A dynamic correction for the seismic analysis of structures

International seismic building codes of practice specify a simplified method based on the first mode for the seismic analysis of regular structures and dynamic analysis for irregular structures. The number of modes to be used in the dynamic analysis of structures should be such that the sum total of the modal masses of all the modes considered is at least 90% of the total structural mass. Previous studies show that the 90% criterion for the number of modes considered may not result in correct responses in all the structural members of an irregular structure. The present study examines the rationale for using the codal provisions for the number of modes to be used for dynamic analysis of irregular building structures using the response spectrum method. Results of this study show that fundamental mode approach for regular structures and 90% modal mass criterion, given by the seismic building codes of practice for the number of modes to be considered for the dynamic analysis of irregular structures, are not adequate. It is observed that the present criterion results in the underestimation of shear forces in the top and bottom storeys according to the numerical examples considered. A simplified method is given for the elastic seismic analysis of irregular and complex structures using a “dynamic correction”, which can be extended to the nonlinear pushover analysis of structures.