SEISMIC RESPONSE OF BUILDING FRAMES WITH VERTICAL STRUCTURAL IRREGULARITIES

Earthquake design codes require different methods of analysis for regu lar and irregular structures, but it is only recently that codes have included specific criteria that define irregular structures. In this p aper, the mass, strength, and stiffness limits for regular buildings a s specified by the Uniform Building Code (UBC) are evaluated. The stru ctures studied are two-dimensional building frames with 5, 10, and 20 stories. Six fundamental periods are considered for each structure gro up. Irregularities are introduced by changing the properties of one st ory or door Floor-mass ratios ranging from 0.1 to 5.0 are considered, and first-story stiffness and strength ratios varying from 1.0 to 0.5 art included. The response is calculated for design ductility levels o f 1 (elastic), 2, 6, and 10 for four earthquake records. Conclusions a re derived regarding the effects of the irregularities on shear forces and maximum ductility demands. It is found that the mass and stiffnes s criteria of UBC result in moderate increases in response quantities of irregular structures compared to regular structures. The strength c riterion, however, results in large increases in response quantities a nd thus is not consistent with the mass and stiffness requirements. Ba sed on these findings, several modifications to the criteria are propo sed, which include a revised formula for estimating the fundamental pe riod for buildings with nonuniform distributions of mass.