Lateral Bracing of Bridge Girders by Permanent Metal Deck Forms;Final rept

摘要

Lateral torsional buckling is a failure mode that often controls the design of steel bridge girders during construction. Bracing in the form of cross-frames and diaphragms are often provided at locations along the bridge length to reduce the unbraced length and increase the buckling capacity. Although they are not currently relied upon for bracing, permanent metal deck forms (PMDF) are frequently used to support the wet concrete for the bridge deck during construction. Similar forms used in the building industry are commonly relied upon for beam bracing. The forms typically behave as a shear diaphragm that restrains the warping deformation in the top flange. The main difference between the forms used in the building and bridge industries are the method of connection. In the building industry, the forms are typically fastened directly to the top girder flange by the shear studs, puddle welds or other mechanical connections. In the bridge industry, the forms are supported on a cold- formed angle that allows the contractor to adjust the form elevation to account for variations in the flange thickness or differential camber between adjacent girders. The support angles lead to eccentric connections that substantially reduce the stiffness of the PMDF systems.