Shear Design for the Confederation Bridge

Abstract

For long span continuous concrete bridges, haunched box girders are the preferred choice of the structural engineers because they not only look most graceful, but they are also more efficient structurally, and more economical. It is therefore not surprising that the designers of the 12.9 km (8 mile) long Confederation Bridge in Eastern Canada chose the haunched girder configuration for the 43 girders of the 250 m (820 ft) long marine spans as well as 21 girders of the approach spans. In the paper it is demonstrated that the conventional approach to shear design of haunched girders is on the unsafe side. This is so because the compression force, the vertical component of which reduces the shear force in the web, is smaller than the bending moment, divided by the internal lever arm. The relevant equations are derived on the basis of equilibrium in a web with diagonal cracks, sloped at an angle. As an example the shear design for the Confederation Bridge is presented.