AD 273: Portal Frame SLS checks and BS 6399-2

Since the issue of Amendment 1 to BS 6399-2, the SCI has received many questions about the serviceability limit state (SLS) checks for duopitch portal frames, when the check includes wind loads based on the external pressure coefficients found in Table 10 of BS 6399-2.

In Table 10, for roof slopes of five degrees and steeper, two loadcases are specified when wind blows across the frame (i.e. at right angles to the apex). On the windward rafter the first loadcase has suctions, whilst the second has either zero or positive pressure. The same principle is found in Table 34, from the Directional Method.

Designers who used CP3 will recognise the first loadcase, but have been surprised by the loadcase that includes positive pressure on the windward rafter. This second loadcase is often found to be critical at SLS, leading to the need for much stiffer frames than those previously designed. The Institute has been asked to advise on the application of this loadcase at SLS. Within a limited scope, our advice is that the second loadcase, with positive pressure on the windward rafter, need not be checked at SLS. The scope of this advice is limited to:

• Portal frames;
• Duopitch roofs (i.e. those falling within the scope of Table 10 of BS 6399-2);
• Modest roof slopes (less than 20 degrees);
• Orthodox, common construction;
• SLS (both loadcases should be checked at ULS).

No new guidance is offered on deflection limits in this Advisory Note.

Several decades of experience has taught designers that SLS checks based on the ordinary loadcases, compared with the normal deflection limits, produce structures that perform satisfactorily in practice. There is therefore no need to introduce a ‘new’ loadcase as an additional SLS check. Whilst the positive pressures may have always occurred, (although not predicted by CP3) the experience over many years is that the SLS checks based on the suction loadcases are satisfactory. When designing structures outside the above scope, or unorthodox in any respect, the designer should consider both loadcases, at both SLS and ULS.

Note: Although Table 34 only has positive pressures indicated at 15 degrees and steeper, positive pressure should be calculated at lower slopes by interpolating between zero pressure at 5 degrees and the quoted pressure at 15 degrees. We are advised that this will be clarified in the next amendment to BS 6399-2.

Contact David Brown, SCI
Telephone 01344 623 345
Email d.brown@steel-sci.com