BRACING OF STEEL BEAMS IN BRIDGES. FINAL REPORT

The report summarizes the research conducted to evaluate the lateral bracing effect of bridge decks. Design requirements for bracing steel beams to control lateral-torsional buckling are developed. Braces are classified as torsional braces (diaphragms, cross frames) or lateral braces (top chord laterals, bridge decks). Analytical studies were conducted which investigated the effect of brace type, size, location, and number of braces on the lateral buckling of beams subject to different loading conditions. Results of tests conducted on twin beams compared favorably with the analytical solution. Both the tests and the theoretical solution showed the cross section distortion had a significant effect on torsional braces. The tests with no bracing or full bracing compared favorably with the new AASHTO lateral buckling formulas. From the tests and the theory, simple formulas for brace force and brace stiffness are presented which are suitable for design and specifications. For lateral bracing, the brace force is 0.8% of the beam force, which includes some provision for beam out-of-straightness. For torsional bracing, the design formulas can be used to determine web stiffener requirements to control cross section distortion. Five design examples illustrate the use of the bracing formulas.