AD 484: Snow design situations

This advisory note aims to offer clarity when considering snow loading – in particular the drifted snow design situations. Snow loads are covered by BS EN 1991-1-3 and the associated National Annex. The most important advice is that NA.2.2 clarifies that the design situations to be considered in the UK are given in Case B2 of Table A.1 of BS EN 1991-1-3. Clause NA.2.3 specifies that Annex B of the core Eurocode should be used to determine the exceptional snow drift loads. If this advice is followed, it should stop designers even considering several design situations in the core Eurocode, as these are not relevant for structures constructed in the UK.

Case B2 of Table A.1 identifies two persistent design situations and one accidental design situation. The accidental design situation covers drifted snow, which is considered in valleys, behind parapets, behind obstructions and on lower roofs abutting taller structures. As noted above, Annex B is used to determine the loading in these accidental design situations. As an accidental case, the design combination of actions should be calculated in accordance with expression 6.11b of BS EN 1990.

Perhaps somewhat confusingly, the second persistent case is also described as a drifted snow design situation – although it is treated as a persistent case and attracts the normal partial factors used in expressions 6.10, 6.10a and 6.10b of BS EN 1990. This second drifted case is the removal of all the snow from one roof slope, so it might be better described as an asymmetric case. In the core Eurocode, clause 5.3.3 and Figure 5.2 indicate that half the snow is removed from one roof slope, but this is amended by clauses NA.2.13, NA.2.18 and Figure NA.3 of the UK National Annex to specify that all the snow is removed from one roof slope.

Some designers appear to ignore this second persistent case. Designers may recall snow loads determined from BS 6399-3 which had exactly the same asymmetric load with no snow on one slope, but this situation only needed to considered for roof slopes greater than 15°. There is no limiting roof slope in the Eurocode, so designers are reminded of this design situation.

Contact: David Brown
Tel: 01344 636555
Email: advisory@steel-sci.com