Seismic performance assessment of steel moment frames with non-parallel system irregularity

Many structures exhibit non-orthogonal systems irregularity based on architectural design, which is a type of torsional irregularity. This paper evaluates the inelastic response of multi-story steel moment resisting frames with this type of irregularity. A parametric study is carried out on six building models exhibiting coupled behavior in lateral and torsional response with various degrees of torsional irregularity. Current code regularity limits for structures appear to be based on engineering judgment rather than on quantitative analyses, which indicates that these limits need to be investigated for different structural systems with different types of irregularities. Another goal of this paper is the evaluation and comparison of the response modification factor values of steel moment resisting frames with non-parallel systems irregularity derived by pushover analysis, as well as nonlinear time history analysis. A new torsional irregularity coefficient is proposed based on the response spectrum analysis results. It is shown that it is essential to undertake nonlinear dynamic analysis to design some structures with high irregularity in plan and to capture nonlinear mechanisms due to non-parallel systems irregularity.