Buckling Behavior of U-Shaped Girders

Trapezoidal box girder systems are becoming a popular form of curved bridge system because of their torsional stiffness and aesthetic appearance. The systems typically consist of twin steel U-shaped girders with a concrete deck acting compositely as the top flange. The top flanges of these U-girders are susceptible to lateral-torsional buckling during transport, erection, and placement of the deck. There is no existing codified design method for bracing of U-shaped girders. Minimizing the amount of bracing used will lead to a more efficient design since this bracing makes up a significant amount of the total costs and is not utilized once the concrete deck has cured.

In order to develop a design procedure for lateral bracing of U-girders, the behavior of unbraced U-shaped girders was first studied. An analytical program was undertaken to study the buckling behavior of unstiffened and transversely stiffened U-shaped girders using finite element models. A series of laboratory experiments were then performed using U-girder scale models in order to verify these analytical results. Finally, the analytical and experimental results were compared with existing design equations for torsionally braced I-shaped beams.