An assessment of the inelastic deformation demand in critical regions of a multistory structure requires a complicated procedures and significant computation efforts. The primary object of this study is to develop a simplified analytical model for seismic response prediction of steel frame structures in terms of cumulative damage that accounts for the effect of all inelastic excursions on seismic performance. The second object of this study is to investigate the effects of design seismic loads on damage of steel frame structures caused by seismic loads and to investigate the reasons of the damage concentration in lower stories of steel frame structures designed according to seismic regulations.
A multistory frame is reduced to an equivalent single-degree-of-freedom (ESDOF) system using a series of deflected shape due to incremental static loads. Story displacements and interstory drifts are calculated from the inelastic dynamic response of ESDOF system using a modified shape vector. The plastic deformation ranges of all inelastic excursions obtained from the rain-flow cycle counting method are utilized to estimate the cumulative low-cycle fatigue damage. The analysis results obtained using the ESDOF Model are compared to those obtained using multistory model. The reasons of the damage concentration in lower stories are investigated from the analysis results of the various frames which is designed according to the various seismic regulations.
In most cases, the results obtained using the simplified model are in good agreement with those obtained from the multistory model. The simplified analytical model proposed in this study can be used for prediction of the cumulative damage as well as the maximum deformations in the preliminary design of multistory frame structures with the advantage of simplicity and low computational costs.