Field Measurements of Intermediate External Diaphragms on a Trapezoidal Steel Box Girder Bridge

Trapezoidal steel box girders are being used more frequently for curved bridges due to their high torsional rigidity and aesthetic appeal. A typical system uses two U-shaped girders with a cast-in-place concrete deck. The deck acts compositely with the U-shaped steel girder, which adds significant stiffness to the bridge as it completes the closed section. A critical design stage occurs during the deck pour since the curved girders are non-composite and may have excessive relative distortions. External diaphragms are commonly installed between the girders to control these distortions.

Using intermediate external diaphragms will increase time and costs through design, fabrication, and construction; therefore, it would be very beneficial to minimize their use on box girder bridges. Forces in three external diaphragms and several top lateral-bracing members were measured during the construction of a three-span trapezoidal steel box girder bridge built in Austin, Texas. The stresses in the bridge were monitored during three main loadings. The measured forces in these members were compared to analysis using a finite element model.