Toughness requirements for plastic design with structural steel

This report deals with the results of an international research project ”Plastotough”, the aim of which is to discover the toughness requirements for structural steel to make it suitable for plastic design for ”static loads” and for loads that vary over time, as from seismic events. These toughness requirements should be specified as the Charpy energy value KVUS in the upper shelf domain of the toughness-temperature diagram and as the extension of KVUS to the minimum temperature TUS for which it is still valid. This report, after recalling empirical rules for toughness requirements, first gives the methodological tools from fracture mechanics and damage mechanics to tackle the task. The use of these tools is connected with experimental tests on specimens that are detailed such that they are representative of local (from notch effects) and global (from plastic hinges with plastic rotations) ductility demands. The steels for the test specimens were taken from regular ”European production”; rolled sections in particular exhibit a quality level that far exceeds the minimum requirements of EN 10025. For the steels considered, it was proved that they follow the EricksonKirk correlation between the T27J values and TUS so that they can be classified in a group of steels together with pressure vessel steels and naval steels. All strain requirements from local strain raisers and from global plastic rotations could be met on the safe side using the fracture mechanics and damage mechanics resistances determined for these steels. For seismic design in particular it could be shown that the cracking associated with ultra-low-cycle fatigue is not relevant for buildings if the behaviour factor q is determined using reasonable limits for inter-storey drift. Therefore, the key conclusion is that for the safe use of plastic design in ”static” and ”seismic” situations, the elevated level of toughness quality of steels usually available from European producers must be used. To this end it is recommended to specify these steels either in the context of brandnames or by additional options in EN 10025.