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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)    LRFD

ßbr = 2 (2 Pr / Lb)    ASD

Nodal Bracing:

Pbr = 0.01 Pr

ßbr = 8 Pr / (0.75 Lb)    LRFD

ßbr = 2 (8 Pr / Lb)    ASD

Pr = required axial compressive 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)    LRFD

ßbr = 2 (4 Mr.Cd / Lb.h0)    ASD

Nodal Bracing:

Pbr = 0.02 Mr.Cd / h0

ßbr = 10 Mr.Cd / (0.75 Lb.h0)    LRFD

ßbr = 2 (10 Mr.Cd / Lb.h0)    ASD

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

Notes:

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