Civil Engineering Reference
In-Depth Information
former is relevant to the weathering
and staining of building façades
(Figure 1.20 on page 28) and the
latter is useful for assessing the risk
of rain penetration through masonry
walls and building features
(35)
.
Design guidance in the UK on the
incidence of driving rain has been
calculated since the early 1960s from
rainfall and all-directional wind
speed measurements. This guidance
has now been revised to take into
account the direction from which the
wind is blowing, and the results can
be shown in map form as in Figure
1.21 (on page 29). This map has been
taken from
Roofs and roofing
(24)
.
Work by the Meteorological Office
on indices of wind-driven rain, or
'driving rain', has overcome some of
the deficiencies of earlier approaches.
Maps for the annual wind-driven rain
index for open sites (described as
airfield conditions) have been
prepared by combining the hourly
products of wind speed and rainfall
during the 33 year period 1959-91.
It then becomes possible to determine
construction suitable for use in
resisting wind-driven rain in all parts
of the UK; for example when using
thermal insulation within the cavities
of external walls. Figure 1.21 shows
four zones, the darkest
shaded experiencing more than
100 l/m
2
per hour, and the others in
progressively lighter shading
showing 56.5-100, 33-56.5, and less
than 33 l/m
2
per hour.
In using this diagram, the exposure
zone for the building is first
determined from the map, and then
modified according to local features
such as height above sea level, and
proximity to the coast which
increases the severity of exposure by
the equivalent of one zone, or shelter
belts of trees or adjacent buildings
which reduces the severity of
exposure by one zone. To give one
example, a site situated near to the
Pembrokeshire coast is in the zone
experiencing the most severe
exposure of more than 100 l/m
2
. If the
building is in a sheltered location, and
more than 8 km from the coast, it can
be assumed that it will be subjected to
driving rain in the 56.5-100 l/m
2
category, that is to say one zone less
than that indicated by the map.
Similar adjustments can be made for
other sites according to the above
criteria.
The final step is to check that the
construction will be satisfactory. This
will depend on a number of factors
such as the basic form of the
construction, the width of the cavity
and whether it is clear or filled,
whether the outer face is impervious,
or is rendered, or if of masonry,
whether the joints are tooled or
recessed, and whether the sills and
copings are flush or projecting.
Guidance on the selection of suitable
construction is available, for example
for the protection of cavity fill it is to
be found in
Thermal insulation:
avoiding risks
(36)
.
By way of illustration, impervious
cladding for the full height of all
walls up to 12 m height would
normally be satisfactory for the
severest exposures, but such cladding
would offer suitable protection above
ground floor facing masonry only if
cavities in the latter exceed 100 mm
in width. For narrower cavities, the
wall must be completely clad. At the
other end of the spectrum, facing
masonry - however the joints and
sills are treated, and irrespective of
whether the cavity is filled or not - it
is only likely to offer satisfactory
protection in the less than 33 l/m
2
zone.
Analysis of available data given in
Climate and building in Britain
(37)
suggests that every ten years on
average, most locations in the UK
experience one hour in which the
total driving rainfall is 30 mm. More
tentatively, there is one hour in every
100 years when 80 mm falls.
Intensities for shorter periods are
illustrated in Figure 1.22, though
these figures are probably
overestimates.
0.1
1.0
10
100
20
100
200
300
Intensity of driving rain in litres/m
2
(= mm/h)
Figure 1.22
Estimated frequencies of driving rain intensities from intense showers (from Holland's rain
intensity frequency graphs for Britain, with additional data for storms lasting 30 minutes
and 60 minutes, and wind speed of 10 m/s)
