Civil Engineering Reference
In-Depth Information
e
(loads not acting axially), as
illustrated by Figure 1.3, normally
invoke special design requirements, as
do concentrated loads (ie large loads
acting over a small area, as in Figure
1.4). Individual loads are the product
of the measured or nominal density
and the volume of the material.
Detailed design advice is given in BS
5628-1, clauses 21, 22 and 23, and in
BS 8110-1, section 2.
areas and wall cladding results in high
internal pressures in buildings
(22)
.
Changes have been made to the
rules for design since the 1939-45
war. In particular the 1952 wind
loading code in BS CP 3
(19)
did not
take account fully of the effects of
building shape and wind directions,
and gave, as a result, lower wind loads
than would be obtained today using
BS 6399-2
(23)
.
Load
Section of wall
bearing load
Joints may go into
tension
Applied loads
Vertical applied loads due to furniture,
fixtures, fittings, live occupants, snow
and wind acting on all surfaces must
be carried to the ground (foundations)
via a structural frame or a loadbearing
wall. The design applied load at any
point is the sum of all the applied
loads acting at that point from above.
γ
Wind speed
Wind speed varies depending on the
geographical location of the site, the
altitude of the site above sea level,
the direction of the wind, and the
season of the year. More local effects
such as height above ground level,
topography and terrain also affect
the speed and direction of the wind
at a particular site. In the immediate
vicinity of a building, or a group of
buildings, the wind changes speed
and direction rapidly, depending on
the form and scale of the building.
Wind speeds in coastal regions are
generally greater than in inland
areas; for instance, the speeds near
the coasts of southern England are
some 10-25% greater than they are
at the same altitudes in places in the
centre of southern England. The
highest wind speeds occur in the
north of the British Isles, the highest
basic gust wind speeds used in
design being 56 m/s in the far north
west of Scotland.
Further information on wind
speed, and factors which affect it,
is given in Chapter 1.1 of
Roofs and
roofing
(24)
from which Figure 1.6 (on
page 18) is taken.
A building modifies the flow of
wind round it. Figure 1.7 (on page
19) shows the case where the wind
blows directly face on to the
building, but the flows will differ
according to the wind direction, and
according to whether the building is
tall or squat in profile (Figure 1.8).
Figure 1.3
Eccentric loads
f, the partial factor of safety is
normally around 1.2-1.6. Eccentric
loads (loads not acting axially) invoke
special design requirements.
Individual design loads are nominal
values based on statistical
probabilities drawn from observations
of building types. Detailed design
advice is the same
as given for dead loads in the
section above.
Seismic loads
Seismic loads generate similar forces
(ie lateral and shear forces on walls) to
wind loads but potentially over a
much greater dynamic range. They are
not explicitly designed for by current
UK practice. They do occur, but
experience suggests that their
intensity in the UK is always less than
wind forces.
Figure 1.4
Concentrated loads
Wind
Gales in the UK cause around £70
million worth of damage to
buildings each year. In the severe
gales of October 1987 around 1.3
million houses were damaged; in
those of early 1990, around 1.1
million houses were damaged when
one third of the costs of the damage
related to walls and their components
(Figure 1.5). Most damage by wind is
of a relatively minor nature;
nevertheless there are still substantial
sums of money spent on rectification
of damage when the failure of glazed
Tall outline
If the height of the building is more
than half the crosswind width B of
the outline, the wind tends to flow
round the sides of the building,
except at the very top of the building.
In the case of the tall building, the
Figure 1.5
Gale damage to the external leaf of a cavity
wall. Few wall ties were in evidence.
(Photograph by permission of East Anglian
Daily Times)
