What Is a Box Beam?

The cross-sectional shape of a box beam is the same as that of a normal box, so it is called a box beam. It is generally composed of a cover plate, a web, a bottom plate and a partition. Its materials are steel and prestressed concrete. It is mainly used for long-span or load-bearing structures.

The cross-sectional shape of a box beam is the same as that of a normal box, so it is called a box beam. Generally consists of a cover plate, a web, a bottom plate and a partition. Its materials are steel and prestressed reinforced concrete. Mainly used for long span or

The box girders will produce shear lag effects when they are flexed vertically. The longitudinal normal stresses in the top and bottom plates no longer conform to the uniform distribution of elementary beam theory along the width of the slab. If the normal stress at the connection between the top plate and the bottom plate is greater than the normal stress at the remaining points along the width of the plate, then the shear lag is called positive shear lag, otherwise it is negative shear lag. In the design of the structure, if the shear lag effect is ignored, the actual stress and displacement of the structure will be underestimated, it will not be able to objectively reflect its true stress and deformation, and even endanger the structure safety. Over the years, many scholars have carried out research work on the shear lag effects of various types of box girder, and have achieved many research results, some of which have been incorporated into bridge design codes ^[1]

Box girder is widely used in bridge engineering, and its shear lag effect is an issue that must be considered in design calculation. Affected by the shear lag effect, the stress on the upper and lower flange plates of the box girder is unevenly distributed along the width of the plate. Usually, the stress near the web is the largest, and its value is much larger than that calculated by the elementary beam. . For many years, many scholars have devoted themselves to studying the shear lag effect of box girder, and many related literatures have been published.

As is known to all, when analyzing the shear lag effect of box girder using the energy variation method and finite beam segment method based on the principle of minimum potential energy, the shear lag warp displacement function of the cross section must be selected in advance, and the correct selection of this displacement function is critical. It is important because it directly reflects the distribution of shear lag warping normal stress on the cross section ^[2]

The selection of shear lag warping displacement functions is confusing. Various forms of warping displacement functions have been used. They mainly include: quadratic parabola, cubic parabola, quadratic parabola, fifth and sixth parabola, cosine curve, For the catenary and elliptic curve, the cubic parabola is the most used. However, as early as 1995, the warp displacement function of the cubic parabola was questioned. Among the many forms of warping displacement functions, seeking the most reasonable form of the function is helpful to theoretically improve the analysis method of the shear lag effect of box girder. In addition, little research has been done on generalized internal forces corresponding to generalized displacements of shear lag. Although the expression of the generalized internal force (shear lag moment) corresponding to the generalized displacement of the shear lag is given based on the first-order variation of the total potential energy of the box beam, it is not easy to recognize the generalized internal force in a physical sense.

JTGD622004 "Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts" provides a practical calculation chart for the shear lag effect of box girder based on the effective width of the flange, but it is mainly applicable to positive shear lag and applicability to negative shear lag And practical analysis methods need further research. Therefore, the discrimination of positive and negative shear lag of box girder is particularly important. In the related literature on the analysis of shear lag effects of box girder, the positive and negative shear lags are discriminated by comparing the relative magnitude between the bending moment and the additional bending moment. This discrimination is mainly applicable to the analysis of shear lag by analytical methods, that is, it is only suitable for simple structures such as simple supported beams and cantilever beams, and it is inconvenient to apply to more complex structures ^[3] .