Structural performance and analyses of S960 ultra-high strength steel welded I-section members

Abstract

Ultra-high strength steel grade S960, with the nominal yield stress of 960 MPa, has been gaining increasing attention and possesses high potential to be extensively utilised in high-rise and long-span structural applications, primarily due to its unique mechanical advantage of superior strength-to-weight ratio, which provides the possibility of design structural members with the reduced dimensions and self-weights. However, its actual use is restricted by the lack of appropriate design rules, since the current international design standards can only be applicable to steel components with material grades lower than S700. The behaviour and design analysis of S960 ultra-high strength steel welded I-section components under different loading cases are already out of their scopes and therefore the focus of this PhD thesis.

A comprehensive testing programme was firstly undertaken, which adopted eight welded I-sections made of S960 ultra-high strength steel. Material testing was conducted to derive the material stress–strain responses of S960 ultra-high strength steel, and membrane residual stresses were measured to determine the residual stress patterns and amplitudes in S960 ultra-high strength steel welded I-sections. At the cross-section level, a total of 16 concentrically loaded stub column tests and 20 eccentrically loaded stub column tests were carried out to investigate the cross-section resistances and local stability of S960 ultra-high strength steel welded I-sections. At the member level, 20 column tests, 8 beam-column tests as well as initial geometric imperfection measurements were conducted to study the global instability and resistances of S960 ultra-high strength steel welded I-sections. The testing programme was supplemented by a numerical modelling programme, where finite element models were initially developed and validated against the test results and then adopted to conduct the extensive parametric studies to generate further numerical data over a wider range of cross-section dimensions, member lengths and loading combinations. The obtained test and numerical results were used to evaluate the applicability of the current design rules for S690 high strength steel welded I-section members under different loading cases to their S960 ultra-high strength steel counterparts (i.e. assessments of slenderness limits for cross-section classification, effective width expressions, column buckling curves and load-moment interaction curves). The evaluation results which demonstrate the applicability of the existing codified rules, and the newly proposed design methods provide the basis for the future revisions of international design codes for high strength steel structures.