Seismic Response of Building Frames with Vertical Structural Irregularities

Earthquake design codes require different methods of analysis for regular and irregular structures, but it is only recently that codes have included specific criteria that define irregular structures. In this paper, the mass, strength, and stiffness limits for regular buildings as specified by the Uniform Building Code (UBC) are evaluated. The structures studied are two-dimensional building frames with 5, 10, and 20 stories. Six fundamental periods are considered for each structure group. Irregularities are introduced by changing the properties of one story or floor. 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 are included. The response is calculated for design ductility levels of 1 (elastic), 2, 6, and 10 for four earthquake records. Conclusions are derived regarding the effects of the irregularities on shear forces and maximum ductility demands. It is found that the mass and stiffness criteria of UBC result in moderate increases in response quantities of irregular structures compared to regular structures. The strength criterion, however, results in large increases in response quantities and thus is not consistent with the mass and stiffness requirements. Based on these findings, several modifications to the criteria are proposed, which include a revised formula for estimating the fundamental period for buildings with nonuniform distributions of mass.