325 Column or Frame Bracing

Column or frame bracing systems can be relative, nodal, continuous, or lean-on. The design recommendations for relative and nodal column bracing are based on an initial out-of-straightness Do = 0.002L, where L is the column length and a brace stiffness equal to twice the ideal stiffness. The initial displacement, Do, is defined as the lateral offset between two adjacent brace points caused by sources other than brace elongations from gravity loads or compressive forces. For example, Do may be a displacement due to wind or other lateral forces, erection tolerances, or column shortening. If Do differs from 0.002L, the brace force, Pbr, will change in direct proportion to the actual Do. In frame systems where a story may contain n0 columns, each having a random out-of-plumbness, an average value for Do can be used [4]

32.5.1 Column Buckling and Design Philosophy

For no-sway columns, the effective length factor, K, will be less than 1.0. Most designs conservatively use K = 1.0 and in most situations achieving K< 1.0 is not economical. For sway permitted columns, the effective length K> 1.0. Bracing used to prevent sway can reduce the effective length to K = 1.0 and achieve significant economic savings.

The bracing design criterion for columns is based on providing sufficient strength and stiffness to allow a sway column to achieve the Euler buckling load corresponding to K = 1.0. For columns that possess nonzero end restraint, this does not correspond to the no-sway buckling load since for these cases K is theoretically less than unity.

For flexural buckling modes, brace points attached in-line with the centroid of the structural member are most effective. For torsional buckling modes, bracing must prevent twist of the cross-section to be effective.

32.5.2 Relative Systems

AISC LRFD brace requirements for relative column bracing are

r fLb

where f = 0.75, Pu is the required compressive strength of the column, and Lb is the required brace spacing.

When the actual brace stiffness provided, bact, differs from the required value given in Equation 32.4, the brace strength requirement can be modified using pbr = °.°04£ Pu^7b1

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