Bending of the beam shown in Fig. 3.90a produces compressive stresses within the upper portion of the beam cross section and tensile stresses in the lower portion. Similar to the behavior of a column (Art. 3.41), a beam, although the compressive stresses may be well within the elastic range, can undergo lateral buckling failure. Unlike a column, however, the beam is also subjected to tension, which tends to restrain the member from lateral translation.
Hence, when lateral buckling of the beam occurs, it is through a combination of twisting and out-of-plane bending (Fig. 3.90b).
For a simply supported beam of rectangular cross section subjected to uniform bending, buckling occurs at the critical bending moment
As indicted in Eq. (3.170), the critical moment is proportional to both the lateral bending stiffness EIy /L and the torsional stiffness of the member GJ/L.
For the case of an open section, such as a wide-flange or I-beam section, warping rigidity can provide additional torsional stiffness. Buckling of a simply supported beam of open cross section subjected to uniform bending occurs at the critical bending moment
where Cw is the warping constant, a function of cross-sectional shape and dimensions (see Fig. 3.89).
In Eq. (3.170) and (3.171), the distribution of bending moment is assumed to be uniform.
For the case of a nonuniform bending-moment gradient, buckling often occurs at a larger critical moment. Approximation of this critical bending moment Mcr may be obtained by multiplying Mcr given by Eq. (3.170) or (3.171) by an amplification factor:
Cb equals 1.0 for unbraced cantilevers and for members where the moment within a
significant portion of the unbraced segment is greater than or equal to the larger of the
segment end moments.
