Buckling Behavior of U-Shaped Girders

Trapezoidal box girder 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 on the deck. There is no existing codified design method for lateral 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-shaped 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.

The experiments verified that the fast buckling mode of these girders is an S-shaped, not an Euler-typed half and that transverse stiffeners can significantly increase the buckling capacity. It was also found that existing design formulas for torsional bracing are unconservative for these girders which have very small top flanges. New design equations must be developed.

Test loads exceeded the theoretical predictions by as much as twenty percent which was attributed to the web corrugations (the web plate was not fabricated flat) in the welded girder.