Stability of Slender Webs of Prestressed Concrete Box-Girder Bridges

Abstract

Long-span, prestressed concrete, box-girder bridges are haunched and have a span-to-depth ratio of 15 to 20 at the piers. This leads to slender webs, particularly for bridges built with high performance concrete. For girders with sloped webs and constant bottom slab width, the web plate is normally warped, which leads to web curvature in the direction of the principal compressive stresses. It is first shown that buckling is not critical as long as the web is uncracked. But, if the webs have shear cracks, the slenderness ratio of the diagonal compression struts can be very high so that the moments and stability of the curved struts need to be studied. It is shown that the tensile forces in the stirrups—determined according to the truss analogy—will counteract the lateral deformations of the slender compression struts. The procedure, which was developed for the design of the Confederation Bridge in Eastern Canada, will be illustrated by applying it to the slender webs of that bridge.