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4.3. Stability Bracing

4.3.1. What constitutes a lateral brace for a beam?

The designer may use one or both of two general options to provide a beam brace: (1) brace the flange subject to compression directly or (2) prevent twist of the cross section. A direct brace may be provided for a primary member by a properly attached floor system itself or by a secondary framing member. Generally, a brace connection, such as the simple shear connection for an infill beam, that is located within the one-third depth of the beam web that is closest to the compression flange can be considered to provide a direct brace. If this is not the case, transverse stiffening can be provided to prevent twist and transfer the bracing effectiveness from a properly attached floor system to the compression flange.

A long-standing rule of thumb is to provide bracing for two percent of the compressive force in the flange or member being braced. Although it lacks an explicit consideration of the required bracing stiffness, this approximation is typically conservative. Note that the two percent rule applies only to compression members that are considered straight within ASTM tolerances. AISC Specification, Appendix 6, addresses requirements for stability bracing of beams and columns.

Yura, J.A. and T.A. Helwig, 2001 Lecture notes for SSRC/AISC seminar “Bracing for Stability”, AISC, Chicago, IL.

Yura, J.A, 2001, “Fundamentals of Beam Bracing,” Engineering Journal, Vol. 38, No. 1, (1st Qtr.), pp. 11-26, AISC, Chicago, IL.