| Reinforced concrete (RC) frame joint is the hinge of frame structures, the frame joint may suffer various kinds of load during a seismic event. The seismic behavior of the joint is an important factor to the strength, rigidity and ductility of a frame structure under severe earthquake. The range of strain rates of reinforced concrete structures under the earthquake action is generally recognized as10-4-10-1. Because both concrete and steel reinforcement are rate-sensitive materials, the RC frame joints behave differently at different strain rates. To study the seismic behavior of the frame joint, the work and contributions in this paper are listed as follows.(1) The general analytical model and simplified strut-and-tie model are summarized to analyze the shear capacity and load-passing mechanism of the joint under earthquake. Through the consideration of the confining effect of the height of compression zone of beams to the joint when subjected to cyclic loading, the improved simplified strut-and-tie model is suggested. The precision of the improved simplified strut-and-tie model’s predicted results is proved in terms of test data, conversation is also proved.(2) Experimental studies of RC frame joints are presented in this paper. The research importance is to investigate the effect of strain rates on the bearing capacity and failure pattern of the joints under different loading rates and different axial compression ratio. It is observed that with the increase of loading speed, the ultimate bearing capacity of the frame joints increases as well, however the bearing capacity after the peak point at higher loading speed drops more rapidly; the frame joints lose bearing capacity at low ductility; the extent of damage of the frame joints becomes greater and the cracking distribution becomes more concentrated. It is also observed that the bond deterioration of beam bars in the joint core under high loading speed is better, while the energy absorbed in the hysteresis loops is larger.(3) With the increase of loading speed, the strain rate of the joint core increases, the magnitude of strain rates of the joints is estimated as10-5,10-4, and10-2respectively corresponding to the loading speeds of0.4mm/s,4mm/s and40mm/s, prevision of the predicted method for the strain rate of RC frame joints is also proved. The difference in axial compression ratio has little effect on the shear bearing capacity of the frame joints. However, the failure patterns of the frame joints at different axial compression ratio distinguish a lot, in the range of axial compression ratio in the experiments presented in this paper, the increase of axial compression ratio can decrease the width of cracks in the joint core, and have benefits on the seismic behavior of the frame joints.(4) The ABAQUS concrete damage plasticity model is used to simulate the seismic behavior of RC frame joints under different loading speed. Higher stress and more damage in the joint core are observed under higher loading speed; through the comparison between simulated results and test results, it can be observed that the envelope curves of simulated load-deflection curves under different loading speed are very close to the curves of test results, they have almost the same bearing capacity. It can also be observed that the simulated bearing capacity of RC frame joints decreases fast after peak point, which is also observed in the test results, so the damage plasticity model can availably analyze the strain rate effect of RC frame joints.(5) Through the comparison among calculated results by the method in standard, calculated results by the improved models and test results, it can be observed that all calculated results are close to quasi-static test results, lower than the high rate results. However, when the dynamic strengths of material, i.e. concrete and reinforcement, were directly substituted into the design model of current standard and improved models, the calculated results are higher than the test results, which tends to insufficiently safe. |
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