Foster, ASJ; (2014) Stability and design of steel beams in the strain-hardening range. Doctoral thesis , Imperial College London.

Abstract

Many of the principal concepts that underpin current metallic structural steel design codes, notably Eurocode 3, were developed on the basis of elastic, perfectly plastic material behaviour, essentially ignoring strain-hardening; such material behaviour lends itself to the concept of discrete cross-section classification. A newly proposed, deformation based approach to structural steel design represents an alternative treatment to cross-section classification that is based upon a continuous relationship between cross-section slenderness and deformation capacity, as well as a rational exploitation of strain- hardening. This method is referred to herein as the Continuous Strength Method. The aim of this research is to develop preliminary guidance for the use of the Continuous Strength Method at the member level, focusing on the behaviour of simply supported and continuous beams. Particular attention will be given to determining the maximum laterally unsupported lengths prior to which the full capacity predictions of the Continuous Strength Method can be achieved, as well as the performance of lateral bracing elements in various structural configurations. Through a programme of experiments, numerical modelling and parametric studies, the implications of allowing for strain-hardening in the design of laterally restrained steel beams is investigated with particular emphasis on the performance of the bracing elements. A total of fourteen tests on simply supported beams and six tests on continuous beams were performed considering two basic scenarios: discrete rigid restraints and discrete elastic restraints of varying stiffness. In all tests, bending resistances in excess of the plastic moment capacity were observed, but generally it was concluded that closer restraint spacing than specified in current design codes to achieve the cross-section capacity may be required to harness significant benefit from strain-hardening and to develop the full CSM bending resistance. The forces generated in the restraints were within current code requirements although some modifications were suggested. Furthermore, the spacings of the restraints were also considered and a new limiting slenderness and transition curve for the CSM was proposed. The results from the experiments were supplemented by parametric studies conducted using analytical and numerical models developed as part of this thesis, as well as through the use of proprietary software packages. A parallel experimental investigation into the material modelling assumptions of the Continuous Strength Method was also conducted, employing an innovative full cross-section tensile test to capture average cross-section material properties. The results from the investigation validated the modelling assumptions of the Continuous Strength Method and improved the accuracy of the predictive capacity equations.

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