Study On Dynamic Characteristics And Impact Factors Of Composite Bridges With Steel Truss Webs

The prestressed concrete composite girder bridge with steel truss webs is a combined structural system which replaces the traditional concrete web with the steel truss web,and is composed of concrete top and bottom slabs,steel truss webs,and internal and external prestressed steel bars.Moving vehicles passing through the bridge will cause vibration of the bridge.In the bridge design,the dynamic impact coefficient IM is usually used to characterize the dynamic impact of the moving vehicle on the bridge.The reasonable value of the impact coefficient is related to the safety of the bridge design.In this paper,the prestressed concrete composite girder bridge with steel truss webs is taken as the research object,and its dynamic characteristics and the vehicle-bridge coupled vibration response under the load of moving vehicles will be analyzed,which will provide a theoretical basis for understanding the dynamic performance of the new composite structure.Details of this study are summarized as follows:(1)By using the thin-walled box girder method,the steel truss web is equivalent to the flat steel web according to the principle of uniform shear stiffness.Combined with the Hamilton principle,the theoretical formula of bending vibration frequency of the composite girder bridge with steel truss webs is derived,considering the shear deformation of bridges and the shear lag effect of box girder sections.(2)The effects of span-height ratio,overhang ratio,width-height ratio,web profile and size on the accuracy of the theoretical formula of bending vibration frequency of the composite girder bridge with steel truss webs are analyzed.The results indicate that the overhang ratio,width-high ratio,and web profile and size have little effect on the accuracy of the theoretical formula.When these three parameters change,the accuracy of the theoretical formula is very high.The span-height ratio has a large effect on the accuracy of the theoretical formula.As the span-height ratio is reduced,the accuracy of the theoretical formula becomes worse.When the span-height ratio l/h is conventionally taken from 8 to 25,the maximum deviation between the theoretical value and the finite element value is within 10%,which satisfies the accuracy requirements of engineering applications.(3)The computation program for analyzing vehicle-bridge coupled vibration is compiled based ANSYS.Through the verification of two examples,the numerical analysis method used in this paper is basically feasible and has high precision.(4)Taking a single cell box girder with the span of 38.4 m as an example,the influence of vehicle loading mode,speed and pavement roughness on the dynamic response at typical positions of this type of bridge is studied.The results indicate that under the two-lane parallel loading,two-lane counter loading and single-lane eccentric loading,the coupled vibration responses of the composite girder bridge with steel truss webs are all around the fluctuation of the response curve under static load.With the deterioration of pavement condition,the fluctuation range of vibration response becomes larger and larger.(5)The structural parameters are changed,and the difference between the calculated value of the dynamic impact coefficient of the composite girder bridge with steel truss webs and the value determined by the General Code for Design of Highway Bridges and Culverts(JTG D60-2015)is discussed in a wider range,Analyzing the accuracy and applicability of the calculation formula of the impact coefficient in the current code for such new bridges.The results indicate that with the deterioration of road surface conditions,the calculated value of the dynamic impact coefficient of the composite girder bridge with steel truss webs increases sharply.The effect of speed on the impact coefficient of each response type of the composite girder bridge with steel truss webs is very complicated,and no obvious law is found.In the single-lane eccentric loading mode,the axial force impact coefficient of the lower side is much larger than that of the upper side.The design and calculation should be paid attention to.The response type has a great influence on the value of the impact coefficient of the composite girder bridge with steel truss webs.Under the same conditions,the IM maximum value is easy to appear on the axial force of the lower side or the torsion angle of mid-span.In order to be safe in engineering calculation,the IM value of the axial force of the lower side and the torsion angle of mid-span can be calculated first.