Evolution of the yield penetration and bond stress in RC exterior beam-column joints under cyclic loading

Beam-column joints are key connections in reinforced concrete frame structures. Under severe earthquakes, the yield of the beam reinforcements and bond deterioration cause changes in the transfer mechanism in the joint core and affect the deformation and bearing capacity of the beam-column joints. The bond characteristic of the beam rebar in the pre-yield and post-yield states were the focus of experiments on five exterior beam-column joints; the joint aspect ratio and volume-stirrup ratio were the key control factors. The strain of the beam reinforcement was measured by electronic strain gauges located in the horizontal segment and the 90-degree hook. The failure mode of joints were discussed, the movement of the yield penetration of the beam reinforcements from the beams to the joint cores was clarified, and the bond stress and rebar slip in the anchorage length were assessed. It was found that the joint aspect ratio is an important factor to the joint failure mode, plastic hinge development at the beam ends and the contribution of rebar slip to overall story drift. When the joint volume-stirrup ratio at a certain range (0.4%~0.63%), it does not have much influence on the final failure mode of the joint, but it has a certain influence on the plastic development and bond performance of rebars in and out of the joint. The joints with a low volume-stirrup ratio exhibited significant bond degradation caused by the Poisson's ratio and rebar slippage, especially after the rebar had yielded. The hysteretic performance of the five beam-column joints was simulated accurately with the consideration of the rebar slip in the joint core.