Strength of compact steel beams with partial restraint

Abstract

Compact steel beams with continuous and complete tension flange restraint are not affected by conventional lateral-torsional buckling, which is based on the Vlasov assumption of the cross-section of the beam remaining rigid when the beam buckles. Because of this, “conventional” theories for the strength of beams governed by overall buckling modes predict that the beam will reach its full plastic moment capacity. Beams with full restraint of the tension flange, however, buckle in a lateral-distortional mode, in which the web must distort in order for the compression flange to displace and twist during buckling. The buckling moment can therefore be less than that required to reach full plastification, significantly so in some cases. The paper addresses this issue by using a special-purpose inelastic finite element method of analysis to study the buckling of beams with continuous and complete tension flange restraint. For beams with low degrees of moment gradient, it is shown that the inelastic critical moment may be substantially less than the full plastic moment. Incorporating the results into the usual rational design methods allowed for in BS5950 and AS4100 is difficult, and a new design proposal that is generally conservative and based on AS4100 is suggested.