Evaluating the code approaches for estimating the seismic drifts of steel frame buildings designed under variable levels of seismicity

Two different approaches are considered by seismic codes for estimating the seismic drifts of moment resisting steel frames (MRSFs). The first approach which is adopted by the European and the Canadian codes is driven by the equal displacement rule and defines the deflection amplification factor (DAF) to be equal to the response modification factor R. The second approach which is motivated by the opinion of professionals who believe that the equal displacement rule often over-estimates displacements is employed by the ASCE 7-10 and the Egyptian seismic code, where the DAF is considered equal to a fraction of R. This study evaluates the two approaches considered for estimating the seismic drifts of MRSFs. Four MRSF buildings having 2-, 4-, 8- and 12-stories have been designed under variable levels of seismic intensity. Five lateral-stiffness scenarios are considered for each building to account for the stiffness increase due to the nonstructural components as well as the modeling approximations. The seismic drifts of the MRSFs are calculated under the effect of twenty-two pairs of far-field ground motion records by time history analysis and with using the code simplified approaches. The results obtained indicate that the current code approaches of amplifying the elastic drifts significantly overestimate the seismic drifts of buildings designed under low level of seismicity. In contrast, the ASCE 7-10 and the Egyptian code approaches of estimating the seismic drifts underestimate the maximum story drift ratios under earthquake loading for long period buildings designed under high level of seismicity.