Elastic distortional buckling of doubly symmetric I-shaped flexural members with slender webs

Intermediate length I-section members, particularly those with slender webs and stocky flanges, have been shown to buckle in a distortional mode, in which the web distorts and the flanges displace sideways with less twist than for the lateral–torsional buckling mode. This, in turn, reduces the torsional rigidity of the member, and hence results in a lower buckling strength. The web distortional flexibility is not addressed explicitly in standards for flexural design of steel I–section members. However, in the 2005 AISC specification it has been tried to account for the influence of web distortional flexibility on the lateral–torsional buckling resistance for slender-web I-section members through implicit use of J=0 in the lateral–torsional buckling equations. This paper evaluates the amount of effectiveness of this consideration within the elastic limit. By comparing the AISC code predictions with the accurate finite strip analysis (FSA) distortional buckling solutions as well as the theoretical predictions of two elastic distortional buckling formulae developed by other researchers, it is found that the assumption of J=0 for slender-web members results in overconservative estimate of the buckling strength in some cases which may cause economic losses.