Comparative Study Of Modal Parameters Of Beam Bridges Incorporating Steel-UHPC Composite Decks And SMA Paved Steel Decks

Recently,steel-UHPC(Ultra-high Performance Concrete)composite orthotropic bridge deck has been widely used as a new type of bridge structure in China.In order to investigate the stiffness variations of the entire structure caused by increasing rigidity of the bridge deck overlay,obtain the modal features of the steel box girder with the steel-UHPC composite system,and furthermore analyze the differences of the modal features compared with steel box girder with traditional stone mastic asphalt(SMA)overlay,it is very necessary to carry out some relevant studies which can provide the basic knowledge of structural modal for dynamic analysis such as wind and earthquake resistance of this kind of bridge.This paper focuses on the following aspects with corresponding contents:(1)This thesis introduced the steel-UHPC composite deck system,and the up-to-date development of time-domain identification methods all over the world.(2)The basic concepts and theories of modal analysis,such as white noise,impulse response function,frequency response function,correlation function and complex modal theory were also introduced.Two kinds of time-domain data preprocessing methods(RTD and NEx T)and four kinds of time-domain modal identification methods(ITD,CEM,ERA and SSI)were described.The method was implemented by MATLAB program which was verified by numerical simulation.Robust performance to different noise level of the method was also discussed,which showed that the SSI is the best one while the ITD is the worst one.(3)The ANSYS finite element models were established using SHELL63 element to analyze the dynamic characteristics of two bridges.The first seven order vibration shapes and their frequencies were obtained.Comparion of the dynamic characteristics between the two bridges demonstrated that each mode frequency of the west bridge with steel-UHPC composite overlay is slightly higher than that of the east bridge with the traditional SMA overlay.The maximum deviation is 1.26%,and the identified results are basically consistent.(4)The ambient excitation method based on reference points was adopted for the Fochen extension bridge.The measuring points were arranged inside the steel box girder of the bridge and continuous samplings were applied in batches.The three methods(CEM,ERA and SSI)which have better noise immunitywere chosen to identify the modal parameters of the two bridges.It can be seen that the frequencies resulted from the three methods of the west bridge is slightly higher than those of the east bridge.The maximum deviation is 3.74%,and the identified results are also consistent.Comparing the identification results of these methods with those calculated by finite element method,the frequencies and mode shapes identified by these three methods are in agreement with those identified by finite element method for these two bridges,which demonstrates the effectiveness of present method.The identified damping ratio is not as accurate as that of frequency and vibration shape.In this thesis,it can be observed that the employed overlays with UHPC or SMA have few differences on the entire dynamic characteristics of the bridge structure.Therefore,steel-UHPC composite deck system can be ignored for the increase of the overall stiffness of the bridge in comparison with SMA overlay for steel box continuous beam bridges with similar span of the Fochen extension bridge.