Determination of fatigue strength: applying code rules on actual structures

Applying the often used Eurocode/EN rules for fatigue strength of steel structures in connection to finite element (FE) calculations poses in general a lot of questions. The rules are largely based on so called nominal stresses, defined as “stress in the parent material or in a weld immediately adjacent to a potential crack location, calculated in accordance with the theory of elasticity, without considering effects of stress concentrations”, with additional note that the nominal stress may be “a normal stress, a shear stress, a principal stress or an equivalent stress”. Furthermore, the fatigue curves are based on connection designs combined with loading directions and even failure modes.

?In an actual structure (or, more or less equivalent, a FE model), the required nominal stresses are often difficult to determine. Furthermore, it is mostly rare that a match is found between the combination of actual geometry/loading and one of the fatigue curves. However, where Eurocode lacks solutions, DNV and ABS (American Bureau of Shipping) give some additional guidelines in these cases.

?The VIRO Engineering Analysis department developed a protocol based on a combination of these rules and guidelines, which allows to assess the fatigue life of steel constructions using FE calculations in a cost/time effective way. It is based on the theoretical knowledge from the rules, but also on practical experience using FE models. The protocol quickly filters-out the details that are prone to fatigue damage and additional detailed screening steps are used for more in-depth analysis if necessary. The whole protocol is discussed beforehand so that no surprises occur during the process and the steps are clear.

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