Shear Demands of Steel Moment-Resisting Frames Under Near-and Far-Fault Seismic Excitations

Strength capacity checking and instability control of building structures require the prediction of story shear demands and other actions produced by seismic excitations. Impulsive feature and large vertical accelerations associated with near-fault ground motions may lead to instability of the structural systems with severe damaging outcomes. The seismic demands depend on structural and ground motion characteristics. In this paper, the effect of ground motion impulsive characteristics on the story shear demands of steel moment frames is investigated for different hazard levels. For this purpose, incremental dynamic analysis is conducted on five steel frames with 3-15 stories subjected to different types of ground motions. Moreover, the accuracy of conventional pushover and static linear procedures is examined and some modification factors are suggested for each analytical approach. Finally, the effect of vertical component of near-fault records is investigated for two case studies. The results of the study demonstrate that story shear demands obtained from static procedures must be amplified for stories located at the upper one-third of the structure by a modification factor of up to 2.5 to find more precise shear demands, depending on parameters such as structure height, story level, analysis method and ground motion type. Among pushover cases, the first-mode load pattern gives more reliable results compared to other load patterns. Also, it was found that the application of vertical component of long-period pulse-like accelerograms increases the column axial forces by up to 100%.