Material
properties
The
material properties for all types of glass are as follows:
• Density = 2 500kg/m²
• Young’s
modulus = 70 000N/mm²
• Poisson’s
ratio = 0.22
The
characteristic strength of glass increases if it is pre-stressed. The values provided
in Table 1 are
based on a single pane of glass.
The
coefficient of thermal expansion of glass depends on its chemical composition.
In
basic annealed glass additives such as alkalines can vary the coefficient from 8-9 x 10-6K-1.
Borosilicate glass has a coeffi cient of 3-5 x 10-6K-1 and purer silicone dioxide glass (i.e. fused
silica or quartz glass) has lower values around 5 x 10-7K-1; this makes it useful in the construction of cooking
surfaces such as ceramic hobs.
Design
criteria
Draft
methodology for determining the design strength of glass (prEN 16612) is based
on applying material factors on the glass itself, and coefficients that address
the load duration and the way in which the glass has
been manufactured. The fundamental tenet of the draft methodology is that the applied
bending stress (E ULS;d) must
not exceed the design bending strength (Rd).
With
the guidance being based on limit state theory, partial factors must be applied
to actions. For permanent actions the partial factor (cg) is
1.35. Partial factors for variable actions (cq) are
based on EN 1990-1 and are summarised in Table 2:
The
calculation of the design strength is based on the design characteristic
strength for basic annealed glass (fg;d) and
is determined using the following equation:
fg;d
=
kmod ksp fg;k / cM;A
where:
fg;k
is
the characteristic strength of basic annealed glass (45N/mm2)
kmod
is
the factor for load duration (Table 3)
ksp
is
the factor for glass surface profile (Table 4)
cM;A is the material
partial factor for basic annealed glass (1.6)
Load
duration has a significant impact on structural glass elements due to the microscopic
flaws on its surface. As loads are applied to glass elements these flaws can
grow and cause cracking to the point of overall failure of the glass. In
recognition of this, coefficient kmod has been developed within prEN 16612 that is
always applied when determining the design strength of glass. Table 3 is
a list of values for kmod with increasing typical load duration
periods.
The
coefficient ksp concerns what post treatment the glass pane’s
surface may have received prior to installation. The values for this coefficient
are listed in Table 4.
When
considering pre-stressed glass (i.e. heat-strengthened and toughened) an
additional expression is installed into the equation for determining the design
strength of basic annealed glass:
fg;d
=
kmod ksp fg;k / cM;A + kv ( fb;k - fg;k )
/ c M;v
where:
kv is
the factor derived from the method of strengthening of the glass (Table 5)
fb;k
is
the characteristic bending strength of pre-stressed glass (Table 6)
c M;v is the material
partial factor for surface pre-stressed glass (1.2)
Table 1:
Characteristic strength of common types of glass
|
|
Glass type
|
Characteristic
strength (N/mm2)
|
Basic
annealed/wired
|
45
|
Heat-strengthened
|
70
|
Toughened
|
120
|
Table 2:
Partial factors for variable actions (cq) on structural glass elements
|
|
Type of
element
|
Partial
factor for variable actions (cq)
|
Primary
structure 1.5
|
1.5
|
Secondary
structure 1.3
|
1.3
|
Infill
panel
|
1.2
|
Low risk
infill panel* 1.1
|
1.1
|
*An
infill panel whose failure would not cause injury
Table 3:
Values for kmod
|
||
Duration
|
Example
|
kmod
|
5 seconds
|
Single gust
|
1.00
|
30 seconds
|
Domestic
balustrade load
|
0.89
|
5 minutes
|
Workplace/public
balustrade load
|
0.77
|
10 minutes
|
Multiple
gust (storm)
|
0.74
|
30 minutes
|
Maintenance
access
|
0.69
|
5 hours
|
Pedestrian
access
|
0.60
|
1 week
|
Snow load
short-term
|
0.48
|
1 month
|
Snow load
medium-term
|
0.44
|
3 months
|
Snow load
long-term
|
0.41
|
50 years
|
Permanent
(e.g. self-weight and altitude pressure)
|
0.29
|
Table 4:
Values for ksp
|
||
Type of
glass
|
As produced
|
Sandblasted
|
Float
|
1.0
|
0.6
|
Drawn sheet
|
1.0
|
0.6
|
Enamelled float
or drawn sheet
|
1.0
|
0.6
|
Patterned
|
0.75
|
0.45
|
Enamelled
patterned
|
0.75
|
0.45
|
Polish
wired
|
0.75
|
0.45
|
Patterned
wired
|
0.6
|
0.36
|
Table 5:
Values for kv
|
|
Manufacturing
process
|
Strengthening
factor kv
|
Horizontal
toughening
|
1.0
|
Vertical
toughening
|
0.6
|
Table 6:
Values for fb;k
|
|||
Base type
|
Values of fb;k of pre-stressed glass (N/mm2)
|
||
Thermally-toughened
|
Heat-strengthened
|
Chemically-toughened
|
|
Sheet float
|
120
|
70
|
150
|
Patterned
|
90
|
55
|
100
|
Enamelled
float
|
75
|
45
|
-
|
Enamelled patterned
|
75
|
45
|
-
|
Cullet – crushed glass that is ready to be melted as part of the manufacturing process of fl oat glass.
Enamel – A glassy material which is melted into the surface of the base glass at high temperatures to form a ceramic coating.
Float glass – Glass which has been manufactured by floating the molten glass on a bed of molten tin until it sets, producing a product with surfaces which are flat and parallel.
Inter layer – The material used to bind plies of glass together in laminated glass.
Pre-stressed glass – method of re-heating basic annealed glass that introduces a surface compressed stress, thus making it stronger in bending.
