ABSTRACT
In the past two decades, the structure with a boxshaped cross-section has been adopted widely in bridge projects in China due to its lightweight, large anti-torsion rigidity and its convenience to advanced construction techniques. The longitudinal force applied to a box-shaped girder is usually transferred from its belly plate to its flange plate when it is flexed. Because the shear strain reaches its maximum at the intersection of the belly plate and the flange plate, that is: the farther away the belly plate, the less the shear strain, the longitudinal displacement difference within the flange plate is resulted and the bending positive stress distributes in certain curved shape along the flange plate. That is called "the shear-lag effect". The study on the shearlag dates back to 1924 when T.V.Karman firstly discussed on the effective width of T-shaped girder with a broad Hange. In 1964, E.Reissner also proposed a numerical solution to a rectangle-shaped girder without a cantilever by the principle of variation, and suggested that the larger spacing between the rib plates, the more noticeable the shear-lag effect in a box-shaped girder. From November 1969 to November 1971, four accidents, structure unstability or failure, took place in Austria, England, Australia and German, respectively. Among the factors that should be responsible for these accidents, one is that the designers did not take account into the shear-lag effect seriously. However, the study on the theory of shear-lag effect in the
box-shaped girder lags far behind that on its structure. There is much less researches or papers on the shear-lag effect in the curved box-shaped girder, esp. one with large curvature.
