Seismic Hazard Evaluation of Torsional Effects in Steel Concentrically Braced Framed Buildings without Any Plan Irregularity

Normally for seismic analysis of buildings without geometric irregularities, torsional effects are not the first to be considered. However, steel braced frames are special since there are uncertainties in brace fracture that may result in torsional irregularities in the inelastic range. During seismic activities, premature fracture of bracing members would lead to an unsymmetrical distribution of lateral stiffness, causing the floors to rotate along the height. In this study, the plan-regular concentrically braced frame (CBF) buildings are modeled to investigate how the additional torsional demands can affect seismic performance. A geometrically symmetrical CBF building is modeled, and a simplified two-dimensional (2D) model is built for comparison; one of the two parallel CBFs is assigned to have less brace ductility capacity to expect premature fracture. By comparing the seismic performance of the three-dimensional and 2D models, it is found that the premature fracture of bracing members becomes the first ring of the chain reaction that initiates the rotation of floors, leading to the early introduction of soft-story mechanism and eventual formation of plastic hinges in columns as the sign of stability loss in CBF buildings. Through studying the seismic response of the CBFs subjected to 18 demonstrative strong ground motions, it is observed that the magnitude of floor rotation is positively correlated with the story drift response, and the amplification of story drift ratio can be as high as more than 100%. The major conclusion for this study is that for CBF buildings without any plan regularities, using a simplified 2D model may underestimate the seismic performance and the torsional effects introduced from premature brace fracture should be considered by the design practices.