The Effect of Non-parallel System Irregularity on Seismic Performance of Braced Steel Frame Buildings

A number of buildings have irregular non-parallel systems due to ground conditions and architectural design, which fall into the category of irregularities in the plan. However, the building codes do not specify a limit as the deviation value of the vertical lateral load bearing components from the major orthogonal axes of the building for this type of irregularities. The building frame system with bracing, on the other hand, is one of the most common systems for low- to mid-rise structures. In this research, the effect of non-parallel system irregularity on the seismic performance of steel building frame with special convergent bracing was investigated. Steel structures with 3, 6, and 9 stories were selected in this study, and in addition to the regular structural models, irregularity of non-parallel systems with different angles to the orthogonal axes from 1 to 5 degrees (i.e., 0.5 m to 2 m deviations for span length) were considered. First, the structures were designed according to the Iranian Earthquake Design Code and the Iranian Steel Structure Design Code. In order to evaluate the effect of irregularity on the seismic performance of the studied buildings, nonlinear static and dynamic analyses of time history for selected models were performed using Perform 3D software and the seismic performance indices of the structures were examined. The results indicate that with increasing irregularity; stiffness, strength, ductility and energy dissipation capacity of the structure decreases while the relative inter-story drift increases