Analysis On Torsion And Distortion Performance Of Variable Cross-Section Concrete Composite Girder With Steel Truss Webs

The steel truss web-concrete composite girder bridge is a new bridge structure with good development prospect,which has the advantages of light weight,beautiful appearance,high spanning capacity and improvement of concrete webs cracking,and has certain practicality,economy and competitiveness in large span bridges.At present,the research on steel truss web-concrete combination girders at home and abroad is mainly based on equal-section girders.However,the force performance of variable-section steel truss web-concrete combination girders differs greatly from that of equal-section girders.Therefore,the theoretical study and finite element simulation analysis of the torsional and distortional properties of the structure are carried out in this thesis with the variable-section steel truss web-concrete composite girder as the research object.The main research contents of this thesis are as follows:(1)Based on the principle of shear equivalence and strain coordination,the equivalence thickness calculation formula of steel truss web is derived and verified.Based on the elastic girder micro-element method,the shear contribution of the concrete top and bottom slabs to the combined girder is considered,and the formula for calculating the shear stress of the steel truss webs of the combined steel truss web-concrete girder with variable cross-section is derived,and the relative error between the theoretical value and the finite element value under the concentrated load is compared and analyzed with the calculation example.The results show that the relative error of the equivalent concrete thickness of steel truss webs obtained by the two calculation methods is 3.9%;the effect of variable section effect in the calculation of shear stress of steel truss web-concrete combination girder with variable section is not negligible.The maximum difference of shear bearing ratio between equivalent steel web of equal section girder and variable section girder under concentrated load is about 79.75%;the closer to the root of cantilever girder,the shear bearing ratio of steel truss web decreases from81.48% to 10.90%.(2)Based on the U.S.second theory and box girder theory,the formula for calculating the restrained torsional quadratic shear stress and the restrained torsional differential equation of the steel truss web-concrete combination girder are derived,and the difference method is applied to solve the variable-section steel truss web-concrete combination girder.The effect of cantilever plate width on the center of torsion and main sector coordinates,and the effect of cantilever plate width and section height on the positive stress of torsional buckling are discussed and analyzed in conjunction with the calculation example.The results show that when the ratio of the cantilever plate to the top plate width is 0～0.64,the distance from the center of torsion to the top plate center increases with the width of the cantilever plate.When the ratio of cantilever plate to top plate width exceeds 1.0,the growth of main sector coordinates at the end point of cantilever plate tends to level off;the increase of cantilever plate has more influence on the positive torsional warpage stress at the intersection of steel truss web and bottom plate;when the ratio of cantilever plate to top plate width is close to 2.4,the decrease of positive torsional warpage stress with the increase of cantilever plate width tends to be stable;with the increase of height-to-span ratio,the positive torsional warpage stress at the three control points gradually increases.With the increase of high span ratio,the values of torsional warpage stresses at the three control points are gradually approached.(3)Based on the principle of energy variational method,the deformation differential control equations of the steel truss web-concrete combination girder are established by using the Euler-Lagrange condition starting from the deformation angle as the basic unknown quantity,and are solved for the variable-section girder by using the weighted residual method of matching point principle.Finally,the effects of cantilever plate width on the deformation warpage moment of inertia and the effects of cantilever plate width and section height of the girder on the deformation warpage positive stress are investigated.The results show that the distortional warpage stress is almost not borne by the web of steel truss,and the distortional warpage stress borne by the bottom plate is much higher than that of the top plate.When the width of unilateral cantilever plate increases to 1.4m,the growth of distortional warpage moment of inertia is gradually stabilized,and the height of girder section is positively correlated with the growth of distortional warpage moment of inertia.When the ratio of cantilever plate width to top plate width of the girder increases from 1 to 1.2,the variation of distortional warpage stress at the intersection of steel truss webs and bottom plate is 68.9%.(4)In this thesis,the calculated values of the warp positive stresses of the variable-section steel truss web-concrete cantilever girder is verified by combining the finite element simulation analysis with the actual project;meanwhile,the stress analysis of the Shuinianbao bridge is carried out to investigate the stress characteristics and distribution of each member during the torsional and deformation of the structure.The results of the study show that the distribution of warpage stresses in the cross section obtained from the theoretical and finite element calculations is basically the same,and the stresses are anti-symmetrically distributed.