Vibration Analysis And Experimental Study Of Concrete Beams Under Fire Based On Finite Element Model Updating

The finite element model updating based on vibration test has long been a hot topic in civil engineering.Nevertheless,the research on vibration-characteristics of structures at high temperature was still infrequent.The research methods of structural vibration-characteristics at high temperature mainly include experimental research and numerical simulation,but the experimental conditions and the establishment of accurate finite element model are troubled by many factors,which lead to less research results.Funded by the National Natural Science Foundation of China(No.5160081693)and the Natural Science Foundation of Shandong Province(No.ZR2016EEB13),this paper conducted research on the finite element model updating method of reinforced concrete(RC)beam,which took into account the influence of boundary conditions.Finally,the method of stepwise finite element model updating was presented base on Support Vector Machines(SVM).In order to obtain the variation of modal information on fire and verify the model updating method,4 simply supported RC beams were made,and modal information was picked up before,during and after the fire tests.Finite element model was modified based on the experimental results.Finally,the bearing capacity of RC beams was obtained by loading tests after exposure to fire,and the experimental datas were collected to verify the rationality of the model updating method.The main contents are as following:(1)The feasibility of finite element model updating of RC beams considering boundary conditions.Firstly,a finite element model with boundary conditions was established,which considered the vertical stiffness by using the element combin-14,and ignored the effect of rotation direction.Subsequently,we used SVM to modify the simply-supported RC beam model based on numerical simulation,which verified the feasibility of the algorithm.(2)Proposed the SVM step by step correction algorithm based on the vibration test.In order to prevent "data explosion" attributed to the excessive number of samples at one time,we proposed a stepwise correction algorithm based on SVM.A three-span continuous beam was used to verify the algorithm,and operational interface of the traditional support vector machine(GUI)was also improved.The correction results demonstrated that the method not only guaranteed the requirement of the evaluation index of structural response but also met the requirement of convergence of physical parameters,which proved that the method has certain applicability.(3)Vibration tests of RC simply-supported beams before and after exposure to fire and bearing capacity tests after exposure to fire were both conducted.4 simply supported RC-beams were made,and the vibration test before and after exposure to fire and the residual capacity test after exposure to fire were carried out respectively.The stepwise algorithm was tested and verified by experimental results which based on SVM input parameters constructed by measured data.This results further illustrated that this method has some practical value in Engineering.(4)Fire tests of RC simply supported beams and vibration tests at elevated temperature.First of all,the simply-supported beam model before exposure to fire was modified by SVM step by step correction method mentioned before.Secondarily,we simulated vibration-characteristics of a simply-supported beam during fire by referring to the empirical formula of temperature field calculation,and compared it with the measured data in fire test.Finally,in order to analyze the influence of the physical parameters on the fundamental frequency on fire,parametric analysis of the physical parameters was conducted through a simply-supported beam model and the formula for calculating the fundamental frequency combined by physical parameters and fire exposure time was fitted.