Numerical investigation into I-shape brace connections of conventional concentrically braced frames

In many low and moderate seismic regions, a low-ductility concentrically braced frame (CBF) is used as the seismic force resisting system for steel structures. The design of such CBFs is straightforward: all members and connections are designed based on the seismic force demand obtained through linear elastic structural analysis; capacity-based design and additional seismic detailing are not required. There is no designated energy-dissipating fuse in the lateral load-carrying path. Such CBFs are referred to as Conventional CBFs (CCBFs) in this study. In CCBFs, the brace-to-gusset connections are inherently weaker in tension than the adjoining gusset plates and braces. This occurs because both the gusset plates and the braces are selected on the basis of their respective compressive buckling resistances, and hence, typically have a much greater resistance in tension. Described herein is a numerical study of the popular flange plate bolted I-shape brace connection configuration. A high-fidelity finite element (FE) simulation procedure was developed and validated against laboratory test results. The resulting numerical models were created with the objective of improving our understanding of the inelastic response of these brace connections. The force transfer mechanism within the two branches of the connection was characterized. Significant nonuniform shear distribution was found to exist within the bolt group in the flange branch, which may be detrimental to the safe functioning of these bolts in the seismic design context. The loading eccentricity on the weld group was quantified. Recommendations on how to avoid brittle bolt shear rupture and premature weld fracture are proposed.