| The beam-column joints are the most vulnerable part of the reinforced concrete framed structure under earthquake loading because the shear forces subjected by beam-column joints is several times than shear forces subjected by beam and column.The addition of steel fibers in beam-column joints is an effective method for enhancing the seismic performance of the joints as well as reducing the quantity of stirrups and constructional difficulties and cost. Calculation methods of the shear performace, restoring force behavior and damage characteristic for steel fiber reinforced high–strength concrete(SFRHC) beam-column joints were studied based on the experiments of two high-strength concrete(HC) beam-column joints, one steel fiber reinforced concrete(SFRC) beam-column joints and ten SFRHC beam-column joints under cyclic loading. The relevant conclusions in this thesis can provide test data and theory basis for compiling industrial standard “Technical specification for steel fiber reinforced concrete structures”. The main research in this thesis as follows:(1) Crack development and distribution, failure characteristic, strains of the reinforced bars and load-deformation hysteresis curves were measured through the experiments of the beam-column joints under cyclic loading which loading point was located at the beam tip. Effects of concrete strength, axial compression ratio of column top, stirrup ratio in the joint core region, volume fraction of steel fibers and the additional length of steel fibers along the beam beyond joint core region on seismic performance of SFRHC beam-column joints were discussed. The results show that adding steel fibers in joints can improve joint seismic performance. With the increasing of the steel fiber volume fraction, the load-deformation hysteresis curve is plumper, the ductility and energy dissipation capacity are enhanced, the deterioration of stiffness and bearing capacity is slower.(2) Based on the analysis of related experimental data collected from home and abroad papers, the effects of axial compression ratio of column top, stirrup ratio injoint core region, volume fraction of steel fibers and concrete strength on shear bearing capacity of SFRHC beam-column joints were discussed. The results show that the shear capacity is increase with the increase of the steel fiber volume ratio,stirrup ratio in joint core region, concrete strength and axial compression ratio of column top. Through the experimental study and theory analysis, the calculation method for shear capacity of SFRHC beam-column joints considering the all effect factors mentioned above was put forward. This shear capacity calculation method also can be used for reinforced concrete beam-column joints.(3) The force bearing mechanism of SFHSC beam-column joints is studied, the results indicate the shear mechanism of SFHSC beam-column joints is comprehensive action of diagonal strut mechanism and struss mechanism. Through the experimental research and theoretical analysis, the relative calculation methods for shear bearing capacity of SFHSC beam-column joints were proposed based on softened strut and tie model, modified compression field theory and octahedral strength model of concrete.(4) The effects of stirrup ratio in the joint core region, axial compression ratio of column top, volume fraction of steel fibers on restoring force behavior of SFRHC beam-column joints were discussed. The results indicate that using SFRHC in the beam-column joint can significantly increase its seismic behavior. With the increasing of the steel fiber volume fraction and stirrup ratio in the joint core region and axial compression ratio of column top, the deformation performance and energy dissipation capacity of the joints are improved. The axial compression ratio of column top, characteristic value of steel fiber and stirrup ratio in the joint core region have great influence on the characteristic points of skeleton curve, the degradation of unloading stiffness and bearing capacity. Based on the results of the study, a restoring force model including stiffness degradation was establishe which expressed by tri-linear toward fixed points and could reflect the restoring force characteristics of SFHSC beam-column joints.(5) The calculating model for damage property of SFHSC beam-column joints under cyclic load was established combining the indexes of the deformation and cumulated energy dissipation. The major affective factors on damage property ofSFHSC beam-column joints were analyzed. The results show that the steel fiber volume fraction, stirrup ratio in the joint core region and axial compression ratio of column top are the major affective factors on damage property of SFHSC beam-column joints. Under the same times of cyclic loading, the damage of the beam-column joints presents decreasing trend as the steel fiber volume fraction,stirrup ratio and axial compression ratio of column top increase. The proposed model can reasonably represent the damage evolution characteristics of beam-column joints,which can be used to describe the damage evolution process of these joints. |
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