Temperature Influence Analysis On Modal Frequency Of Bridges Based On Cointegration Theory

In the past few decades,bridge structural health monitoring(SHM)in civil engineering has been a hot topic that has attracted much attention.The health monitoring of bridge structures is usually based on the change in dynamic characteristics of the bridge structure.The modal parameters of the structure,such as natural frequencies,mode shapes and damping ratios,can be obtained through vibration measurements by using modal parameter identification.However,the modal parameters measured by the vibration-based SHM method are sensitive to damage as well as the changes of operational and environmental conditions including temperature,humidity,illumination,wind and so on.Among those environmental factors,environmental temperature is one of the main issues affecting structural damage detection.In fact,the influence of ambient temperature may be large enough to the influence of structural damage as the bridge structures are always exposed to environmental conditions.This may lead to the possibility of false alarms of structural damage detection,which restricts to implementation of SHM in actual engineering.Therefore,it is particularly important to separate the temperature factors in the bridge SHM.This thesis is aimed at exploring the influence of environmental temperature on modal frequency through finite element simulation and temperature varying experiments in the laboratory.The cointegration vector is formulated between multiple frequency sequences and the common trends between sequences are removed by linear combination between processes.In such a way,the temperature factors between sequences are separated successfully.The main contents of this thesis are as follows:1.The importance issues of SHM in the field of bridges and the current status studying the influence of temperature on structural modal frequencies in actual engineering are addressed.And then,the research progress of separating temperature factors in SHM is discussed.2.To explore the influence of changes in environmental temperature on the modal frequency changes of the bridge structure.The modal analysis of a reinforced concrete beam under different temperature conditions is carried out through finite element simulation to separate the temperature factors by using co-integration algorithm.The first four order modal frequencies are obtained to investigate the influence of temperature on the modal frequency of the bridge structure.And then the co-integration equation is established after the analysis of the first four order frequencies.It is demonstrated that the resulting residual error successfully eliminates the influence of the temperature factor on the structural frequency change,which can greatly improve the accuracy of the damage identification of the bridge structure and reduce the possibility of false alarms.3.To further verify the feasibility and correctness of the co-integration theory to eliminate environmental temperature effects on the structural modal parameters,the varying temperature experiments are carried out in the load chamber.The experiment steel beam is fixed at both ends with a number of acceleration sensors and temperature sensors amounted on it.With designated temperature levels,the vibration measurements are performed so that the modal parameters of the tested beam can be obtained accordingly at the varying temperature conditions.The experimental results are used to verify the proposed co-integration algorithm to eliminate environmental temperature effects on the structural modal parameters.