Research On Stiffness Degradation Of Semi-rigid Joints Of Prefabricated Concrete Frame Structures

Using prestressing steel bars to connect precast beam and column is a new connection method of prefabricated beam-to-column connections,which combines the advantages of the prefabricated frame and the prestressed concrete structure.In the design and analysis of prefabricated concrete frame structures,the research on beam-column joints mainly focuses on their seismic performance such as energy dissipation and ductility,while the related research on stiffness degradation of joints is rare.On the basis of experiments,the influencing factors of stiffness degradation and the degradation law are studied by combining theoretical calculation with finite element analysis,and the method of stiffness reduction of the semi-rigid joint is further proposed.The main research results are as follows:(1)The possible factors affecting the stiffness degradation of semi-rigid joints of prestressed precast concrete frames are analyzed.The parameters of prestressing,epoxy cement paste,concrete strength,axial compression ratio,beam-column linear stiffness ratio and bracket reinforcement are analyzed by finite element analysis software.The results show that the stiffness degradation of joints is mainly affected by prestressing force and concrete strength.Prestressing force affects the degradation process of joint stiffness,while concrete strength determines the final failure state.(2)According to the trend of joint stiffness degradation curve,the stiffness degradation of semi-rigid joints of prestressed precast concrete frames can be approximately divided into three stages: for the interior joint,the first stage is the first to third periods,the second stage is the forth to sixth periods,and the third stage is the seventh to ninth periods;the degradation speed of the exterior joint is slightly lower than that of interior joints,the first stage is the first to fifth periods,the second stage is the sixth to eighth periods,ninth cycle begins to be the third stage.(3)The moment-rotation skeleton curves of semi-rigid joints of prestressed precast concrete frame with different parameters are calculated according to the existing theoretical formulas and compared with the results of finite element parametric analysis.The difference between theoretical calculation and finite element analysis is studied.The results show that the degradation of joint stiffness has a great influence on the structural forces.In the theoretical calculation and analysis,the existing theoretical calculation formula of semi-rigid joints only reduces the stiffness of joints once when considering the degradation of joint stiffness.It can only meet the actual situation in a specific stage,and is only a partial safe value reflecting the overall stress state.(4)According to the three-stage degradation law of stiffness of prefabricated beam-column joints,the stiffness of prefabricated beam-column semi-rigid joints is reduced in three stages and then calculated by theoretical formulas.The calculation results show that the calculation error can be controlled within 10% when the same reduction coefficient is applied to the exterior joints and the interior joints;considering the stiffness degradation speed difference between the exterior joints and the interior joints,different reduction coefficients are used for the exterior joints and the interior joints.The calculation error can be controlled at about 5%,which fully illustrates the rationality of the stiffness reduction method.In summary,this paper focuses on the factors affecting the stiffness degradation of semi-rigid joints of prestressed precast concrete frame.The three-stage reduction method of joint stiffness proposed in this paper can provide theoretical basis for the structural stress calculation of cracked prestressed precast concrete frame beams,and provide new ideas for joint stiffness reduction in the future.