Research On Design Method Of High Strength Concrete Spatial Joints

To study the seismic design of reinforced concrete spatial joints constructed using high strength materials,the softened strut-and-tie model were used.In this paper,a modified softened strut-and-tie model was put forward by improving the angle of diagonal strut and taking the influence of unloaded transverse beam and cast-in-place floor.The shear strength of existing tested 77 reinforced concrete frame joints were calculated by using this modified model,and the concrete work is as follows:(1)A more reasonable shear model was obtained for that the calculation method of diagonal strut angle at different axial compression ratios was improved.The shear strength of existing tested 48 reinforced concrete planar frame joints were calculated by using the modified softened strut-and-tie model,so that the rationality of this modified model was verified.And the accuracy of calculation results by using the modified model and the original model were analyzed.(2)The influence of unloaded transverse beam and cast-in-place floor were considered to obtain a more reasonable out-in-place joints model,which based on the modified softened strut-and-tie planar joints model.The shear strength of existing tested 29 reinforced concrete out-in-place frame joints were calculated by using this modified model,so that the rationality of the modified softened strut-and-tie model was verified.(3)The modified softened strut-and-tie model proposed in this paper was used to analyze the shear behavior of 77 frame joints.The effects of compressive strength of concrete,axial compression ratio and hoop characteristic values in the core area on the seismic shear capacity of the frame joints were discussed.(4)The seismic design methods of high-strength concrete spatial joints with concrete grades C50 ~ C80 were studied by using the design codes of concrete structures of China,USA and New Zealand,including the calculation formula of shear capacity of joints,the control conditions of maximum shear-compression ratio of joints,the restriction conditions of minimum stirrup amount of joints and the bonding and anchoring conditions of longitudinal reinforcement of beams and columns through joints.The methods and advantages and disadvantages of establishing seismic design control conditions of joints in different codes were discussed.It was found that the shear forces calculated by the modified softened strut-and-tie modelare in good agreement with the experimental results,which is in good agreement with the calculation results of the current code method.When the axial compression ratio is greater than 0.2,the modified softened strut-and-tie model can predict the joint shear strength more accurately than the softened strut-and-tie model.When the axial compression ratio is greater than 0.4,the calculated shear forces of the softened strut-and-tie model is obviously larger,while the calculated shear forces of the modified softened strut-and-tie model is in good agreement with the experimental data.The modified softened strut-and-tie model can better reflect the influence of concrete compressive strength,axial compression ratio and hoop characteristic value on the shear capacity of the joint.