Critical buckling strains of round tubes in flexure

The results of a series of 11 tests on single and two span circular hollow tubular beams are used to compare with inelastic bending and axial compression theories of buckling. Predictions of critical strain from both analyses exceed the values obtained from the experimental results, the bending critical stress being somewhat higher than that for axial compression. Due to its simplicity, the latter case is used as a model to assess the form of the diameter-thickness ratio (Dt) limitation for the various categories of design. For given Dt, the critical buckling strain is found to be inversely related to yield stress to a power exponent n which generally lies between 0·5 and 1. Since n tends to unity for steels possessing nearly flat yield plateaus and high Dt, the CSA-S16.1 (1974) prescriptions for Dt limitations for compact and non-compact sections are reasonable. A slightly lower value of n for plastic design sections may, however, be appropriate.