47 Biaxial Bending

Members subjected to bending about both principal axes (e.g., purlins on an inclined roof) should be designed for biaxial bending. Since both moment about the major axis Mux and moment about the minor axis Muy create flexural stresses over the cross-section of the member, the design must take into consideration these stress combinations.

4.7.1 Design for Biaxial Bending Allowable Stress Design

The following interaction equation is often used for the design of beams subject to biaxial bending

Sx Sy

where Mx, My are service load moments about the major and minor beam axes, respectively, Sx, Sy are elastic section moduli about the major and minor axes, respectively, and Fy is the specified minimum yield stress.

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