where M1 and M2 are end moments of the unbraced segment and M1 is less than M2. The ratio Mj/M2 is positive for double curvature bending and negative for single curvature bending. Also, if any moment within the segment is greater than M2, Cb is taken as 1.0.

Figure 7.3 shows curves for Fbx for compact and noncompact sections. Note that there are two curves. The first (Fig. 7.3a) is based on the assumption that buckling of the unbraced compression flange is initiated by warping torsion. The second (Fig. 7.3c) shows Fbx controlled by St. Venant's torsion.

The minor direction allowable bending stress Fby is taken as

Fby = 0.60Fy except in the case of compact I-sections where it is taken as Fby = 0.75Fy

This is because a compact I-shape bent about its minor axis does not have to satisfy the criteria for unsupported length since the major axis stiffness provides a continuous lateral support. Fox box sections and rectangular tubes, the allowable bending stress in both the major and minor directions is taken as

Figure 7.3. (a) Values of Fb for compact and noncompact I-sections: Fb versus l/r; Fb controlled by warping torsion; (b) beam section showing area for the calculation of rT; (c) values of Fb for compact and noncompact I-sections: Fb controlled by St. Venant's torsion.

provided the section is compact and the unbraced length ly is less than the greater of

1950 +1200

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