Civil Engineering Reference
In-Depth Information
therefore equation (21.3) can be re-written as
k
mod
= 0.633
t
-1/16
where
t
is the stress duration in hours.
Figure 21.13
shows this
stress corrosion with as a function of time. A simple time-step
function corresponding to the three stress durations suggested
in the European standard (prEN 13474-1, 2007) is superim-
posed on
Figure 21.13
and described further in
Table 21.4
.
toughened glass (also known as tempered glass). In the United
States, the minimum allowable far-fi eld pre-compression for
fully toughened glass is 69 MPa (Overend, 2010), whereas
in Europe the minimum far-fi eld pre-compression equates to
approximately 90 MPa (BS EN12600:2002). For heat strength-
ened glass the far-fi eld pre-compression ranges between 24
MPa and 52 MPa.
In toughened glass, surface cracks may only propagate after
the surface pre-compression has been overcome. Equation
[21.1] may therefore be extended to toughened glass as
follows:
21.3.3 Secondary processing and mechanical properties
21.3.3.1 Toughened glass
Annealed glass can be either chemically or thermally treated
to reduce the infl uence of surface fl aws. Thermal toughening
is more economical and involves heating the glass to 625º C
followed by rapidly cooling the surfaces. As the inner core of
the glass cools and contracts it puts the outer surface into com-
pression. This results in a parabolic residual stress distribution
through the thickness
h
of the glass, where the glass surface is
in compression
f
rk
(
Figure 21.14
).
The magnitude of surface pre-compression,
f
rk
, is governed
by the rate of cooling and by the proximity of free edges to the
point of interest. Two distinct classes of thermally treated glass
are available: heat strengthened glass and the stronger fully
(
)
m
(21.4)
P
=− −
(
1e
=
1e
=−xp
(
kA
)
f
PP
−
σ
f
f
f
f
f
−
f
f
f
r
r
r
r
k
f
r
where
f
rk
is the residual compressive stress on the glass
surface.
21.3.3.2 Laminated glass
Laminated glass consists of two or more glass plates bonded
together by a transparent polymer interlayer, normally poly-
vinyl butyral (PVB). The nominal thickness of a single PVB
foil is 0.38 mm and it is normally built up into two layers (0.78
mm) or four layers (1.52 mm). Laminating the glass has no
observable effect on the crack propagation, but has a signifi -
cant infl uence on the post-fracture behaviour.
PVB is a viscoelastic material and is susceptible to creep.
The stiffness of the interlayer and the fl exural behaviour of
laminated glass are therefore infl uenced by the magnitude
and duration of loading and temperature (
Figure 21.15
). At
2.00
stress corrosion with
t
o
= 5s;
n
= 16
step function
proposed in Ta ble 21.4
1.75
1.50
0.1s
1.25
1s
10s
1.00
1min
10min
1hr
0.5
f
rk
f
rk
f
rk
f
rk
0.75
1d
1wk
6wk
1yr
5yr
50yr
0.50
+
0.2
h
0.25
0.6
h
h
-
0.00
1.E-02
1.E+01
1.E+04
1.E+07
1.E+10
0.2
h
+
Stress Duration (s)
Figure 21.13
Relative tensile strength as a function of stress
Figure 21.14
Residual stresses in thermally toughened glass
duration
Design combination
Stress duration
Stress duration factor k
m o d
Long-term combination,
F
d L
e.g. self weight
t
f
> 6 weeks
0.29
Medium-term combination,
F
d M
e.g. sustained imposed loads,
seasonal temperature, snow
and self weight
6 weeks ≥
t
f
> 10 minutes
0.43
Short-term combination,
F
d S
e.g. wind, access loads, sustained
imposed loads, wind, temperature,
snow and self-weight
t
f
≤
10 minutes
0.74
Table 21.4
Load duration combination proposed by draft European Standard (prEN 13474-1, European Committee for Standardization, 2007)
