Seismic Design of Vertically Irregular Reinforced Concrete Structures

Abstract

Seismic building codes, such as the Uniform Building Code (UBC) do not allow the equivalent lateral force (ELF) procedure to be used for structures with vertical irregularities. The UBC defines a structure to be irregular based on the ratio of magnitudes of either strength, stiffness, mass, setback or offset of one floor to that of an adjacent floor. The criteria defining the limits of irregularity are somewhat arbitrary, but are introduced in the code to provide unambiguous, enforceable provisions. The purpose of this study is to quantify the definition of irregular structures for four different vertical irregularities - stiffness, strength, mass and nonstructural masonry infills. A total of 87 building structures with interstory stiffness and strength ratios ranging from 0.09 to 1.89 and 0.27 to 1.07, respectively, and mass ratios of 1.0, 2.5, and 5.0 are considered for a detailed parametric study. The lateral force resisting systems (LFRS) considered are special moment resisting frames and shear walls. These LFRS's are designed based on the forces obtained from the equivalent lateral force procedure. An ELF) analysis. Finally, nonlinear dynamic analysis is performed in order to assess the seismic performance of these buildings. The results show that the restrictions on the applicability of the equivalent lateral force procedure are unnecessarily conservative for irregular structures. Most structures considered in this study, designed on the basis of the ELF approach, perform reasonably well. In some cases, however, there is an initiation of an undesirable collapse mechanism. It is recommended that capacity based criteria in the design phase be appropriately used in the vicinity of the irregularity in order to ensure desired performance and behavior.