When compression is induced in an element of a cross section, e.g., a beam or column flange or web, that element may buckle. Such behavior is called local buckling. Provision to prevent it should be made in design because, if it should occur, it can impair the ability of a member to carry additional load. Both the AISC ASD and LRFD specifications for structural steel buildings recognize the influence of local buckling by classifying steel sections as compact, noncompact, or slender-element.
A compact section has compression elements with width/ thickness ratios less than p given in Table 6.29. If one of the compression elements of a cross section exceeds p but does not exceed r given in Table 6.29, the section is noncompact. If the width/ thickness ratio of an element exceeds r , the section is classified as slender-element.
When the width/ thickness ratios exceed the limiting value r , the AISC specifications require a reduction in the allowable strength of the member.
The limits on width/ thickness elements of compression elements as summarized in Table 6.29 for LRFD and ASD depend on the type of member and whether the element is supported, normal to the direction of the compressive stress, on either one or two edges parallel to the stress. See note a in Table 6.29.
In seismic applications, refer to the width/ thickness ratio limitations provided in the AISC Seismic Provisions for Structural Steel Buildings