| Buildings
occurs over the first few miles,
meaning
that locations on the coast
experience much higher wind
loading than sites only 1 or 2 miles
inland. Clearly, cliff top sites that
combine a coastal location with
altitude experience particularly
high wind speeds.
Town or country
Agricultural buildings are
generally built in exposed
locations that do not benefit from
the shelter provided by a
surrounding town or city. This
results in higher wind speeds than
would be experienced by
comparable buildings located on
an urban site.
Topography
Topographical features such as
hills can increase wind speed as
the air is forced over them. For this
reason, it is important for the
person calculating the wind
loading to have some familiarity
with the site and not simply rely on
a postcode.
Wind direction
Wind speed is dependent on
direction, with the strongest winds
generally blowing from the south
west. For this reason, when
considering other factors such as
distance to the sea or to the edge
of town, it is important to consider
the direction in which this distance
is measured. A common approach
adopted by engineers is to
consider the wind blowing from
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several points around the
compass and to calculate the wind
speed for each direction.
Building height
Taller buildings are exposed to
stronger winds and this needs to
be reflected in the wind loading
calculations. For single storey
buildings it is common practice to
calculate the wind speed for the
ridge height.
Standards and software
Wind loading should be
calculated using a recognised
code of practice, which in the UK
means BS EN 1991-1-4. This is
one of the structural Eurocodes
and is applicable across Europe,
although each country has its own
National Annex containing
nationally determined parameters
and specific national
recommendations. The calculation
method in BS EN 1991-1-4 is
complex and requires specialist
technical knowledge, so it is
essential that wind loading
calculations are undertaken by a
qualified structural or civil
engineer.
By far the simplest approach is
to use one of the many software
tools currently available. These
range from commercially available
packages that take account of all
of the factors noted above to free
online tools that produce
reasonable but conservative
results with minimal input from t he
user. Several steel purlin
manufacturers include wind
loading tools as part of their
specification software (free to
customers). In many cases, the
precise site location may be
specified in the software by its
postcode or grid reference.
Alternatively, various online
resources may be used to obtain
the grid reference, altitude and
other location data. Thanks to
Google, even the local topography
and surrounding terrain may be
surveyed without leaving the
office.
Concluding remarks
The design of any steel or
timber framed building is
dependent on the magnitude of
the wind loading acting on the
building. Without knowledge of the
wind loads, it is impossible to
design the frame or to specify the
fasteners for the roof and wall
cladding. Since the wind loading
depends on so many geographical
factors in addition to the shape
and size of the building, it should
be calculated for each and every
building project, since no two
buildings will be identical. A
Eurocode standard (BS EN 1991-
1-4) provides recommendations
for the calculation of wind loading
on structures, but these
calculations need to be performed
by a qualified engineer.
Alternatively, the wind loading may
be calculated using software,
including free online tools.
November 2017 | Farming Monthly | 27