Civil Engineering Reference
In-Depth Information
U
F
NFAVOURABLE
AVOURABLE
Zipper
Column
L
L
L
L
Figure A.9
Unfavourable and favourable brace arrangements for inverted V-bracing (
left
) and K - bracing (
right
)
post-buckling residual strength is typically 30% of the initial compressive strength. By contrast, forces
increase in the brace in tension up to the yielding point. The net result is an unbalanced vertical force
on the intersecting beam. The unfavourable effects of the unbalanced force can be mitigated by employ-
ing confi guration of braces as, for example, those given in Figure A.9. The presence of zipper columns
has been found to be very effective for systems with V- or inverted V- braces (Khatib
et al
., 1988 ; Yang
et al
., 2006 ).
Vertical confi guration irregularities affect structural response at the various fl oor levels in buildings;
elastic and inelastic demands at these levels are signifi cantly different from uniform distributions. If
there are abrupt changes in stiffness-strength or mass irregularities in elevation, high demand concentra-
tions will ensue, as indicated in Figure A.10 .
Soft storeys occur in buildings whenever the stiffness of a storey to resist lateral demands is signifi -
cantly less than that of adjacent storeys. This is because structural systems with this confi guration tend
to develop inelastic behaviour at the most vulnerable storey. As a result, signifi cant changes in load
paths and deformation patterns arise (
see
Sections 2.3.1.2 and 2.3.2.2). Soft storeys experience large
earthquake-induced displacements and, in turn, cause extensive damage and even collapse. Global
instability in many multi-storey buildings is initiated by soft storeys. Similarly, if two or more adjacent
storeys exhibit large variations in strength, this leads to the effect known as 'weak storey'. Soft and
weak storeys often occur simultaneously due to the close relationship between strength and stiffness,
as discussed in Sections 2.3.1.2 and 2.3.2.2 .
Typically soft and weak storeys are located at the ground fl oor of buildings and are caused by exces-
sive inter-storey heights due, for example, to large shop windows or garages, changes in stiffness and
strength above the fi rst fl oor and discontinuity in lateral resisting systems. Braces in framed structures
and shear walls are often interrupted at the ground fl oor for architectural reasons. Soft storeys are also
generated by components which are non-structural. For example, infi lls create unexpected bracing
actions (also known as ' stiffening ' ) of upper fl oors in buildings; large concentrations of inelastic
demand are thus imposed at ground fl oor level. Clearly, irregular distributions of infi lls along the build-
ing height can cause unfavourable failure modes, which are not necessarily localized at the base.
Bracing due to partially infi lled frames, mezzanines and hillside sites may lead to short column effects,
which are highly unfavourable (
see
Section 2.3.1.2 ). Infi lls are frequently made of heavy masonry or
RC panels such that non-uniform arrangements affect the mass distribution (mass irregularity in eleva-
tion). As a general rule, differences of more than 20-25% in mass or stiffness and strength between
consecutive fl oors can cause unfavourable failure modes. This not only infers that column dimensions
should be reduced with caution, but also suggests the necessity for restrictions on linkages between
adjacent buildings (such as walkways) as well as on setbacks.
Vertical continuity for earthquake-resisting systems is essential for regularity in elevation. Cores,
structural walls, frames with or without braces, and all other lateral resisting structures should run
