Efficient lifting equipment with extra high-strength steel

The over-all objective of the research project Efficient Lifting Equipment With Extra High Strength Steel (LiftHigh) was to obtain a better utilisation of extra high strength steels, in this context defined as steels with yield strengths in the interval 460 - 1100 MPa, and thereby expand the limits for structural steels available for crane manufacturers as well as for other applications. The work within the project has included actions focusing on the fatigue resistance, the resistance to global and local buckling and development of methods for advanced design with non-linear FEM. A specific aim for the project was to show that using extra high strength steel makes it possible to obtain a 20-40 % reduction of weight for a given function. This aim is in accordance with the desire amongst the crane users, which also was proven by an enquiry performed in the early beginning of the project. The enquiry showed that the opinion among crane users was that a decreased crane weight should be the first priority for future development. Examples on how to use FEM for design are given in Chapter 5 of this report, covering generation of finite element models, determination and application of appropriate geometrical and structural imperfections, conduction of static calculations and evaluation of the results. In Chapter 6 it is shown, by redesign of an existing trolley structure and a telescopic boom, that it is possible to achieve a substantial weight reduction by using extra high strength steel. In these two particular examples the weight reductions were 38 % and 18 %. Concerning global and local buckling and fatigue, the conclusions are summarised in Sections 7.2 and 7.3. For global and local buckling, the experimental and theoretical investigations has covered cold formed rectangular and circular hollow sections, as well as welded square hollow sections. In short, the results obtained gives no indications that the design models already in use, for instance in EN 1993-1-5, for steels with regular yield strengths can not be applied also for high strength steels with yield strength up to 1100 MPa. However, regarding welded plates, the design rules in EN 1993-1-5 seem to slightly overestimate the resistance (concerning a modified slenderness greater than 0.9). For fatigue, it is concluded that the investigated steel grades fit to the detail categories given in EN 1993-1-9 and can be classified as good as mild steels although, for some details differences appear in the calculated slope of the S-N-curves. For these details a classification with slope m = 4 can be considered in EN 1993-1-9. As it is known from former investigations, the fatigue resistance is influenced by the design of the weld detail on the one hand and the quality and the execution of the weld on the other hand. In constructions where fatigue is the decisive factor, elevated concentration should be laid on the geometry of details and quality of the weld. In this context advantage can be also achieved using post weld treatment. Nevertheless, it has to be mentioned that with the same stress variation range, the fatigue loads of high-strength steels can be applied on a correspondingly higher level than it is possible for mild steels. This fact does not arise from the classification in detail categories, as it is carried out in EN 1993-1-9.