Study On The Dynamic Mechanical Properties Of Reinforced Concrete Beam-column Joints

Owing to the material rate sensitivity, reinforced concrete members exhibit generally rate-dependent behavior with varying stiffness and carrying capacity under different loading speeds. However, the majority of previous studies are focused more on the rate effect of concrete and reinforcement, but less on beam-column joints under high speed. Based on the former considerations, dynamic tests for reinforced concrete beam-column joints are carried out under the displacement-controlled loading, aiming at attaining a better understanding of the effect of loading speed on the dynamic mechanical properties of beam-column joints. The main sections of the research work are summarized as follows.(1) The depth of concrete compressive area of beam section under reciprocating loading can be obtained by the ductile design criterion of reinforced concrete structures. Adjustments for the simplified softened strut-and-tie model including modifications of the height of the diagonal compression strut, the inclination angle of the diagonal compression strut, the yield stress of steel bars embedded in concrete, as well as the softening coefficient are made, while adjustments for the general analytical model including modifications of the model coefficient and the exterior joint type coefficient are made. According to the Coulomb failure criterion, Mohr’s circle theory and mechanical model of the joint assembly, a model to calculate the shear strength of concrete is derived under the shear compression state. In addition, failure modes of beam-column joints and development of cracks of the joint core area are predicted by a binomial logistic regression model and simplified softened strut-and-tie model, respectively.(2) The feasibility of different methods to calculate the strain rate level and the rationality of different formulas to calculate the shear carrying capacity of the interior beam-column joint are compared, while the influence of loading speed and axial compression ratio on the dynamic mechanical properties of interior beam-column joints are studied. An empirical formula to predict the dynamic increase factor of horizontal shear carrying capacity of the interior beam-column joint is proposed through multiple linear regression analysis. The research results show that the angle between diagonal crack and vertical axial force in the joint core area decreases as the axial compression ratio increases. The number of cracks in the joint assembly gradually decreases as loading speed or axial compression ratio increases. Also, as loading speed or axial compression ratio increases, damage of concrete in the joint core area decreases but damage of concrete in the plastic hinge regions of beam ends increases. With the increase of loading speed, the carrying capacity of the joint assembly may be improved, and the improvement of the yield carrying capacity is more significant than that of the ultimate carrying capacity. With the increase of loading speed, the bond slip between concrete and reinforcement becomes more obvious, but the ductility of the joint assembly remains almost unchanged. Owing to the increase in loading speed or axial compression ratio, the stiffness of the joint assembly may decline sharply. And the energy dissipation of the joint assembly under a higher loading speed or axial compression ratio is larger than that under a lower loading speed or axial compression ratio. It is found to be unsafe to substitute the dynamic strengths of concrete and reinforcement directly into the quasi-static design formulas to calculate the shear carrying capacity of the interior beam-column joint, because the shear carrying capacity of the interior beam-column joint is overestimated. The similar influence tendency of loading speed and axial compression ratio on the mechanical properties of exterior beam-column joints may be found.(3) The effect of strain rate on reinforced concrete beam-column joints can be effectively simulated by means of the finite element analysis software ABAQUS under high loading speed, when the plasticity damage model for concrete is adopted. The load-deflection skeleton curves and shear carrying capacity of the joint assembly obtained by the finite element analysis are in good agreement with the test results.In addition, the stirrup reinforcement ratio has an effect on the stress distribution and damage degree of concrete in reinforced concrete beam-column joints. With the increase of stirrup spacing in the joint core area, the yield carrying capacity and ultimate carrying capacity of the joint assembly may reduce, and the reduction of the ultimate carrying capacity is more significant than that of the yield carrying capacity.