| Buildings
Snow loading on agricultural buildings
The United Kingdom is blessed with a fairly mild winter climate, but it is not uncommon for a significant snowfall to
occur at least once every winter, bringing with it the usual transport chaos and school closures.
n Scotland and
North East
England, heavy
snow is more
common and the
problem increases
with altitude. For farmers,
especially those with livestock, the
snow is a cause for concern and, if
possible, many farmers will try to
bring their animals indoors during
the worst of the weather. For their
sake, it is essential that the
buildings erected on farms and
elsewhere are sufficiently strong to
withstand the worst winter snow,
including drifted snow where
appropriate. Fortunately, there is a
relatively simple design method
that frame manufacturers can
follow to ensure that their buildings
are safe. This article aims to
describe this method and highlight
the key issues.
Snow loading on buildings
There are two fundamental
types of snow loading that can act
on a building structure and it is
important to distinguish between
them:
• Uniform snow loading
• Snow drift
I
All buildings should be
designed for uniform snow load as
there will always be an element of
distributed snow, even if some
drifting occurs. Some buildings,
especially those with obstructions
against which the snow could
accumulate such as parapets or
walls, should also be designed for
snow drift loading. While snow drift
loading is usually much higher
than the uniform snow load, it is
limited to a relatively small area
and is subjected to lower safety
factors by the design standards.
Snow drift loading may also occur
on buildings that are susceptible
to snow sliding off a higher roof
onto a neighbouring lower one.
All snow loading ultimately has
to be carried to the foundations by
the cladding, purlins and building
structure. It is, therefore essential
that building designers take
account of the snow when
designing all elements of the
building (cladding, purlins, rafters,
columns and foundations) and the
connections between them. For
steeply pitched roofs, the building
designer should also give
consideration to the down-slope
16 | Farming Monthly | February 2018
component of the snow loading
acting on the roof cladding and
ensure that sufficient fasteners are
provided.
Factors affecting snow loading:
Location
Some parts of the country are
more susceptible to snowfall than
others. A number of
meteorological factors influence
the depth and frequency of
snowfall at a given location,
beyond the expertise expected of
a steel frame manufacturer or
structural engineer.
Fortunately, data from many
years of meteorological records
have been analysed to produce
simplified guidance in the form of
a snow map and a simple
equation to allow for site altitude.
Both are published in the
National Annex to the BS EN 1991-
1-3. The snow map, which has
been reproduced in Figure 1,
divides the country into a number
of zones, according to the
predicted intensity of the 1 in 50
year snow event. As one might
expect, the highest snow loading
occurs in Scotland and eastern
areas of England.
Altitude
The depth of snow fall naturally
increases with altitude. This is
catered for by a simple equation in
BS EN 1991-1-3 that gives site
snow load in terms of site altitude
and snow zone (taken from the
map). It is therefore a relatively
simple matter to obtain the ground
snow loading for any location in
the UK. Multiplying by another
factor, known as the shape factor,
gives the uniform snow loading on
the roof of the building.
Parapets and obstructions
Parapets and other obstructions
against which the snow may
accumulate cause snow
driftsresulting in localised high
snow loads well in excess of the
uniform snow load. These need to
be taken into account in the
design of the roof cladding and its
supporting structure and may
require additional local
strengthening of the roof (e.g.
purlins at closer centres). Where
snow drifts against a wall or
parapet, there is also the
possibility of a horizontal force
from the snow onto the cladding
and structure. In extreme cases
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