Elastic Instability of Flanges and Webs:

Failures Due to Buckling

Such as any compression member as explained in the previously, the compression flanges and webs of beam may also fail due to buckling for which the following is recommended.

For Flanges

The overhang of the compression flange of a beam may buckle locally. To prevent this width/thickness ratio of outstanding legs is limited to 16 for structural steel with f_y = 250 MPa.

For Webs

The web of beams may fail either due to shear buckling or flexural buckling. The former is caused due to a state of pure tension/compression existing along 45^o planes. The code IS: 800 recommends d₁/t ratio 85 for steel with f_y = 250 MPa (for steel with f_y = 340 MPa and 400 MPa this ratio is 75 and 67 respective).

For larger ratios of d₁/t vertical stiffener has to be provided.

Flexural buckling is caused by the triangular compressive stress due to bending and d₁/t ratio is kept at 200. For ratios larger than these, horizontal stiffeners are to be provided. Such cases occur only in deep plate girders, etc. For general rolled steel sections such cases rarely occur. Therefore, a check must be made, although designing.