SummaryNonlinear time history analysis (NTHA) is widely considered to be the most accurate and robust method of seismic analysis and, when performed with force analogy method (FAM), can result in significant reduction in computational time, as compared to the conventional displacement‐based finite element (FE) solution technique. Taking this advantage of efficient FAM, this paper investigates the effects of location (along building height) and intensity (150–225%) of single‐floor mass irregularity on seismic response of mid‐rise moment‐resisting steel frames. Three building heights (6, 8, and 10 storeys) have been considered, and the analysis results are presented in the form of floor displacements, storey drift ratio, and plastic energy dissipation. Based on the analysis, it has been observed that critical location (for the maximum seismic response) of mass‐irregular floor corresponds to the level that exhibited the maximum seismic response (i.e., storey drift ratio) in the reference mass‐regular frame. Further, criticality of the irregular frame to seismic excitation has been found to decrease when mass‐irregular floor moves towards the top of the building. The criticality of the irregular frame has been found to increase with the increase in mass‐irregular intensity. Collapse of the irregular frame has been seen to initial first near the storey level where mass‐irregular floor is located.

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