Seismic Performance Evaluation and Retrofit of Existing Steel Frame Buildings

Existing steel frame buildings in Switzerland that have been designed from the early 1970s typically utilized a steel lateral load resisting system with practically no seis- mic design requirements. However, Switzerland is a moderate seismicity zone and the seismic safety and lateral stability of such buildings should be carefully examined. In particular, such buildings could develop undesirable collapse mechanisms because ca- pacity design principles were not employed. This Master thesis project investigates the behavior of Civil Engineering building at EPFL, an existing steel frame building consisting of concentrically braced frames (CBFs). Its connections do not comply with the current seismic design practice of capacity designed steel CBF connections. The three most critical CBF buildings were identied and a nonlinear model for each one of them was developed. A set of seven ground motions was selected in accordance with the seismic hazard on EPFL site. On the basis of nonlinear response history analy- ses results, the seismic performance of the building was throughly examined according to three target structural performance levels: Immediate Occupancy, Life Safety and Collapse Prevention. Dierently from previous studies performed on Civil Engineering building, this Master thesis pinpoints the nature of its vulnerability as the result of the combination of loading eccentricity together with the non-implementation of capacity design principles leading to the gusset plate connections failure. Rigorous nonlinear analyses highlighted the violation of the hierarchical order of yielding in elements of the braced frames: not only the connections would not be able to withstand the defor- mations and loads that come from brace yielding and buckling, but in the rst instance the braces cannot yield and buckle because of the eccentricity in the gusset plate con- nection. Those ndings implied the proposal of the optimal seismic retrot solution which was nally evaluated through a nonlinear response history analysis.