Torsion Effect Analysis Of Steel-concrete Composite Box Girder

With the increase of steel output in China,steel-concrete composite box girder is widely used in medium and small span bridges because of its advantages of reasonable stress,simple structure and convenient construction.As a kind of thin-walled structure,its torsional effect is more obvious under eccentric load,so it is of great significance to improve its relevant calculation theory and strengthen numerical simulation for promoting the development of composite beam bridge.In this paper,the torsional effect of steel-concrete composite box girder is studied by combining theoretical analysis with numerical simulation,and the main contents are as follows:(1)Based on the theory of spatial effect of thin-walled bar,the main fan characteristic value of thin-walled member section is obtained by selecting auxiliary poles properly,and a set of formulas suitable for calculating the torsional geometric characteristics of thin-walled section is summarized.After detailed calculation with an example,parametric analysis is carried out,which paves the way for the subsequent analysis of the torsional theory of composite beams.(2)Based on Umanski’s second theory,combined with the torsional geometric characteristics of composite box girder section,the constrained torsional control differential equation of composite box girder is established,and the initial parameter method is used to solve the differential equation,and the torsional characteristic values of composite beams under concentrated torque and uniformly distributed torque are derived,that is,the distribution law of torsional angle,deformation coefficient,torsional double torque,total torque,bending torque and free torque along the beam length direction.Through an example of a composite beam,the analytical solutions of the restrained torsional warping normal stress and shear stress of a certain cross section are obtained,and compared with the stress values extracted after modeling by finite element method,it is found that the stress variation laws obtained by the two methods are basically consistent,which further verifies the correctness of the deduced formula.(3)The finite element software ANSYS is used to model and analyze the 35 m composite simply supported box girder with equal width,focusing on the deformation(torsion angle)and the change law of warping stress under eccentric load.By changing the width-span ratio,height-span ratio,stiffener thickness and the number of diaphragms,the torsional performance of the box girder is parameterized,and the reasonable height-span ratio is between 0.04 and 0.06,and the width-span ratio is0.07.Setting thicker stiffeners increases the dead weight of the structure,and the thickness is about 16 mm.The effective torsional stiffness will continue to increase with the increase of the number of diaphragms,but setting too dense diaphragms will cause great waste,and the reasonable number of diaphragms is significant to ensure sufficient torsional resistance.(4)In view of the variable cross-section(widening)box girder in engineering,based on the above-mentioned composite box girder with equal width,the finite element models of equal width box girder and three kinds of variable width box girder are established respectively,and the variation law of warpage stress under eccentric load is studied.The results show that the warpage stress of cantilever part of equal width box girder is the smallest,and increases with the widening of bridge.At the center line of bridge,the stress values of four kinds of box girders are basically close,and the variation range of stress is far from the center line.For simply supported box girders of equal width,the normal stress amplification factor is symmetrically distributed about the center line of the bridge,and gradually increases from the mid-span position to both sides.For widened box girders,its minimum value appears in the short side span,which is about 2/5 span,and gradually increases to both sides.The shear stress amplification factor changes more actively along the longitudinal direction of the bridge,which is reflected in the fact that the shear stress amplification factor of the box girder with equal width is between several kinds of box girders with different widths.