Nonlinear Static and Dynamic Analysis of Irregular Steel Frames: A Case Study on Performance Levels

Abstract

This study evaluates the seismic performance of irregular steel structures in plan using linear and nonlinear analyses. The main objective was to compare the performance of Special Moment Frame (SMF) and Special Dual System (SDS) under various earthquake records and assess compliance with seismic performance levels (IO, LS, and CP). Models of 3, 5, 7, and 10 stories were analyzed using ETABS and SAP2000 software. Results indicated that Nonlinear Static Analysis (Pushover) was insufficiently accurate in predicting inter-story drifts in some cases, and only the 3 and 5-story dual system models achieved the IO level. In contrast, Nonlinear Dynamic Analysis (Time History) showed that all models were at the LS level but did not reach IO. Additionally, calculated seismic coefficients (such as behavior factor and ductility) differed significantly from the values recommended by Standard 2800, indicating a need for revising design codes. Examination of plastic hinges revealed that in the moment frame system, beams exceeded the IO limit, while in the dual system, the main vulnerability was concentrated in braces and some columns. These findings confirm the necessity of modifying design provisions for beams in moment frames and braces in dual systems to improve seismic performance. The results highlight the need for revising design codes and seismic rehabilitation guidelines to design structures with better performance and higher safety against earthquakes.