Bracing Requirements for Elastic Steel Beams

Two types of bracing are studied to control the lateral-torsional buckling of steel beams; namely, lateral bracing at the compression flange and torsional bracing. A computer program, BASP, was used to study the effects of brace type, size and number of braces on the buckling strength of beams subject to different loading conditions. It was found that cross section distortion at the brace point significantly affects the efficiency of torsional bracing. Properly attached web stiffness can minimize the distortion.

Based on the analytical solution two bracing equations are developed; one for lateral bracing and one for torsional bracing. The torsional bracing formula accounts for cross section distortion and the presence of web stiffness. Top flange loading requires larger bracing members and the equations consider the effect as well as the variation of moments along the span.

Seventy-six lateral buckling tests were conducted on twin W12X14 beams with a 24-ft. span with a top flange concentrated load at midspan to compare with the analytical solution and the bracing equations. Lateral bracing or torsional bracing of different magnitudes were attached to the beams to determine the required bracing to force the buckle between the end and the midspan. Different beam initial out-ofstraightness and web stiffeners were considered. It was found that the bracing equations and buckling program, BASP, compare very well with the test results.