Study On The Mechanism Of Temperature Effect Of Modal Parameters Of The Main Beam Of Tied Arch Bridge

The research of bridge structural health monitoring and damage identification is to determine the dynamic characteristics of the structure through modal tracking,regression analysis and statistical pattern recognition under operating state,so as to give corresponding evaluation to the overall health status and structural changes of the structure.However,under the natural environment excitation,the modal frequency of the bridge structure identified by the system response modal parameters is not only related to its own stiffness,mass and other parameters,but also related to the environmental impact of temperature,traffic load and wind.Temperature is the most important factor,and the fluctuation of modal parameters caused by the change of temperature can cover up the minor damage of the structure,resulting in the distortion of the dynamic characteristics of the structure,which leads to the misjudgment of damage identification,damage location and damage degree.Therefore,it is of great significance to study the influence mechanism of temperature on the modal frequency of bridge structure in order to predict and eliminate the influence of temperature,and to distinguish the variation of modal parameters caused by the change of temperature and structure itself.Based on the bridge of beam and arch composite structure as the research object,which is equivalent to multi-span elastically supported beams with tied beams and arch cables as elastic supports.Through establishing differential vibration equation,Elastic modulus,boundary conditions and the temperature distribution which is sensitive to the changing temperature are taken as variables respectively to theoretically clarify the influence affected by temperature.In order to explore the temperature influence mechanism of modal frequency on structural material characteristics,by establishing the modal frequency equation of the multi-span elastic support beam and solving the variational calculation of the temperature sensitive variable in the expression of modal frequency,it is concluded that the elastic modulus is the main reason for the influence of the temperature change on the modal frequency of the structural material properties.In order to explore the mechanism of temperature effect when the structure is under redundant boundary conditions,the equivalent axial force under the temperature effect was applied to the multi-span elastic support beam structure model,and the modal frequency equation including axial force at both ends was established to determine the relationship between axial force and modal frequency.The two temperature sensitive conditions of modal frequencies areverified by numerical analysis of the temperature effect of modal frequencies in a three-span continuous beam numerical model.In order to explore the temperature influence mechanism of the temperature distribution mode of modal frequency,the stiffness variability of the cable as the equivalent elastic support should be considered.Different temperature distributions,including the main beam temperature gradient and the steel arch temperature,can change the mechanical properties of the cable by deforming the structure,which in turn affects the modal frequency of the structure.The relationship between the temperature deformation and the stiffness variation of the equivalent elastic support of the cable is determined by theoretical deduction,is verified by the temperature influence analysis of the simple beam-arch combination numerical model.It is concluded that the temperature gradient of main beam dominates modal frequency change of beam-arch composite structure.The finite element model of Dasha River Bridge in Shenzhen is modified by modal experiment results,and the numerical study of the temperature influence of modal frequency is carried out to verify the correctness of the temperature influence theory proposed in the previous paper.In the numerical analysis,there are two temperature sensitive conditions affecting the Dasha River Bridge,namely the elastic modulus and the section temperature gradient.According to the result analysis,it is determined that the variation of the mode frequency caused by the cross-section temperature is much larger than that caused by the change of the elastic modulus.Through the experimental study on the temperature influence of modal frequency on the Dasha River bridge structure,it is verified that the correlation between the modal frequency and the section temperature gradient of the main beam is consistent with the positive correlation conclusion obtained above due to the negative correlation trend caused by the masking of the elastic modulus.Through the deformation monitoring and cable frequency identification,the influence of the temperature gradient of the main beam on the modal frequency is clarified.It is proposed that the elastic modulus of the structural material caused by the long-term average temperature change is the main reason of an overall shift in the structural modal frequency.