Strength of compact steel beams with partial restraint

Compact steer beams with continuous and complete tension flange restraint a re not affected by conventional lateral-torsional buckling, which is based on the Vlasov assumption of the cross-section of the beam remaining rigid w hen the beam buckles. Because of this, "conventional" theories for the stre ngth of beams governed by overall buckling modes predict that the beam will reach its full plastic moment capacity. Beams with full restraint of the t ension 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 requi red to reach full plastification, significantly so in some cases. The paper addresses this issue by using a special-purpose inelastic finite element m ethod of analysis to study the buckling of beams with continuous and comple te tension flange restraint. For beams with low degrees of moment gradient, it is shown that the inelastic critical moment may be substantially less t han the full plastic moment. Incorporating the results into the usual ratio nal design methods allowed for in BS5950 and AS4100 is difficult, and a new design proposal that is generally conservative and based on AS4100 is sugg ested. (C) 2000 Elsevier Science Ltd. All rights reserved.