13

Therefore, use a W21 x 62 section.

4.7.1.2 Load and Resistance Factor Design

To avoid distress at the most severely stressed point, the following equation for the limit state of yielding must be satisfied:

where fun — (MuxISx + MuyISy) is the flexural stress under factored loads, Sx, Sy are elastic section moduli about the major and minor axes, respectively, fb — 0.90, and Fy is the specified minimum yield stress.

In addition, the limit state for lateral torsional buckling about the major axis should also be checked, that is, fbMnx > Mux (4 . 80)

fbMnx is the design flexural strength about the major axis (see Section 4.5). To facilitate design for biaxial bending, Equation 4.79 can be rearranged to give

fbFy fbFy W fbFy fbFy V bfJ

In the above equation, d is the overall depth and bf the flange width of the section. The approximation (Sx/Sy) « (3.5d/bf) was suggested by Gaylord et al. (1992) for doubly symmetric I-shaped sections.

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