTATION A denotes cross-sectional area C (with suffix 1 and 2) denotes out-of-balance fixed-end moments c is a stability function DC denotes double-curvature E ,, Young’s odulus K (with appropriate sm denotes stiffness L denotes length Llr ,, slenderness ratio M (with appropriate suffix) denotes bending moment * Crown copyright reserved. 69  70 WOOD ON THE STABILITY OF TALL UILDINGS m, and o are stability functions P with appropriate suffix) denotes load S is a stability fuktion SC denotes single-curvature X and Y re alternative stability functions to S, c, m, o X (with appropriate suffix) denotes load factor GENERAL URVEY OF DESIGN PROBLEMS THE DESIGNER is nowadays well acquainted with the fact that first yield of the most highly stressed member is not usually followed immediately by collapse.‘ The behaviour of various structures may be divided into three principal types, according to their load/deflexion characteristics. Thus in structures normally envisaged in simple “plastic” design, a variation in moment redistribution takes place between first yield and attainment of a complete collapse mechanism at constant load. If, however, there is a favourable change f geometry (e.g. if the shape of the deflected surface of a slab leads to membrane action), then the ideal plastic-collapse load is exceeded. Conversely if the deflexions result in unfavourable change of geometry, such as in the buckling of struts, then an instability-type of failure may occur, where a peak load is reached short of the ideal plastic-collapse load. Multi-storey frames unfortunately may come into this third class. 2. The instability problem encountered in multi-storey frames may be conveniently subdivided as follows :- U) Instability of individual stanchions by bending about the minor axis, (b) Torsional instability of beams or stanchions when failure is combined (c) Local crinkling of flanges. (d) Instability due to the type of stress/strain curve of the materials them- selves, e.g. plastic hinges n reinforced concrete have a limited otation before the moment of resistance falls off. e) “Frame” instability, consequent upon unfavourable deflexions develop- ing in the frame as a whole, the mutual interaction of all members being the important feature here. 3. Various aspects of the instability problem have been he object of intensive research of recent years, particularly when the aim was to provide rapid design methods.2. 3, 4 Without such esearch certain slender stanchions and light sections could not have been used with safety. It does not necessarily follow that such sections are being used under uneconomical conditions for it is the ratio of the ideal plastic-collapse load factor A to the corresponding elastic critical (overall buckling) load actor &it which really decides whether members are being used economically. Moreover, the whole concept of the “equivalent length” (of stanchions) is here involved. Perhaps the best procedure is to give a brief historical account of the work of the various research centres since the Strictly speaking this is a special case of item e) below. with twisting. 1 The references are given on p. 101.  WOOD ON THE STABILITY OF TALL BUILDINOS 71 time of the Steel Structures Research Committee 1929-36). In doing so it is as well to remember that any new design methods must face p to the same high standards of rapidity demanded of that Committee by designers-a task of some magnitude. EARLY OST-WAR UPID DESIGN ETHODS FOR RIGID-JOINTED MULTI-STOREY 4. The earliest post-war rapid design methods51 6 relied considerably on the design of stanchions proposed by the Steel Structures Research Committee (S.S.R.C.). These methods, and the “Draft Rules” themselves, depended on the side-sway of the frame being eliminated by separate means (as is made clear in the introduction o the Draft Rules).’ 5. Although seemingly restrictive, his was a wise decision, for it s impossible for designs for multi-storey building frames, where side-sway is allowed, o equal, in point of economy, modern no-sway designs. Moreover, many types of city buildings are exempt from an analysis of wind stresses. 6. The no-sway design methods appearing mmediately after the 1939-45 war were:- U) A combination of the S.S.R.C. method of designing stanchions, to- gether with a new method of designing beams using nomograms as devised by Horne.5 (b) A modification of the basic S.S.R.C. method of designing stanchions, together with a new graphical method of designing beams as devised In both cases the difficulty was to predict rapidly either maximum support moments or alternatively central-span moments in beams, since either might control. Horne found that if the far ends of the adjacent beams were assumed pinned, and the far ends of the adjacent stanchions fixed, hen his simple substitute frame gave accurate predictions of maximum beam moments, which could be read from nomograms. More extensive substitute frames were simi- larly employed by Wood, nd graphs were used o predict the influence of whole groups of beam loads (Fig. 1 *). There is little o choose between these methods, and together they made elastic (no-sway) design simple, for no mathematical analysis whatever was needed in the design process itself. 7. In spite of the considerable proportion of steel that was saved in the beam design, the stanchion design lagged behind n terms of economy, although to effect any improvements has subsequently meant long and arduous research carried out at various centres. FRAMES by Wood.6. 8 L, 11 and 22, denote fixed-end moments for live loads in central, .H., and R.H. ays. * Fig. 1 is taken from B.R.S. Notes A:47 and A:58. D l and d2, denote fixed-end moments for dead loads in central, .H., and R.H. bays. Stiffness = Moment of inertia length Worst possible loading arrangement when beam upport moment is required at point 1. arrangement) to opposite hand). (For support moment at point 2, turn the whole diagram (and the corresponding loading Add together the three contributions.  72 WOOD ON THE STABILITY OF TALL BUILDINGS E B 0 E G d

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Jul 23, 2017
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