| Buildings
this could give rise to sway in the
frames.
Neighbouring roofs
Where a building is erected
adjacent to an existing taller one,
building designers need to be
aware of the possibility of snow
falling off the higher roof onto the
lower building causing local
overloading of the roof cladding
and supporting structure. The
same could also be true if a low
mono-pitch extension were built
against a duo-pitch shed. The
snow load in this case will depend
on the roof slope area of the taller
building and the area of the lower
roof onto which the snow is likely
to fall (i.e. the size of the heap
formed on the lower roof). If there
is a significant height difference
between the two roofs, there is
also the risk of impact loading due
to the momentum of the snow as it
hits the lower roof. This loading is
likely to be far greater than the
equivalent static load. RIDBA is
aware of examples where such
roofs have collapsed in practice.
Snow loading calculations
Snow loading is site and
building specific so should be
calculated for each building
project.
This is not as onerous as it
sounds, since the only two factors
affecting the uniform snow loading
are the location (expressed as a
snow zone) and altitude. It does
however mean that a steel frame
www.farmingmonthly.co.uk
designed for a farm in Cornwall is
unlikely to be able to withstand the
snow loading of a typical winter in
North Yorkshire (although the wind
loading is probably worse in
Cornwall). Taking account of the
influence of neighbouring
buildings and obstructions, the
snow drift loading could differ
between two buildings on the
same farm!
The magnitude of the snow
loading should be determined
using BS EN 1991-1-3 (part of
Eurocode1) and its UK National
Annex. For agricultural buildings, it
may be permissible to apply a
reduction to the snow loading
obtained from the Eurocode,
depending on the use and
occupancy level of the building.
For further details, see BS 5502-
22. When designing the structural
frame, the snow loading is
assumed to act at the same time
as the dead load (e.g. self-weight)
and any positive (downward) wind
loading. BS EN 1990 gives the
appropriate safety factors to apply
to this combination of loading.
Access for maintenance (imposed
load) is cons