Local Buckling:

The purpose for the above limitations in the effective width of flange or web plates is the phenomenon of local buckling which causes wrinkles in the thin plates subjected to compression in their own plane (Figure 2).

                          (a) Thin Plate Under Compression      (b) End View of the Thin Plate under Compression

Because of this phenomenon, the web (or flange) plate will fail before the failure of the actual composite member. Since buckling depends upon the b/t ratio, the width of these elements is restricted to the values given above. Any width greater than the maximum permissible value is to be ignored in structural computation.