Probabilistic seismic demand assessment of steel moment-resisting frames with mass irregularity in height

Ductility with direct effect on the response modification factor of buildings can influence their seismic performance. Moreover, some factors such as geometry and different types of irregularity can affect the ductility and seismic performance of structures. In this study, the effects of mass irregularity in height on the over strength, ductility, response modification factors and probabilistic seismic performance in steel Moment Resisting Frames (MRFs) are assessed. Then, the obtained results are compared with those of regular structures. For this purpose, incremental dynamic analysis (IDA) is implemented using ten records out of past worldwide earthquakes. The location of mass concentration in height is studied assessing 8-, 12- and 16-storey buildings with their mass concentrated at the first floor, mid-height and roof. Then, the probabilistic seismic responses of these structures are evaluated using the outputs of IDA. In this regard, probabilistic seismic demand analysis is conducted on each model. The obtained results are used to plot the seismic fragility and demand curves for both regular and irregular models. Based on the findings, mass irregularity causes the reduction of ductility and response modification factors. This effect increases when the heavier storey is located either at the first floor or at roof. Moreover, mass irregularity brings about the increase in probability of damage occurrence or its exceedance from a certain level.