| Concentrated loads from wheel loads or gantry girders, purlins or beams onto the main frame members of buildings, and roller loads during launching of plate or box girders can cause local failure of the web into the form of elastic or inelastic web instability (web crippling) or web yielding. Since the early 1930's, about 653 tests have been conducted on built-up rolled sheet steel shapes and welded plate girders to determine the effects of web thickness, flange rigidity, load eccentricity and material properties on the web's capacity. Many formulae have been developed from this work for determining the ultimate capacity of the web and over the years, the empirical and semi-empirical expressions by Graham et al (1959) and Roberts (1981) have been adapted and adopted as design rules by specifications and standards in North America. The modified Roberts solution for web crippling contains a contribution to web buckling capacity from both the flange and the web where only the contribution of the web should be accounted for as the flanges may be fully utilized in bending. It was developed for interior loads only and is based on section sizes and failure mechanisms that bear no resemblance to the range of W and WWF shapes produced by Canadian and American mills. The lack of a unified approach to describe the web capacity for a range of behaviour (including elastic web buckling which is currently not existent) and the application of the modified Roberts solution that is suspect have generated the need for this research.; Therefore, the objective of this study was to develop rational expressions to predict the ultimate load-carrying capacity of webs of these rolled shapes. A state-of-the-art review is presented along with a summary of an analytical finite element study of the web crippling behaviour of a wide range of rolled W and WWF shapes. Based on the numerical results, the equivalent column concept was developed, and subsequently verified experimentally for both interior loads and end reactions, for a large range of W shapes. Use of the equivalent column concept and the equivalent slenderness parameter, {dollar}lambdasb{lcub}e{rcub}{dollar}, in one of the column curves given in Clause 13.3.1 gives a simple, easy to comprehend, and unified approach for determining the web yielding, inelastic and elastic web buckling capacity of sections subjected to concentrated loads. The mean test to predicted ratio and the corresponding coefficient of variation for the proposed design rules, with the simplified K value of 0.6 which represents realistic load and boundary conditions, are 1.063 and 0.098, respectively. |
