| In order to satisfy the needs of route design and terrain,variable cross-section continuous box girder with good mechanical properties,structural characteristics and applicability is widely used by bridge designers in the design of bridge superstructure.However,due to the variation of the height of box grider,the mechanical behavior of the continuous girder bridge with variable cross-section becomes more complex than that of the straight cross section bridge.The thin-walled structure has significant shear lag effect,with the increasing number of large span continuous box girder bridges with variable cross section,the shear lag effect is still the focus in academic and engineering research.Therefore,the calculation theory and method considering the shear lag effect of continuous box girder with variable cross-section are studied as follows:1.A coordinate system(x,y,z)for space physical coordinate description of variable height box girders is established for single box single room variable cross section box girder model.The two parabola function,which satisfies the axial force balance condition,is chosen as the displacement function for the shear lag warping.The expression of the spatial displacement of the arbitrary point in the box beam is derived through 8 basic deformation modes and the relative displacement parameters.The geometric relationship between the strain and displacement of the arbitrary point in the box girder is further derived base on the box beam theory,the space problem of the stress calculation of the variable height box girder is transformed effectively into one dimensional problem.2.In order to calculate shear lag effect of thin-walled box girder with variable section,a three nodes board beam segment method considering shear lag effect is proposed in this paper.The proposed three nodes isoparametric element has 3 nodes and 24 degrees of freedom.Basing on the relation among strain,displacement and deformation in box girder section,and regarding C0 continuous shape functions as interpolation functions of variable height within element,the determinant of isoparametric finite element corresponding to beam segment method has been established by using the minimum potential energy principle and the Gauss Legendre numerical integration algorithm.Finally,the finite element analysis program of related theory is written by MATLAB.3.The finite element calculation program is used to analyze three examples of simply supported box girder,variable cross section cantilever box girder and variable cross-section continuous box girder.In this paper,a three spans continuous composite box girders bridge on a railway traffic line in Tianjin is selected as the analysis object.The sensitivity of the shear lag effect on the typical cross section and the natural frequency of the composite box girder are studied by the factors such as the change of the form of the beam,the strength of the concrete,the slope of the web,the ratio of the width of the wing and the width span ratio,and the common influence of the shear lag effect analysis.In this paper,a board beam segment method is established for calculating the shear lag effect of continuous box girder with variable cross section.The correctness and reliability of the theory and formula derivation are verified by an example,and the efficiency of this method is proved.The analysis results of the parameters are of practical value,which will provide some reference value for the design of variable cross-section continuous box girders in the future. |
