Estimation of floor response spectra using modified modal pushover analysis

A common approach to designing nonstructural components against seismic excitations involves the use of floor response spectra (FRS). FRS can be accurately computed only through a nonlinear time-history analysis of the structure subjected to a specific earthquake ground motion. However, for multi-storey structures, which are usually modeled as multi-degree-of-freedom (MDOF) systems, this analysis becomes computation-intensive and time-consuming and is not suitable for adopting in seismic design guidelines. An alternative method of estimating FRS on MDOF systems is presented here. The proposed method uses multiple 'generalized' or 'equivalent' single degree of freedom (ESDOF) systems to estimate FRS on a MDOF system within the context of a 'modal pushover analysis (MPA)'. This is a modified version of the previous MPA procedure as it considers the contribution of the first mode to yielding of higher modes when obtaining multiple ESDOF systems. FRS values for each mode are obtained through nonlinear dynamic analysis of each ESDOF system and then the total FRS values are calculated for the considered modes according to the SRSS combination rule. The efficiency of the modified procedure is tested by comparing FRS based on this method with results from nonlinear dynamic analyses of MDOF systems, as well as estimates based on ESDOF systems built from the traditional MPA method, for several ground motion scenarios. Three steel moment frame structures, of 3-, 9-, and 20-storey configurations, are selected for this comparison. Bias statistics that show the effectiveness of the modified method are presented.