Modified response spectrum analysis to compute shear force in tall RC shear wall buildings

Abstract

Among various analysis methods, response spectrum analysis (RSA) is widely used to compute design forces of structures subjected to earthquake loading. Previous studies indicate that using a single response modification factor (R) to reduce elastic forces contributed from all modes in the RSA procedure is the primary cause for underestimating the design force demands on structures. In this paper, two modified response spectrum analysis (MRSA) methods, MRSAHE based on a higher-mode elastic approach and MRSAHI based on a higher-mode inelastic approach are proposed for computing design shear forces in tall reinforced concrete (RC) shear wall buildings. Here 15-, 20-, 31-, and 39-story tall buildings subjected to earthquake excitations in Bangkok are first designed using the RSA procedure, and then nonlinear response history analysis (NLRHA) is carried out. It is found that the RSA procedure provides good estimates for floor displacements and story drift ratios, but significantly underestimates force demands, when compared with NLRHA. The MRSAHE method provides good estimates of shear force for the 15- and 20-story buildings and conservative estimates for the 31- and 39-story buildings; while the MRSAHI method provides good estimates for all cases. However, MRSAHI requires a nonlinear inelastic structural model but MRSAHE does not. In both MRSA methods, the bending moment demands are computed in the same way as the conventional RSA procedure, with ductile detailing in RC walls or columns required at locations where the combined axial and bending strain exceeds a certain limit. In order to determine the locations that require ductile detailing in RC walls or columns, a novel method based on the equal displacement concept using strain from elastic analysis to predict inelastic strain is found to provide good agreement with inelastic strain computed from NLRHA.