Study On Truss-box Equivalent Theory Of Composite Truss Bridge With Steel Truss Webs

Composite truss bridge with steel truss webs is a type of composite structure bridge which has been widely used in domestic and foreign projects in recent years.The bridge is mainly composed of upper and lower concrete deck,internal(external)prestressed steel bundles and steel truss web members.Compared with the previous concrete box bridge,the bridge can avoid the cracking of concrete web,further reduce the weight of the concrete,and realize a larger span.At present,domestic and foreign theoretical and experimental research focuses on joint connections,and there is little research on overall force analysis,due to the bridge has many members,discontinuous structure and load transfer in two different materials,it is very difficult to analyze the structural stress,which affects the development of this kind of bridge.In this paper,the method of equivalent steel truss webs to concrete webs is proposed to study the mechanical properties of bridge structures.The theoretical derivation is proposed to calculate the equivalent thickness of the steel truss web of the composite truss bridge,the bending theory considering the shear lag effect,the shear theory considering the shear distribution rate of different parts and the calculation of the constrained torsion theory.the accuracy of the proposed calculation theory is verified by the finite element calculation results.Finally,the truss-box equivalent theory of composite truss-web girder bridge is formed.The main research work and achievements of this paper are as follows:1.Based on the principle of equal shear strain energy and shear deformation before and after equivalent,The formula for calculating the equivalent of steel web as a diagonal web of composite truss bridge is derived.Based on the principle of equal shear strain and shear force,the formula for converting steel web into concrete web is derived.Through the above two steps of conversion,the truss portion of the original structure is equivalent to a continuous concrete web,Forming a similar box girder structure with a closed section Furthermore,the comparatively perfect box girder theory can be used to study the mechanical behavior of the equivalent structure.2.Based on the equivalence theory,the appropriate shear lag war page displacement function is selected.Based on the principle of virtual work and the principle of minimum potential energy,the shear lag differential equation of composite truss bridge with steel truss webs is deduced,the formulas for calculating shear lag effect,vertical deflection and axial force of steel truss webs of composite truss girder bridges are obtained,the influence ofdifferent loading positions on shear lag effect is analyzed,considering the influence of different geometric parameters on shear lag of composite truss bridge with steel truss webs,the formula for calculating the effective width of bridge deck is obtained.Finally,a comparative analysis with the actual finite element model is made,the accuracy of the theory is verified.3.Considering axial force balance of upper and lower concrete deck,steel truss webs and equivalent concrete webs are only subjected to shear and do not bear bending moment and axial force.The differential equation of shear stress for composite truss web girder bridge is established,the formula for calculating shear stress of equivalent concrete webs is obtained,the shear force on the steel truss webs member is obtained.Finally,the shear distribution rate is calculated.The correctness of the theoretical derivation is verified by comparing with the results of finite element analysis,the shear distribution rate of steel truss webs is about 0.7,the shear distribution ratio of concrete bridge deck is 0.2~0.3,the shear resistance of concrete bridge deck is not considered in design,it is safe to only consider the shear resistance of steel truss webs members.4.Based on the force balance,the calculation formula of the constrained torsional shear stress of the equivalent thin-walled box girder structure is established,the formulas for calculating the torsion angle,warping degree,double moment and warping moment of cross section under concentrated torque applied at one fixed end and one fixed end are obtained,the formula for calculating the axial force of oblique webs is derived,the torsional characteristic parameters are calculated.The comparison with finite element method shows the accuracy of theoretical calculation in this paper.