Ductility of steel connections in concentrically braced frames under seismic forces

Steel Concentrically Braced Frames (CBFs) designed for seismic forces in accordance with current Australian practice are assumed to possess a level of ductility and overstrength sufficient to justify a response reduction factor (R) of five. This assumption implies that some earthquake energy will be dissipated through inelastic behaviour, specifically the buckling of the bracing members. However, the Australian steel code AS4100 does not require that the connections in the majority of CBFs be designed for the actual strength of the bracing members. Instead, the bracing member design load is used. A summary is presented of work currently under way to test the actual strength and ductility capacity of connections in a CBF designed for conditions typical of Melbourne. The results of a preliminary Finite Element Analysis (FEA) predict that the strength of the selected critical connection is less than the theoretical buckling capacity of the brace member. The series of practical investigations outlined is expected to confirm these FEA results.