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Re: LRFD Seismic Provisions

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I believe these special load combinations are intended to be applied in
circumstances where the isolated independent failure of an element could
result in partial structural collapse or the loss of a continuous lateral
force resisting system. Situations include axial loads on columns supporting
discontinuous lateral force resisting elements above, the axial forces on
column splices that have less capacity than the column sections themselves
and other conditions specifically indicated in the provisions. The factor .4R
is proposed by the BSSC PUC to be replaced by a factor omega in the 1997
NEHRP Provisions. A new column of overstrength ratio numbers is proposed to
be added to future R tables that will allow for some variation from 0.4R
which doesn't reflect appropriate levels of overstrength for some building
systems. Alternatively, designers will be encouraged to determine
overstrength ratios based on simple nonlinear static procedures.