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Steel Designers' Manual - 6th Edition (2003)
Anatomy of structure 113
by which horizontal loads can be satisfactorily transferred to braced bays or other
vertical stiff elements. General design requirements and some practical suggestions
are also given in section 3.2.2 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 impossible
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 in section 3.2.2. 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).