Research On Mechanical Properties Of Concrete Composite Box Girder With Steel Truss Webs

Concrete composite box girder with steel truss webs is a new type of steel-concrete composite box girder structure,and the bridge type has found an increasing wide utilization in modern bridge construction because of its advantages such as light weight,convenient erection,non-existent cracking on webs,etc.Concrete composite box girder with steel truss webs is different from traditional solid web box girder,its oblique web members which mainly bear the axial force are connected with the concrete flange plates through the joint structures,it inevitably makes the mechanical properties of concrete composite box girder with steel truss webs different from that of solid web box girder.At present,the research literatures on mechanical properties about concrete composite box girder with steel truss webs are relatively rare,therefore,in order to study the following aspects,the paper aims at the structural characteristics of concrete composite box girder with steel truss webs and takes Nanking Jiangshan Bridge as the engineering background.(1)The formula for calculating the longitudinal elastic modulus of concrete composite box girder with steel truss webs' web members is deduced,and the flexural behavior of the composite box girder is studied accordingly;the shearing performance,steel truss webs' force condition and the longitudinal deformation distribution of cross section is satisfying “plane cross-section assumption” or not of concrete composite box girder with steel truss webs are studied through the finite element simulation analysis.The research results show that: for concrete composite box girder with steel truss webs,the section's bending moment is bore by concrete flange plates,and the section's shearing force is bore by steel truss webs members;the steel truss webs members' force conditions of concrete composite box girder with steel truss webs are similar to that of the plane truss system in the paper,and the steel truss webs members' rod forces can be calculated approximately by using the plane truss system;the longitudinal deformation distribution of cross section satisfies “plane cross-section assumption”.(2)The formulas for calculating the longitudinal bending stress and longitudinal additional stress on flange plate of concrete composite box girder with steel truss webs are derived based on the principle of energy variational method;in the process of derivation,the modification of the longitudinal displacement function of composite box girder's cantilever plate is considered,the necessity of modifying the longitudinal displacement function of composite box girder's cantilever plate and the modification coefficient's reasonable value of the composite box girder considering the influence of cantilever plate's boundary constraint are studied through finite element simulation analysis and related theoretical calculation.The research results show that: for concrete composite box girder with steel truss webs,the longitudinal stress on the lower flange plate can be calculated by the formula for longitudinal bending stress,and the longitudinal stress on the upper flange plate can be calculated through the superposition of the longitudinal bending stress and corrected longitudinal additional stress on flange plate;the modification of the longitudinal displacement function of composite box girder's cantilever plate can effectively improve the calculation accuracy of the longitudinal stress on roof,and the modification coefficient's reasonable value of the composite box girder considering the influence of cantilever plate's boundary constraint can be taken as 1.4.(3)The formula for calculating the deflection of concrete composite box girder with steel truss webs which considers the steel truss webs' shearing deformation and shear lag effect synthetically is derived based on the principle of energy variational method,then the accuracy of derived formula,the relationship among the depth-span ratio,horizontal inclination angle of steel truss webs member and additional deflections of the composite box girder caused by the steel truss webs' shearing deformation and shear lag effect are studied.The research results show that: the formula for deflection in the paper has better accuracy;the additional deflection of concrete composite box girder with steel truss webs caused by the steel truss webs' shearing deformation and shear lag effect increases with the increasing of composite box girder's depth-span ratio,and the additional deflection of composite box girder caused by the steel truss webs' shearing deformation and shear lag effect can not be neglected in deflection's theoretical calculation;horizontal inclination angle of steel truss webs member only affects the additional deflection caused by the steel truss webs' shearing deformation of concrete composite box girder with steel truss webs.(4)The formula for calculating shear lag coefficient of concrete composite box girder with steel truss webs,changing regulation of shear lag coefficient along the longitudinal bridge and calculation method for the effective distribution width of the flange plate are studied through the finite element simulation analysis and related theoretical calculation.The research results show that: the effect of flange plate's additional axial force on the calculation of longitudinal stress on flange plate of concrete composite box girder with steel truss webs calculated by elementary beam theory should be taken into account;the shear lag effect of continuous concrete composite box girder with steel truss webs under even load is the most obvious,its changing regulation is also the most complex,and there is a kind of phenomenon of negative shear lag effect;for concrete composite box girder with steel truss webs,it is suggested to use American code and British code when calculating its effective distribution width of the flange plate.