Tube-in-tube rigid beam to CFT column connection in moment-resisting frames: An experimental study

Using concrete-filled steel tube (CFT) columns in high-rise buildings has attracted considerable attention in recent years. Aside from its advantages, rigid beam to CFT column connections are followed by some challenges. Difficult construction and implementation, sophisticated details of connection, and most importantly, placing and welding internal stiffeners (continuity plates) are among the common challenges. In the present study, a novel connection called "Tube-in-Tube (TIT) connection" has been proposed. In TIT connection, an inner tube and a pair of vertical straps are utilized in the panel zone area. To investigate the proposed connection, two fullscale specimens were tested using cyclic reversal loading. The two specimens were identical in dimensions and detailing, except for their strap width and the diameter of inner tube. Specimen 1 and Specimen 2 were detailed with inner tube diameter equal to half and one-third of the column dimension, respectively. The inter-story drift ratio, joint's shear deformation, plastic hinge formation, and energy dissipation capacity were analyzed. Moreover, mechanical parameters such as yield and ultimate strengths, and ductility of the specimens were calculated. Specimen 2 showed 3% and 8% increase in the peak flexural strength and cumulative energy dissipation in comparison with Specimen 1. The results showed that the panel zone remain elastic during the test. In order to further investigate the proposed connection, finite element (FE) analysis was conducted on the specimen. The results of FE analysis confirmed the elastic behavior of the tube and straps.