The Bracing Requirements of Steel Beams of Intermediate Slenderness

Steel beams, whether rolled or built-up, contain unavoidable initial imperfections and residual stresses and are subject to unintentional eccentricity of applied loading. Such beams which also possess inadequate lateral restraint are prone to failure as a result of lateral-torsional instability, which occurs under elastic or inelastic conditions depending on the slenderness of the member. A review of the literature pertaining to the bracing requirements of steel beams revealed little published work concerned with the restraint of beams of intermediate and low slenderness which fail inelastically. The provision of adequate midspan restraint for the prevention of inelastic instability in centrally loaded, single span I-beams formed the basis of this study. The non-linear analysis capabilities of the finite element programmes MSC/NASTRAN and FINAS were employed to provide theoretical verification of the results of a series of tests on small-scale, fabricated, steel I-beams. Measured initial geometrical imperfections of the test beams were modelled in the finite element idealisation by suitable adjustment of nodal coordinates and both geometrical and material non-linearites were accounted for in the analysis. Numerical instability and convergence difficulties were encountered in both analyses, although their occurrence was less frequent in FINAS. In FINAS analyses where these difficulties did not arise, collapse loads were determined and post-buckling behaviour followed with relative ease. A bracing fork device for the provision of a predetermined stiffness of midspan restraint was developed and subsequently employed in all tests. Strain gauges attached to the prongs of this device permitted bracing forces to be measured at any stage in the tests. In general, satisfactory correlation was achieved between finite element and experimental results, allowing bracing criteria for single span, centrally loaded and restrained beams to be proposed. As anticipated, the bracing requirements of inelastic beams proved more onerous than those demanded by the classical bifurcation analysis employed in problems of elastic beam buckling. A subsequent series of comparative designs in accordance with the three current (1985) British steelwork codes (BS 449, BS 5950 and BS 5400) revealed that bracing members designed as struts in compliance with the minimum strength and maximum slenderness criteria of these documents provided adequate stiffness and strength of restraint