Finite element simulation of Special Concentrically Braced Frame Tests

Special Concentrically Braced Steel Frames (SCBFs) are a desirable structural system for seismic design. They are stiff and strong which reduces damage during small, frequent events, and they can provide good inelastic performance during severe, infrequent earthquakes if the brace and connections are properly designed. Braces for SCBFs place severe force and deformation demands on the brace-beam-column connections. Gusset plate connections are used for this application in the US and many other countries. However, the design of gusset plates is difficult, because of the many potential failure modes of complex demands on the connection. An analytical and experimental study into the seismic performance of SCBF gusset plate connections is in progress. To date, 11 large-scale experiments have been completed. The experiments are single story, single bay braced frames which were subjected to cyclic loading with boundary conditions intended to simulate multi-story frame behavior. The specimens are approximately full size for a lower story of 3 to 4 story building. The experimental results are being combined with nonlinear analyses. The current methods in the US are based upon variations of the AISC Uniform Force Method. Experiments show that improved seismic performance of SCBFs may be achieved by permitting greater flexibility and inelastic deformation in the gusset plate and by designing the gusset plate welds based upon the plastic capacity of the gusset plate. To achieve the former, a new clearance requirement for out-of-plane brace rotational capacity is proposed. A series of inelastic finite element analyses, which were performed in support of the experimental research program, are summarized. Validations of the finite element model through comparison of the predicted and experimentally observed behavior are noted.