Evaluation of seismic performance of torsionally-stiff and torsionally-flexible steel buildings under far-field and near-field ground motions

Near field ground motions with forward directivity and fling step effects, can impose high seismic energy to the building. In this paper, the seismic performance of symmetrical steel of 5-, 10- and 20-story buildings with dual lateral load resisting system, in both torsionally-stiff (TS) and torsionally-flexible (TF) archetype buildings has been investigated. For this purpose, the buildings are subjected to nonlinear time history analysis, which are performed with applying two horizontal components of three sets of far field (FF), non-pulse (NP) and with forward directivity pulse (FD) records. The results are presented in the form of peak floor displacement, inter story drift ratio, story shear as well as ductility demands of braces, beams and columns for three categories of earthquake records. The results show that in torsionally-stiff and torsionally-flexible buildings of 5- and 10-story under the FD records, the maximum occures in the lower stories and the first mode is predominant. Also, for the 20-story torsionally-stiff building, the maximum demands occurs under the FD records in the lower stories, which the maximum difference between FD with NP and FF earthquake records, for the story drift ratios are 115% and 79%, respectively, and for the ductility of the columns, they are 894% and 798%, respectively. Also, contribution of higher modes of FF records for 20-storey buildings are greater than the FD and NP records; therefore, the maximum demands in the upper stories occurs under the FF records.