Experimental and numerical investigations on square CFST column to U-shaped steel-concrete composite beam joints with internal T-shaped diaphragms

An innovative joint detail with internal T-shaped diaphragms for concrete-filled steel tubular (CFST) columns to U-shaped steel-concrete composite (USCC) beams is proposed in this study. Five beam-column joint specimens are tested under cyclic loading to evaluate the seismic per-formance. The test parameters are the connection form between top reinforcing bars (re-bars) and steel tubes, the total cross-section area of top re-bars, and the strengthening method for the joint. Test results show that welding the U-shaped steel directly to the steel tube may lead to premature fracture of bottom flanges in the connection vicinity; the H-steel corbel and the strengthening plate help delay the fracture of bottom flanges, and increase the joint's positive bending capacity effectively. The joint shows superior negative bending capacity, and the plastic stress distribution assumption can be utilized to calculate the joint's negative bending capacity with reasonable accuracy. While the joint shows lower positive bending capacity when premature fracture of bottom flanges occurs, thus the elastic stress distribution assumption is suggested for calculating the joint's positive bending capacity. Additionally, a finite element (FE) model is developed and verified. The effects of axial load ratio, the total cross-section area of top re-bars, the width to thickness ratio of steel tubes, and the thickness of the strengthening plate on the joint's bending capacity are investigated numerically. It is found that the joint's bending capacity decreases with the increase of axial load ratio and width to thickness ratio of steel tubes. The strengthening plates increase the positive bending capacity, and the top re-bars enhance the negative one.