Steel Buildings in Europe

Part 2: Concept Design 2 - 63 6 OTHER DESIGN ISSUES The following design issues will affect the overall building concept, including the structural design aspects. 6.1 Accidental Actions EN 1990 requires that structures be de signed for accidental design situations. The situations that need to be consider ed are set out in EN 1991-1-7, and these relate to both identified acciden tal actions and u nidentified accidental actions. The strategy to be adopted in either case depends on three “consequence classes” that are set out in EN 1990; for buildings, one of those classes has been subdiv ided and th e catego ries of building in each class are set o ut in EN 1991-1-7, Table A.1. For identified accid ental actions, desi gn strategies in clude protectin g th e structure against the action but more generally, and for unidentified actions, the structure should be designed to have an appropriate level of “robustness”, defined as: “The ability of a structure to withstand events like fire, explosions, impact or the consequences of human error, without being damaged to an extent disproportionate to the original cause.” For unidentified actions, the strategy fo r achieving robustness is set out in EN 1991-1-7 § 3.3, which says that: “… the potential failure of the structure arising from an unspecified cause shall be mitigated … by adopting one or more of the following approaches: a) designing key elements on which the stability of the structure depends, to sustain the effects of a model of accidental action Ad; b) designing the structure so that in the event of a localised failure (e.g. failure of a single member) the stability of the whole structure or of a significant part of it would not be endangered; c) applying prescriptive design/detailing rules that provide acceptable robustness for the structure (e.g. three dimensional tying for additional integrity, or a minimum level of ductility of structural members subject to impact).” 6.1.1 Consequence classes EN 1990 defines three consequences classes:  CC1 Low consequences of failure  CC2 Medium consequences of failure  CC3 High consequences of failure