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Minimum Brace Strength and Stiffness
Minimum Brace Strength and Stiffness

1. Columns:

   Relative Bracing:

   Pbr = 0.004 Pr
   ßbr = 2 Pr / (0.75 Lb)

   Nodal Bracing:

   Pbr = 0.01 Pr
   ßbr = 8 Pr / (0.75 Lb)

   Pr = the factored column axial load
   Lb = distance between braces

2. Beam Lateral Bracing:

   Relative Bracing:

   Pbr = 0.008 Mr.Cd / h0
   ßbr = 4 Mr.Cd / (0.75 Lb.h0)

   Nodal Bracing:

   Pbr = 0.02 Mr.Cd / h0
   ßbr = 10 Mr.Cd / (0.75 Lb.h0)

   Mr = factored bending moment
   Lb = distance between braces
   h0 = distance between flange centroids
   Cd = 1.0 for bending in single curvature; 2.0 for double curvature


   The brace stiffness affects the brace force, the larger the brace stiffness, the smaller the brace force

   In above equations, bracing is assumed to be perpendicular to the member to be braced.; for inclined or diagonal bracing, the brace strength (force or moment) and stiffness (force per unit displacement or moment per unit rotation) must be adjusted for the angle of inclination

   Relative brace controls the movement of the brace point with respect to adjacent braced point

   Nodal brace controls the movement at the braced point without direct interaction with adjacent braced points

   Pbr shall be combined with the lateral forces from other resources such as wind or seismic loading

   When Lb is less than Lq, where Lq is the maximum unbraced length for the required column force with K=1, then Lb is permitted to be taken equal to Lq

   Beam lateral bracing shall be attached near the compression flange, except for a cantilevered member, where an end brace shall be attached near top (tension) flange. Lateral bracing shall be attached to both flanges at the brace point near the inflection point for beams subjected to double curvature bending along the length to be braced.

Reference: AISC 14 th Edition
Check Torsional Bracing for Beam (torsional-bracing.xls)