Toughness requirements for plastic design with structural steel

Abstract

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.

Investigation of the cyclic inelastic capacity of steel beams through the use of the plastic collapse mechanism

“…Considering the maximum effective number of cycles, it is obvious that the effects of the cycles produced by seismic excitation belong to the lower limit of the ultra-low-cycle fatigue (Kanvinde and Deierlein 2004). As a consequence, the damage curves for earthquake effects are similar with the one of monotonic loads (Feldman et al 2011). The low cycle fatigue is characterized by a slope, corresponding to the reducing effects of the cyclic loading.…”

“…The fracture of the buckled flanges of the wide flange steel sections under a cycle loading has been studied by Yamada et al (1988); Kanvinde and Deierlein (2004); Feldman et al (2011). The fracture strains decrease when the number of cycles increases due the low cycle fatigue.…”

“…4). Recently, a new category, namely, ultra low cycle fatigue (ULCF) (named also extremely low cycle fatigue, ELCF) is proposed (Xue 2008;Feldman et al 2011;Kanvinde and Deierlein 2004), thus recognizing the importance of the reduced number of cycles. Moreover, Kanvinde and Deierlein (2004) based on ULFC models simulated the inelastic earthquake induced fractures by emphasizing the ductile crack initiation due to monotonic and the cyclic loading for less than ten seismic cycles.…”

Current Steel Structures Research No. 30

“…It can be shown that the cracking associ ated with ultra-low-cycle fatigue is not relevant for buildings, as long as the behavior is determined using reasonable limits for inter-story drift. For seismic issues these material performance investigations demonstrate that an elevated level of toughness is an appropriate solution (Feldmann et al, 2011a). Bridges with integral abutments are characterized by two unique features.…”