Distortional Buckling of Overhanging Monorails (No. R906)

Abstract

This paper is concerned with the elastic lateral-distortional (LD) buckling of overhanging steel monorail I-beams and its influence on their design strengths. Distortion of a slender web reduces the elastic buckling resistance of an intermediate length beam below its flexural-torsional (FT) resistance. A finite element computer program was used to study the elastic LD buckling of overhanging monorails with bottom flange loads at the free ends. The ratio MLD / MFT of the elastic LD to FT buckling moments decreased significantly as the flange width-thickness ratio bf /tf decreased, but varied only slightly with the web depth-thickness ratio bw/tw. The value of MLD varied almost linearly with the ratio of the supported span to the overhang length, and the effect of top flange torsional restraint at the exterior support on MLD was similar to that on MFT. Distortion effects were very small when there was no torsional restraint, but increased significantly with the torsional restraint stiffness. The full plastic moment was often reached before MLD, especially for low values of bf /tf and high values of the torsion parameter K. Conservative approximations were developed for predicting the elastic LD buckling moments for ranges of sections, slendernesses, span ratios, and torsional restraint stiffnesses.