09.13: The right choice of steel: Towards an optimized choice of the steel grade for toughness assessment

The choice of steel is generally ruled in EN 1993‐1‐1: 2005. The standard specifies several requirements for the choice: the material properties, the ductility, the toughness properties and the through‐thickness properties. The simplified design rules in the different parts of EN 1993 are based on the principle of limit state design. It assumes that the resistance of cross‐sections and members are determined in tests which comply with product standards (e.g. EN 10025‐4) and execution standards (e.g. EN 1090‐2: 2008) and which are evaluated according to EN 1990. Since the tests are conducted at room temperature, the design rules and the safety concept apply to the upper shelf region of the toughness‐temperature curve where the steel behaves ductile with sufficient toughness. Hence, the design rules only cover ductile failure modes and are only related to the material properties. For brittle failure in the lower shelf region, the assumptions for the design rules and safety concepts are no longer met. Therefore, brittle failure is accounted for with the right choice of the steel with sufficient toughness (Table 2.1 in EN 1993‐1‐10: 2005). This two‐way safety assessment ensures appropriate safety of steel structures in the full temperature application range. Table 2.1 has been mainly derived for welded construction details in bridges subject to fatigue. In consequence, the rules in EN 1993‐1‐10: 2005 could be extremely conservative if applied to elements subject to quasi‐static loading. According to EN 1993‐1‐10: 2005, an alternative approach to Table 2.1 consists in the application of fracture mechanic theory. However, the procedure in the standard is only generally described. Therefore the approach is rarely used and Table 2.1 is applied, so to speak, for elements in buildings and bridges. Finally, this often leads in general structural applications to an uneconomic choice of steel for toughness assessment. This paper gives guidance on the right choice of the steel grade according to the Eurocode and highlights the advantages of thermomechanical steels for each criterion. In addition, a new table for the choice of the steel grade for elements under compression and subject to quasi‐static loads will be proposed in this paper. The table is based on the same fracture mechanic approaches and standard detail than Table 2.1, but with the difference that crack growing is adapted to quasi‐static loading.