Study on Shear Resistance Mechanisms of RC Beam-Column Joints and Development of Seismic Design Method

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The aseismatic provision for reinforced concrete beam-column joints has been made recently in ACI Building Code and New Zealand Code. The development of the rational design method for the beam-column joint has become a matter of concern also in Japan. In the ACI and New Zealand codes, the provisions for the development length of beam bars passing through a joint and the limitation to the joint shear stress are very important, but they are not based on the sufficient test data and the rational theory. In this study, the interrelation between the bond situation of beam bars and the joint shear stress level were discussed for the experimental results.Eight half-scale interior beam-column joints were tested. Small-size low-strength bars were used in the improved bond type, and large-size high-strength bars were used in the bond deterioration type. The joint shear stress level was varied in the region of the restricted value in ACT Code.In the first test series with a small amount of lateral shear reinforcement in a joint, the lateral reinforcing bars yielded and the opening of joint shear cracks was remarkable. The joint shear failure occurred after beam yielding. The story shear-story drift hysteresis loop showed the pinching effect independently of the bond situation of beam bars. In the second test series in which a joint was reinforced so that half of the entire shear could be resisted by the lateral reinforcement, the opening of the joint shear cracks was restrained and the joint shear failure was prevented effectively. The hysteresis loop was fat spindle-shaped in the improved bond type, but the bond deterioration type specimens showed a pinching effect.From these experimental results, the effect of the bond situation of beam bars on the shear resistance mechanisms was investigated. Finally the limitation to the joint shear stress and the restriction of beam bar bond index were presented.