Seismic behavior of circular steel tube confined reinforced concrete column-steel beam frames considering the joint effect

Reliable beam-column joints and rational design methods are crucial to ensure the safety and economy of frames, particularly for steel-concrete composite structures. This study numerically investigates the seismic behavior of the frame consisting of circular steel tube confined reinforced concrete (CSTCRC) columns and steel beams, with particular emphasis on the performance of an improved type of CSTCRC joint within the structure. To consider joint effects in the frame analysis, methods for characterizing and simulating the CSTCRC joint were proposed first, followed by the parametric analysis. Then, based on the joint model, sixteen frames were designed and assessed through time-history and incremental dynamic analyses, considering the parameters of joint modeling methods, seismic grades, and column types. The study results indicate that the genetic algorithm can efficiently and objectively determine the hysteretic parameters of joint. Except for the frames with seismic grade III, the joints within a CSTCRC frame with seismic grades I and II remain elastic, even at the collapse state. The nonlinear joint deformation does not compromise the collapse resistance of frames; therefore, an elastic joint model (EJ) is recommended for simplified modeling and a calculation method for joint shear stiffness is proposed. Compared with RC frames, the relatively large elastic inter-story drift limit of CSTCRC frames allows reduced member dimensions, leveraging the high strength and ductility of CSTCRC columns to improve the overall structural economy. Particularly, in the 8° seismic zone, CSTCRC frames achieve an average reduction of 24.8% in material cost and an 80.2% increase in collapse resistance relative to RC frames. Lastly, the effects of key affecting parameters are elaborated and design suggestions for both joints and frames are proposed.