Bracing, Stiff Walls or Cores - Anatomy of Steel Structure.

General design requirements and some practical suggestions are also given previously for braced steel bay design.

The layout on plan of vertically stiff elements is frequently difficult even in conventional and regularly framed structures. General guiding principles are that the centre of resistance of the bracing system in any direction should be coincident with the centre of action of the horizontal forces in that direction. In practice this means that the actions and resistances should be evaluated, initially qualitatively, in the two directions perpendicular to the structural frame layout.

Another desirable feature which is also common to many structures is that the braced bays or stiff cores should be located centrally on the plan rather than at the extremities. This is to allow for expansion and contraction of the structure without undue restraint from the stiff bays, and applies equally to a single structure or to an independent part of a structure separated by movement joints from other parts. It is difficult to achieve this ideal in a regular and uniform structure, and almost impos- sible in a typically highly irregular industrial steelwork structure. Fortunately, steelwork buildings, particularly those without extensive reinforced concrete floors and with lightweight cladding, are extremely tolerant of temperature movements and rarely suffer distress from what may be considered to be a less than ideal stiff bay layout.

The procedure for design purposes should be as follows. First a basic means of transferring horizontal loading to foundation level must be decided, and guidance on this is given previously. In either or both directions, where discrete braced bays or stiff walls or cores are being utilized, then initially a geometric apportionment of the total loading should be made by an imaginary division of the structure on plan into sections that terminate centrally between the vertically stiff structural element. The loadings thus obtained are used to design each stiff element.

When this process has been completed and if the means exist, by  horizontal diaphragm or adequate plan bracing, to force equal horizontal deflections on to each element, a second-stage appraisal may be needed to investigate the relative stiffness of each stiff element. Then the horizontal loading can be distributed between vertical stiff elements on a more accurate basis and the step process repeated.

Considerable judgement can be applied to this procedure, since it is usually only of significance when fundamentally differing stiff elements are used together in one direction on the same structure. For example, where a horizontal diaphragm or plan bracing exists and where some of the stiff elements are braced steelwork and some are rigid frames, it will normally be found that the braced frames are relatively stiffer and will therefore carry proportionately more load than the rigid frames. Similarly, where a combination of concrete shear walls (or cores) and braced frames is used, then a relative stiffness distribution will be needed if an effective horizontal diaphragm or plan bracing ensures sensibly constant horizontal deflection or sway
(Fig. 3.10).

Apportioning horizontal loads to vertical stiff elements
Fig. 3.10 Apportioning horizontal loads to vertical stiff elements

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