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Flexural-Torsional Buckling

This project is concerned with the extension of practical advanced analyses of the in-plane behaviour of 2-D steel frames under in-plane loading to out-of-plane behaviour. These have been largely unsuccessful in the past because of difficulties in modelling the effects of yielded zones, load heights, interactions between twist and axial force and moments, and end warping restraints. Practical advanced analyses of out-of-plane behaviour need to be able to account for the influences of moment and axial force distributions, load heights, and end restraints on elastic and inelastic lateral buckling, and to be consistent with the code formulations of beam and column out-of-plane strengths.

It is proposed in first part of this project that the advanced analysis of 2-D frames for which local buckling is prevented be simplified by first carrying out an in-plane analysis using one of the presently available plastic hinge methods, and then by using a practical advanced analysis of the out-of-plane behaviour which is based on an inelastic lateral buckling analysis which includes allowances for residual stresses and initial crookednesses and twists.

The second part of the project develops a simple advanced method of designing steel members against out-of-plane failure, in which reduced elastic moduli are used in an out-of-plane buckling analysis to model the effects of high moment, residual stresses and geometrical imperfections on yielding. The reduced moduli are derived from the basic beam and column strength curves of the Australian steel code AS 4100 (SA, 1998).

The strengths predicted for simply supported beams in uniform bending are exactly the same as those of AS 4100, while those for simply supported columns are extremely close. The strengths predicted for simply supported beam-columns with equal and opposite end moments are a little higher than the less conservative predictions of AS 4100, and are very close to the basic beam and column strengths when these are plotted against a consistent generalized slenderness. The strengths predicted for simply supported beams under double curvature bending are somewhat less than those of the AS4100 method of design by buckling analysis, while those for beams with central concentrated loads acting at or away from the shear centre are very close, and those for end restrained beams under uniform bending and for sway columns are generally a little higher.

While the method has been developed from and compared with the Australian code AS 4100, it may be modified for any other modern code for the design of steel structures. It may be more widely applied to two-dimensional frames with in-plane loading, as part of a simple method of advanced analysis in which separate assessments are made of the in-plane and out-of plane strengths.

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