Civil Engineering Reference
In-Depth Information
face-sealed systems. BS 8200
(28)
suggests a limit depending on the
material, in the range 1/90 to 1/500
span for deflections in opaque infill
panels in secondary framing. It will
be possible to calculate the effects on
joints of differential movements on
adjacent panels, and those
responsible for the final detailed
design should carry out the
necessary calculations.
peak suctions higher up the building
which are critical, and the Digest
gives special pressure coefficients for
these peak suctions.
In addition to pressures on the
external faces of buildings, the
design also has to take into account
the effects of wind on the inside
faces of the external wall (Figure 1.9).
The internal pressure inside a
building depends on the sizes of the
openings in its walls.
A conventional building with all
its large doors and windows closed
has a low porosity. This has a
damping effect. If windows get
broken on one face during a severe
storm, or if there are other large
openings on one face, internal
pressures can rapidly approach
external pressures, and the net load
on the opposite external wall will
then greatly increase.
As long as a multi-layer skin acts
structurally as a single unit (for
example a properly tied cavity wall),
the wind distribution through the
layers is irrelevant. But there are
cases where the pressures between
the layers can be important,
depending on how permeable the
cladding system is, and on whether
there are voids and how large these
are. In the case of impermeable
cladding with voids, another key
factor is the pressure at the location
to which the voids are vented. For a
description of the principal loading
cases, see pages 16-18 of the BRE
report on overcladding
(26)
.
In many cases, the degree of
permeability of the building surface
is indeterminate and it is safest to
assume that the walling must
transmit the full wind loads through
any adhesive bond and mechanical
fixings. Bond and fixing strengths
may be determined by testing small
sections or by applying a proof
suction load to the prototype panel
using a test rig
(27)
.
A further factor to consider will be
local deformation of the surface of
the walling under wind loads which
may alter the geometry of a joint.
Certain kinds of walling joints, for
example unfilled joints in rainscreen
systems, are more tolerant of
changes in joint geometry than are
Pressure
Suction
Figure 1.7
Wind blowing directly on the face of a
building
Seasonal factors
Seasonal factors can be relevant to
construction work. The highest
extreme winds in the UK are
expected in December and January.
In the summer months of June and
July, extreme winds may be
expected to be only about 65% of the
winter extremes; for the six month
summer period from April to
September, normal wind speeds are
only expected to be 84% of those in
the six month winter period,
October to March. Tables of these
seasonal factors are published for
one month, two month and four
month periods starting in each
month of the year
(25)
.
BRE can provide realistic test
regimes; for example, using
computer controlled simulation of
actual wind flow patterns (see
Chapter 4.2).
Cr osswind width B
H
more
than
B/2
Tall outline
H
less
than
B/2
Squat outline
Figure 1.8
Effect of the shape of a building on wind
Fatigue
There are two types of fatigue that
may need to be considered: high-cycle
fatigue, and low-cycle fatigue.
High-cycle fatigue
High-cycle fatigue occurs when the
structure is subjected to very many
thousands of load cycles at a small
proportion of the ultimate capacity
of the structure caused by
oscillations at resonant frequencies.
High-cycle fatigue is essentially a
serviceability problem, provided the
structure is properly inspected and
maintained, and that fatigue cracks
can be repaired or components
replaced before their fatigue life is
exhausted. This is common practice,
for example, for the holding-down
bolts of slender steel chimney stacks.
Wind
Figure 1.9
Internal pressures due to one dominant
opening
