Structural System And Long Term Behavior Of Multi-span Composite Cable-stayed Bridges

Multi-span composite cable-stayed bridges have developed rapidly in China and abroad,but the researches on structural system and long term behavior of multi-span composite cable-stayed bridges are not fully addressed.This dissertation investigates the stiffness system and long term behavior of multi-span composite cable-stayed bridges,focusing on mechanism analysis,program development and engineering application.The main results and achievements of the research are summarized as follows:(1)The theoretical model of longitudinal stiffness of multi-span cable-stayed bridges is established and the program is developed accordingly.The longitudinal stiffness theory is based on multi-spring model,which reflects the characteristics of stiffness transfer between towers,and explains the mechanism of stiffness interaction between cables.The accuracy of the program is verified by comparing the results of the program with those of the finite element models.(2)The vertical stiffness system of multi-span cable-stayed bridges is established and the program is developed accordingly.Based on the longitudinal stiffness theory,the vertical stiffness of cables considering system modification is deduced.Based on the mechanical characteristics of multi-span cable-stayed bridges,a calculation method for the iteration of beam-cable-tower system is proposed.The geometric nonlinearity of cable sag effect and moment-axial force coupling effect are also considered in the model.Based on the calculation results of a multi-span cable-stayed bridge and an extradosed cablestayed bridge,the accuracy and validity of the program are verified.The program can provide a convenient analysis platform for the preliminary design and research of multispan cable-stayed bridges.(3)Taking Dongting Lake Bridge of Menghua Railway as an example,the mechanism of stiffness lifting measures for multi-span cable-stayed bridges is analyzed,and the effects of various stiffness lifting measures on structural performance are quantitatively compared.The mechanism and efficiency of oblique stable cables,crossing cables and horizontal stable cables are explained in detail.The analysis method of stiffening cable system is integrated into the vertical stiffness program.The integrated program can analyze the influence of different stiffness lifting measures on the structure,thus broadening the application of the program.(4)Based on ABAQUS software platform and exponential algorithm,a threedimensional concrete shrinkage and creep analysis subroutine is developed.By comparing long-term test results of concrete beams and composite beams with the results of the subroutine calculation,the validity of the subroutine is verified.(5)Taking Erqi Yangtze River Bridge as an example,the long-term behavior of multi-span composite cable-stayed bridges is analyzed,and the difference between multipylon system and two-pylon system in the long-term state is clarified.Affected by the structure system,the additional deflection of the main girder caused by shrinkage and creep of the three-tower composite cable-stayed bridge is smaller than that of the twotower composite cable-stayed bridge under the same span.The main girder compression caused by the shrinkage and creep effect of the main girder is the main factor affecting the long-term behavior of multi-span composite cable-stayed bridge.In the design,attention should be paid to the influence of the unloading of side-span back cables and the increase of negative bending moment of the main girder near the pylon on the structure in the long-term state.