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I have a W-shape steel beam that is fixed-fixed and the top flange is fully braced for lateral torsional buckling. The negative moment at the end controls the design and there is no lateral bracing for the bottom flange. What will be the unbraced length ($L_b$) for the bottom flange? Also, if I provide brace points at the points of moment inflection then what will $L_b$ be? In general, what is the rule for measuring $L_b$ for bottom flange under negative moment? If possible, please provide proof from AISC code.

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  • $\begingroup$ I would suggest examining Euro codes for this kind of design. $\endgroup$ – Rhodie Jan 20 at 3:29
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On page 16.1-306 of AISC 360-10... "The unbraced length is defined as the spacing between locations where twist is restrained." Without the addition of any bottom flange bracing, this would imply that the unbraced length is the distance between the supports.

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