A comparison of the present SEAOC /UBC torsional provisions with the old ones

Abstract

The seismic response of single storey asymmetric structures modelled as two-degree-of-freedom elastic-perfectly plastic oscillators and designed using the design eccentricity formula of the present Uniform Building Code (UBC) torsional provisions, which first appeared in the 1988 edition, is compared with designs based on the earlier UBC 1979. Changes in the total strength requirement and its distribution among the resisting elements are examined, and the effects of these changes on the ductility demand of three-and five-element systems are evaluated. Jt is shown that for two levels of the strength reduction factor R (= 4;1) the ductility demands predicted for systems designed according to the UBC 79 provisions are in most cases similar to those for UBC 88 systems, with the exception of torsionally flexible systems - mainly mass eccentric and, to a much lesser extent, stiffness eccentric systems. In view of the relative complexity of the UBC 88 provisions, a modification of the simpler UBC 79 ones is proposed which results in the UBC 79 torsionally flexible systems behaving in a similar and often better manner than their UBC 88 counterparts.