Research On Calculation Method Of Bearing Capacity Of Exposed Rigid Joint

The steel structure storey adding transformation of existing buildings can guide the transformation of urban and rural construction to green and low-carbon,optimize the building energy consumption structure,and meet the overall goal of "double carbon" and the requirements of sustainable development in China.However,there is a sudden change in the upper and lower mass and stiffness of the storey added hybrid structure,which is easy to form the whiplash effect.Therefore,the storey added hybrid joint connecting the upper and lower structures is particularly important.Taking the exposed rigid joint,which is most widely used in hybrid joints,as the research object,carry out the research on the influence of the change of joint parameters on the mechanical performance of the specimen,and clarify the influence law of the bearing capacity of the joint,and considering the structural performance degradation caused by the service time of the lower existing building,and the bearing capacity calculation method and restoring force model of joints are deeply studied.The specific research contents and achievements include;(1)ABAQUS finite element software is used to establish the numerical calculation model of exposed rigid joint.The calculation results are verified and compared with the experiments data in literature[70].It shows that the finite element modeling method is correct’and feasible,with sufficient accuracy,and can be used for subsequent research.(2)Study on the influence of relevant parameters on the mechanical performance of exposed rigid joint.Change the axial compression ratio,beam longitudinal reinforcement ratio,beam stirrup ratio,concrete strength grade,beam column linear stiffness ratio and building service time,establish multiple groups of variable parameter analysis models,and deeply explore the influence of parameter changes on the mechanical performance of joints.According to relevant research results,it is suggested that the value of the axial pressure ratio should be controlled at 0.16～0.32,which can not only ensure the reliable connection of the upper and lower structures,but also play a good role in improving the ultimate bearing capacity of the joints;the value range of longitudinal reinforcement ratio of beam is 0.79%～1.40%,which can give full play to the flexural performance of concrete beam,meanwhile,the ultimate load of joints has also been significantly improved;the beam stirrup ratio has little influence on the bearing capacity of this joint,so it can meet the structural requirements of the code;when the concrete strength grade is in the range of C30～C45,the ultimate bearing capacity and failure load of the joint are improved,while maintaining good plastic deformation capacity;combine this parameter to improve the contribution to the bearing capacity,it is suggested that the value of beam column linear stiffness ratio should not be greater than 0.570;degradation of material properties due to building service time is manifested in the continuous development of concrete carbonation and steel corrosion.Comprehensively considering the research results,when the exposed rigid joint is used for the storey adding transformation of the steel structure,it is suggested that the impact of the performance degradation of structural materials should be considered when evaluating the overall performance of the existing buildings that have been in service for more than 32 years.(3)Research on the calculation method of flexural capacity and shear capacity of joints.Select the five parameters that have a great impact on the joint bearing capacity in the variable parameter research and analysis:axial compression ratio,beam longitudinal reinforcement ratio,concrete strength,beam column linear stiffness ratio and service time of existing buildings.The integration research and analysis of finite element numerical simulation is carried out,and the influence law of various parameters on the joint bearing capacity is summarized.Through the regression fitting method,the relationship between each parameter and the flexural capacity and shear capacity of the joint is obtained,and the multi parameter calculation formula is integrated.The fitting correlation between the curve fitting and the finite element calculation results is good,indicating that the established calculation formula for exposed rigid joints has good applicability and accuracy,which can provide relevant reference for subsequent research.(4)A joint restoring force model based on building service time is established.According to the relevant data of finite element numerical analysis,under the premise of certain loading and unloading rules,the dimensionless skeleton curve model and node stiffness degradation law are constructed by numerical fitting method,and the three fold line restoring force model of exposed rigid joint considering the service time of buildings is established.