The Research On Shear Lag Effects In Box Girder Bridges

Shear lag effect is a significant mechanics character of thin walled box girders. Several years recently in our country,many landscape cracks are found in more and more thin walled box girders, and this phenomena shows, to neglect the shear lag effect will cause the insecurity of design work of construction.This thesis particularly expatiates the conception of the shear lag girder about thin-walled box and its disadvantage to structure, makes a summary of effect a variety of theories and methods to solve the shear lag problem in engineering group. The applicability and limitation about above theories and methods are particularly comment in this thesis and it gives the research prospect.The shear lag effect in box girders under symmetrical bending load if studied by means of the variational principle, transfer matrix and finite strip method. Postulate three shear lag warp displacement function with consideration of different warping amplitude of upper plate, floor plate and projecting slab in box girder, shearing deformation and self-balancing warping stress of cross section, the governing differential equations and boundary conditions of box girder are derived by energy-variational principle. The closed-form solutions of the equations are thus obtained. Conventional method which approximate selects the shear lag warp displacement function with same warping amplitude of upper plate, floor plate and projecting slab in box girder is a special case of method given in this article without consideration of self-balancing warping stress of cross section and same physical dimension of upper plate, floor plate and projecting slab in box girder. The method and postulations shows universal of the method given in this paper as well as exact.A method of transfer matrix, which tries to study the Shear lag effect, is put forward. Based on the homogeneous solution derived from the governing differential equations and differential relationship of endogenous force with displacement, detailed formulae of field transfer matrix and point matrix of bridge are derived respectively. The solution is carried out by recurrence calculation. The numerical computational programs (BGSFL1) for calculation of the shear lag effect in box girders is worked out with Mathematics languages.The homogeneous solution derived from the governing differential equations is used as the displacement patterns of finite segment. The stiffness matrix and load column matrixes are obtained in terms of directed stiffness method and working-energy principle. A box girder finite segment model with consideration of shear lag is established and the numerical computational programs (BGSFL2) is worked out. Furthermore, it exploring the influence on shear lag with different load effect pattern, and width-to-span ratio,and reaches some regular patterns and conclusions about the shear lag of box girders.The equations of this paper are the continuation of the theory of shear lag, thus enriching and developing theories of the shear lag in box girder. The analysis results would provide reference for engineering designing.