NUMERICAL INVESTIGATION INTO I-SHAPE BRACE CONNECTIONS OF CONVENTIONAL CONCENTRICALLY BRACED FRAMES

Steel concentrically braced frames (CBFs) are effective seismic force resisting systems which are widely used in North America. In high seismic regions, capacity design principles and numerous additional seismic detailing provisions are explicitly required in design to confine the inelastic behaviour to the bracing members, i.e. yielding in tension and buckling in compression. However, in areas of moderate or low seismicity, the design of CBFs may be based on a conventional approach, in which the principal requirement is for the factored resistance to be equal to or greater than the factored load effect obtained from linear structural analysis; capacity protection is not required. The conventional design approach is predominantly used because of its simplicity in terms of design and fabrication, and therefore economy. Such CBFs are referred to as Conventional CBFs (CCBFs) in this paper. The type ‘Conventional Construction’ (CC) CBFs in the National Building Code of Canada (NBCC) and CBFs ‘not specifically detailed for seismic resistance’ in accordance with the American Society of Civil Engineers (ASCE) 7 fall into the CCBF category. Along the lateral load path in CCBFs, the brace connection is usually the weakest link and may be prone to fracture when subjected to tension. In Canada, due to the lack of data to characterize the brace connection’s inelastic seismic performance, the seismic design load for the connections of type CC CBFs is required to be amplified by 1.5 unless ductile behaviour can be guaranteed. However, no explicit demand, nor any design provisions regarding how to obtain ductile behaviour, are readily available since little research has been done on this issue. Hence, an extensive experimental and numerical research project regarding the I-shape brace connection design in CCBFs has been launched at Polytechnique Montréal and McGill University. In this paper, a numerical simulation procedure was used to investigate the behaviour of a typical I-shape brace connection configuration, i.e. the flange plate connection. The accuracy of the numerical models was validated through comparison between the simulation results and the laboratory test results. Two force transfer branches (the flange branch and the web branch) within the brace-to-gusset connection were identified. The numerical analysis reveals that the flange branch will develop its capacity prior to the web branch since it needs less bolt slippage to achieve the bearing condition. Regarding the force partition between the two branches, a parametric study was conducted with variation of the flange lap plate thickness and the web lap plate thickness. The results indicate that the force sharing at the ultimate limit state is determined by the ultimate strength of each branch. To avoid the low-ductility bolt shear rupture and weld fracture, the bolts and welds are recommended to be designed based on the ultimate strength of each branch. Another welded web lap plate attachment was also studied, but it did not have an impact on the ultimate strength and force partition within the connection as long as it does not change the failure mode in the web branch.