Increasing demand from flourishing construction markets led to the successful development of high strength steels (HSS). The new structural steel has exceptional high strength, high fracture toughness, long fatigue life, high corrosion resistance, and better weldability making the material attractive for structural design applications in the modern steel buildings and bridges. With their high strength, typically in the range of 500~700 MPa, and reduced weight/dimensions, it frees imaginations of modern designers and opens up new possibilities. Although HSS cost more, this is more than offset by reduced fabrication and erection costs. The advantage of the intrinsic properties of the HSS makes it possible to achieve successful applications in a cost-effective manner. At present, the Australian steel design standard, AS 4100 (SA, 1998), is limited to conventional low strength steels (LSS) with yield stress less than 450 MPa, (i.e. fy . 450 MPa). As a result steel structural members fabricated from HSS in Australia are usually designed according to overseas specifications, such as AISC-LRFD (AISC, 2003) which allows the design for structures fabricated from HSS materials. However, the design provisions of AISC-LRFD were mainly based on experimental and analytical studies on standard LSS. HSS exhibits mechanical properties that are quite different from conventional LSS. On the other hand, the design procedure and approach of the American specifications (AISC, 2003) are unfamiliar with Australian design engineers, which explains why practising engineers in Australia are reluctant to use AISC-LRFD specification in the design of HSS members. Therefore research into the behaviour of HSS members is essential to address this shortcoming. However, since the use of HSS often leads to smaller sections, hence thinner plates, the elastic and inelastic instability of these thin-walled and HSS members become highly critical. Conservatively, the local instabilities of the constituent plate element interactions in the cross-section have been ignored in the current steel practices. Increasing the slenderness of either plate elements within a cross-section leads to a significant reduction in the section capacity of the structural member. Therefore, the interactive effects between flange and web plate elements have to be considered in the strength, stability and deformation studies of HSS members. Furthermore, the current definitions and values of the plate slenderness limits also vary among major steel design codes (AS4100, 1998; AISC, 2003; EN1993, 2003; BS5950, 2000). The main aim of this research project is to investigate the structural behaviour of Ishaped HSS members subjected to local buckling effects in the elastic and inelastic ranges. For this purpose, it will use advanced numerical analyses and laboratory experiments to study the structural behaviour of these HSS members in compression and bending, respectively. The critical review has found that various inconsistencies among the major steel design specifications (AS4100, 1998; AISC, 2003; EN1993, 2003, BS5950, 2000) in the current practice produce conflicting design predictions of section capacities. The experimental measurements of residual stress distributions have confirmed that the ECCS recommendation (1984) is inappropriate for crosssections fabricated from typical HSS materials (i.e. BISPLATE80). The experimental measurements and numerical studies carried out in this project have produced a better understanding of the structural behaviour of HSS members subjected to local instabilities. The study has enabled to provide a series of proposals for proper assessment of plate slenderness limits for structural members made of HSS materials. It may also enable the inclusion of future version of the AS4100 code for HSS materials to be used in the design of steel building and bridge constructions. It is believed that the use of HSS in building and bridge constructions will increase significantly in the very near future, and to fully-facilitate this, the future versions of national and international steel design specifications must include rational and reliable design rules for members made of all steel grades by including the effects of HSS special characteristics and true interactive local buckling behaviour of HSS members. This research project has contributed towards this.
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