Abstract :
[en] The cross-sectional capacity is limited by material yielding on one hand and local buckling on the other hand. Most current metallic structural design codes use the concept of section classification for determining the section capacity of hot-rolled profiles. In this system, there exists a discontinuity between the classes 2 and 3 under pure bending load condition, while for the class 4, the effective width method (EWM) must be used to account for local buckling phenomenon. With the appearance of the more complex cross-sections, especially for thin-walled steel structures, the calculation using EWM becomes more and more intolerable and complicated. In order to overcome these difficulties when evaluating the cross-section strength, the alternative Direct Strength Method (DSM) approach, developed only for cold-formed thin-walled members, has gained importance over the last few years, and has been adopted by the North American and Australian standards for the design of cold-formed steel structural members recently. This thesis investigates the possibility to extend the DSM method to hot-rolled and welded structural steel members, and the objective is to derive a new formulation that would cover the whole field of section slenderness without section classification system used in Eurocode 3, using the same procedure for the DSM developed for cold-formed thin-walled steel structural members. Hot-rolled and welded H sections are mainly considered in this thesis.
The DSM is premised on the assumption that the strength can be predicted from the yield strength to the elastic critical load ratio used in conjunction with a strength curve; i.e. the same concept than the one used in Eurocode 3, in terms of member buckling phenomenon. In the DSM approach applied for cold-formed thin-walled sections, local buckling and distortional buckling modes should be considered. For common hot-rolled or welded sections, only local buckling needs to be considered since distortional buckling does not occur for these sections.
All available test results in literature are collected and evaluated with the current methods for calculating the cross-section resistance such as EWM, DSM, and the recent Continuous Strength Method (CSM). A short comparison is made and their domains of applicability are commented. A consistent approach has been adopted to develop numerical models for hot-rolled and welded sections subjected to compression and bending respectively. The FE model was built in FineLg and verified against test results of the “Semi-comp: Plastic member capacity of semi-compact steel sections - a more economic design” project, which was carried out earlier with the objective of improving the utilization of plasticity in rolled and welded sections with medium cross-section slenderness. Measured material and geometric properties, initial geometrical imperfections and residual stresses distributions have been highlighted. Once verified against the test results, the FE models have been used to carry out a parametrical study aimed at providing more FE results and highlighting the local buckling interaction between the webs and the flanges.
All the experimental and FE numerical results constitute data base for the derivation of the new DSM curves. On the basis of Winter Formula for plate buckling and the generic DSM strength curve, different formulas are proposed for hot-rolled and welded H cross-sections under axial and bending load respectively. Also the statistical evaluation and safety study is conducted to obtain the safety factor γ_M, which is required for practical design.
