Study on seismic performance and restoring force model of framed CFST column-composite box beam joints

Quasi-static tests are conducted on framed concrete-filled steel tubular (CFST) column-composite box beam joints to study the deformation characteristics and restoring force performance under seismic loads. The joints' seismic performance is evaluated. This study shows that the joints with three different connection forms (single-limbed diagonal brace, cross brace, and diaphragm) in the core area have good bearing and energy dissipation capacities. As a result, the joints exhibit excellent seismic performance. The theoretical and fitting analyses of test results reveal the stiffness degradation law for this type of joint. Additionally, a fixed-point directional degradation trilinear restoring force model is established and compared with the test results. The research results indicate that the proposed restoring force model accurately predicts the nonlinear recovery behavior of the joint under seismic loads. This model is in good agreement with the hysteretic curve of the test, providing a theoretical basis for the seismic design and performance evaluation of the joint and for the seismic design and toughness optimization of a structure.