Steel Buildings in Europe

Part 4: Detailed Design 4 – 53 In some circumstances, the floor units cantilever past the edge beam. Tying in these situations is not straightforward, and a solution must be developed in collaboration with the frame supplier and floor unit manufacturer. 6.3.5 Vertical tying EN 1991-1-7, A.6 provides guidance on the vertical tying of framed structures. It recommends that column splices should be capable of carrying an axial tension equal to the largest design vertical permanent and variable load reaction applied to the column from any one storey. It does not specify which storey but it would be appropriate to use the largest value over the length down to the next splice, or to the base, if that is nearer). In practice, this is not an onerous obligation and most splices designed for adequate stiffness and robustness during erection are likely to be sufficient to carry the axial tying force. Standardised splices are covered in Multi-storey steel buildings. Part 5: Joint design [14] . 6.4 Key elements EN 1991-1-7 [13] , § A.8 provides guidance on the design of “key elements”. It recommends that a key element should be capable of sustaining an accidental design action of A d applied in horizontal and vertical directions (in one direction at a time) to the member and any attached components. The recommended value of A d for building structures is 34 kN/m 2 applied to the surface area of the element in the most onerous direction. Any other structural component that provides “lateral restraint vital to the stability” of a key element should also be designed as a key element. Equation 6.11b of EN 1990 [5] defines the combination of actions which needs to be considered. When considering the accidental loading on a large area (e.g. on a floor slab supported by a transfer beam), it is reasonable to limit the area that is subjected to the 34 kN/m 2 load because a blast pressure is unlikely to be this high on all the surfaces of a large enclosed space. The maximum area to be considered is not defined but could be inferred from the length of load-bearing wall to be considered (see EN 1991-1-7, § A.7) which is 2,25 times the storey height, say 2,25  2,9 = 6,5 m. Therefore, a maximum area that would be subjected to the 34 kN/m 2 load could be a 6,5  6,5 m square. For the design of a key element, it is necessary to consider what components, or proportion of components, will remain attached to the element in the event of an incident. The application of engineering judgement will play a major part in this process. For framed construction, the walls and cladding will normally be non-structural. Therefore, it is likely that the majority of these will become detached from the key element during an incident, as shown in Figure 6.8. For the column member key element shown in Figure 6.8, an accidental load of 34 kN/m 2 should be applied over a width b eff for accidental loading about the major axis. The column section should be verified for the combination of moments and axial force using the design case given above. The accidental