The European technical rules for the design of steel structures (Eurocode 3) offer a method to select steels to avoid brittle fracture. This concept is available for members subject to fatigue loads and is based on a safety assessment using fracture mechanics, applying stress intensity factors (K-concept). In order to extend these rules to members with predominant static loads, investigations have been carried out for typical details as used in buildings, both experimentally and theoretically. The details investigated were welded connections of solid bars or I-profiles with slotted gusset plates. The experimental studies included large scale tests using test specimens with initial cracks at locations of high stress concentrations. For the theoretical studies, the K-concept was applied. Formulae for K-requirements were developed for such details with numerical simulations using BE- (Boundary Element-) methods. The results allow an extension of the rules in Eurocode 3.