Nonlinear Finite Element Model Updating For RC Frame Using Local Monitoring Data

Nonlinear Finite Element Analysis is an important method to study the seismic response of structures.However,the finite element models include many uncertainties.Most of the existing structure model updating methods are based on linear elastic parameters of structures and materials,including stiffness,frequency,mode and frequency response function,making it difficult to reflect the nonlinear properties under seismic damage.Piezoelectric based smart aggregate(SA)can be used to monitor the stress behavior in the whole process of concrete damage under seismic loads.In this paper,the plan of RC frame shaking table test based on local damage monitoring is designed,and the nonlinear finite element analysis result is compared with local monitoring data,and a nonlinear finite element model updating method is proposed.The main contents of this paper are as follows:(1)Propose the plan of RC frame shaking table test based on local damage monitoring.Aiming at the contradiction between the small compression zone caused by the load limitation of the small shaking table and the large compression zone which is essential for SA damage monitoring,the reinforcement of cross-section is decided by analysis.First,the size of the column cross-section is calculated according to the placement and dimension of SA,and then the geometrical dimension of the frame is designed.Second,reinforcement design of the frame is caculated by the PKPM software and two feasible column reinforcement schemes are found.Based on the principle of the maximum damage of concrete in the compression area,the nonlinear finite element analysis is carried out by using OpenSees software and section reinforcement and ground motion input are optimized.The results show that,for the two-story RC frame with the column cross-section dimension of 200 mm × 200 mm and C30 concrete strength,the middle bars of the column cross-section can effectively increase the height of the compression area.Under the 0.8g Loma Prieta seismic wave,Column concrete can enter the softening region.(2)Determine the key factors that affect the accuracy of nonlinear finite element model of RC frame.Based on the material parameters tested and monitoring acceleration time history,the specimen of shaking table test is simulated by OpenSees software.The output including structure displacement and acceleration,concrete stress and steel strain in the critical section of the 1st column.Comparison of simulation results and monitoring data is carried out to find out the key parameters of finite element model updating.The research shows that the key factors that affect the precision of finite element analysis include underestimation of concrete strength,base rotation of frame model and excessive residual deformation of steel.(3)The nonlinear finite element model parameters of RC frame structure are modified and evaluated based on local damage monitoring data.In order to simulate the relative rotation between the frame foundation and the shaking table,a zero-length element is added to the bottom of the column in the finite element model.The double-line moment-rotation relationship of the zero-length element is determined according to the structural cycle obtained in the elastic and elastoplastic stage monitoring The compressive strength,the loading and unloading rule,the tensile strength,the tensile softening stiffness of the concrete and the reasonable value of joint assumption in the fiber model are determined based on the minimum error to the concrete stress,the steel bar strain and the story drift obtained from the test monitoring.The results show that the model updating based on the local damage monitoring data can significantly improve the simulation precision of the nonlinear finite element of the RC frame and reflect the real stress state of the complex stress region which is difficult to model accurately.