Steel Buildings in Europe

Part 4: Detailed Design 4 – 49 6.3 Design for the consequences of localised failure in multi-storey buildings 6.3.1 Design strategy In multi-storey buildings, the requirement for robustness generally leads to a design strategy where the columns are tied into the rest of the structure. This should mean that any one length of column cannot easily be removed. However, should a length be removed by an accidental action, the floor systems should be able to develop catenary action, to limit the extent of the failure. This is illustrated diagrammatically in Figure 6.2. The recommendations in EN 1991-1-7 [13] , Annex A in relation to horizontal tying actions and vertical tying actions are related to this form of partial collapse. Annex A does not prescribe a complete design model for this form of partial collapse – for example, the reaction to the horizontal forces in Figure 6.2 is not addressed. The rules in the Annex are best considered as prescriptive rules intended to produce structures that perform adequately in extreme circumstances and are not meant to be fully described systems of structural mechanics. The illogical practice of designing certain connections for considerable force, yet not making provision to transfer the forces any further, illustrates this point. It is important to note that the requirements are not intended to ensure that the structure is still serviceable following some extreme event, but that damage is limited and that progressive collapse is prevented. 1 1 Column removed Figure 6.2 Concept of robustness rules 6.3.2 Limit of admissible damage The limit of admissible damage recommended in EN 1991-1-7, Annex A is shown in Figure 6.3.