Seismic response of mid-rise steel MRFs: the role of geometrical irregularity, frequency components of near-fault records, and soil-structure interaction

Abstract

Near-fault (NF) ground motions may induce higher demands on structures compared to far-fault records. This is often due to the velocity pulses, called forward-directivity pulses, which are commonly observed in the fault-normal ground motion components. Hence, some researchers accept such pulses as the representative of pulse-like motions. This paper is devoted to studying the nature of NF events by evaluating their impact on the structural response. To this end, a group of ten-story steel moment-resisting frames with regular and setback configuration were subjected to NF pulse-like records. The records were decomposed to distinct frequency components, including the extracted pulses (low-frequency components) and the residual (high-frequency) parts. The effect of TP/T1 (the ratio of pulse period to the first-mode structural period) was also evaluated by classifying the earthquake records into four groups, indicating TP/T1≈0.5, 1, 2, and > 2. Furthermore, soil-structure interaction (SSI) was considered through the Beam on the Nonlinear Winkler Foundation theory. Incremental dynamic analyses were conducted under the action of the original ground motions, extracted pulses and residual parts. The results deeply question the possibility of using directivity pulses as the representative of original excitation in dynamic time history analyses, especially for setback structures. The residual component plays an important role in seismic response of such structures in most cases of TP/T1. The considerable effect of this frequency component is also observed in regular structures when TP/T1≈1. Meanwhile, SSI can increase structural drift demands, especially in setback structures, when subjected to NF pulse-like records.