Civil Engineering Reference
In-Depth Information
Detailed guidelines on loading is given in (EN1991-1-1), but
generally consists of a line load no greater than 1 kN/m applied
at no higher than 1.2 m from the fi nished fl oor level.
providing adequate movement joints as shown in the point-
supported glazing panel in
Figure 21.17
.
And/or
(b) Thermal gradients across the glass surface generally result-
ing from different exposures to solar radiation, for example,
the solar energy absorbed by the unshaded central regions
of a facade panel will be signifi cantly higher that the energy
absorbed by the shaded edges of that panel. The tempera-
ture gradient is a function of the absorption coeffi cient of
the glass, the incident radiation, the glass emissivity and
the ambient temperature. The French code [23] provides
guidelines on how to calculate the thermal gradients which
must not exceed the allowable maximum temperature diffe-
rence shown in
Table 4.1
. Further guidelines are available
in a technical note by CWCT [24].
21.4.1.2 Wind
Wind induced pressures on the building envelope are a function
of the mean wind velocity and the turbulence intensity. National
and international wind loading codes of practice (e.g. EN1991-1-
4) are available, but they are limited to simple building geometries
and offer limited guidance on complex geometries or intricate
facades. As a result, wind pressures obtained from these codes
are often supported with wind tunnel testing when the building
has an unusual geometry. Furthermore load amplifi cation can
occur when the natural frequency of the glass structure is less
than 1 Hz (e.g. large span and/or slender facades).
BS 6262-3 provides an abbreviated method for determining
the design wind pressure on glass facade panels and includes a
series of design charts for sizing rectangular glass plates with
simple supports along the four edges.
The draft European standard for Glass in Buildings (pr EN
13474) recommends that the stress corrosion caused by repeated
wind loading equates to the gust design pressure applied as a
10 min static pressure on the glass. This was found to be safe,
but in some cases overly conservative (Zammit
et al
., 2008).
Accurate mathematical predictions of the sub-critical crack
growth (and hence the strength) of a glass panel subjected to
real-world wind pressures is not a trivial task and is the subject
of ongoing research.
21.4.1.5 Human impact on vertical glazing
This is a key requirement for barriers and partitions and is intended
to minimise injury caused by persons falling through the gaps in
barriers and/or by contact with glass fragments. The test involves
a soft body pendulum test. The imapactor consists of two rubber
tyres wrapped around 50 kg steel cylindrical impactor. The test
described in EN12600 is used to classify the impact resistance by
observing the mode of breakage. Other national codes of practice
(e.g. BS 6262-4) must be consulted for the recommended classi-
fi cation for particular applications/locations.
21.4.1.6 Impact from hail and windborne debris
These are not normally specifi ed in the UK, but are required
in locations such as Florida (wind borne debris) and the Alps
(hail resistance). ASTM E886 and ASTM E1996 describe tests
for fi ring timber missiles at glazing panels to simulate fl y-
ing debris. BS EN13583 describes tests for assessing the hail
resistance of fl exible roofi ng materials, and can be adapted to
21.4.1.3 Internal pressure in insulating glazing units (IGUs)
A pressure difference, also known as the isochore pressure,
between the sealed cavity of an IGU and the environment will
arise when there is either a difference in altitude and/or a diffe-
rence in temperature between the place of production and the
place of installation of the IGU.
Assuming a constant volume, the net pressure
p
net
in kPA is
given by:
T
( )
H
( )
p
03
4
T
( )
T
( )
T
( )
( )
T
T T
T
T
( )
( )
0
012
H
( )
H
( )
H
( )
(21.8)
+
+
0
H
( )
H
H
−
( )
−
=
03
T
( )
T
( )
T
T
T
T T
−
( )
−
( )
−
−
ne
t
03
.
03
03
4
4
T
( )
T
( )
T
( )
T
( )
T
( )
T
( )
T
T
T
T T
T
T
T
T
T
T
T
( )
( )
( )
( )
( )
T
T
T
T
( )
T
( )
T
T
T
T
T
T
T
T
T
( )
T
( )
T
( )
T
( )
T
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
+
+
+
+
0
0
.
p
4
T
( )
T
( )
T
( )
( )
T
T
T
T
T
T T
( )
( )
p
p
p
T
( )
( )
T
( )
( )
0
0
0
.
.
012
ne
t
where
T
p
and
H
p
are the cavity temperature in Kelvin and the
altitude in metres at the place of production and
T
and
H
are
the cavity temperature in Kelvin and the altitude in metres at
the place of installation.
In reality the cavity volume changes as the glass panes
deform under pressure. The isochore pressure is therefore
reduced by the fl exibility of the panes. Guidelines for calculat-
ing this effect in rectangular double glazed units is given in pr
EN 13474 (Haldimann
et al
., 2008).
close-fit
slotted
slotted
oversize
21.4.1.4 Thermal stress
Thermal stresses arise from:
(a) Diurnal and seasonal temperature variations leading to dif-
ferential movement between the glass and its sub-frame.
These stresses are normally prevented altogether by
Figure 21.17 Provision for movement in point supported glazing
panel (Haldimann
et al
., 2008)
