(Glass failures)
| 08.06 | GLASS
AND GLAZING
(Glass failures) |
|
Introduction
The breakage of glass impairs
the safety, security and comfort of buildings. Matching and installing
replacement panes of glass or glass units can also be expensive.
This Section describes the causes of glass breakage and presents measures
to minimise its occurrence and diagnose its cause(s).
Breakage characteristics
depend on the glass type. Annealed glass breaks into sharp - edged shards,
although these may remain in the frame, depending on the glazing method.
Toughened glass, when broken, disintegrates into small, relatively blunt
particles (‘dice’) that will generally fall out of the frame, preventing
examination of the breakage pattern. With laminated glass the broken
panes will remain adhered to the laminate, enabling the breakage pattern
to be examined and the cause of breakage to be identified. The manufacture
and properties of different glass types are described in Section
08.01.
Causes of glass breakage
Glass is potentially very
strong as shown by its use in glass fibre reinforced plastic however in
sheet form the strength is reduced by the presence of invisibly small defects,
known as Griffith cracks, which cause stress concentrations allowing cracks
to propagate. Strength may be reduced further by larger visible defects.
The potential causes of breakage of annealed glass are as follows:
Failure of structural glazing
may occur due to excessive stresses in the glass. Further details
are given in Section 10.01.
Annealed glass
Uniform load
The most significant form
of uniform loading on glass is normally wind load although other causes
such as snow may occur in some situations. The safe load on a pane of glass
is related to the size of the pane and glass thickness. BS 6262 gives guidance
on the selection of glass thickness.
If the glass has been incorrectly
selected (i.e. it is not thick enough) or if the peak wind load has been
incorrectly assessed, then the glass may not have adequate bending resistance
and will ultimately break. This image
shows the glass fracture pattern due to excessive, uniform load for four
edge supported glass. Two edge support gives simple bending behaviour with
failure as shown in this image.
Impact
Glass may break as a result
of impact. The impact may be accidental, (e.g. from human collision),
or deliberate, (e.g. through vandalism). Breakage may also be caused
by thrown or wind borne projectiles (e.g. gravel blown from roofs).
The breakage pattern depends on whether the impact is soft (e.g. human impact), image, or hard (e.g. small projectile), image. Under certain circumstances of impactor and velocity of impact a Hertzian cone results from the impact, image.
Part
N of the Building Regulations places limitations on the use of glass
in locations vulnerable to impact and BS 6262: Part 4 gives recommendations
on the use of glass in areas liable to human impact. Section
08.07 covers the safety requirements for the use of glass.
Edge damage
Edge damage, which is usually
caused by poor handling, can impair the bending strength of a pane of glass
by creating stress concentrations. Edge damage can also occur when
cutting glass (difficulty increases with thickness) although this is much
less likely to lead to breakage than damage due to poor handling.
There are no standard acceptance
criteria for the edge condition of glass, although glass with edges that
are severely feathered or have deep, pointed shells/vents are generally
unacceptable, Section 8.03, especially in glass
types that absorb a high percentage of solar energy.
Poor glazing
Insufficient clearance between
the glass edge and glazing frame will cause the glass to come into contact
with the frame under frame movement or glass expansion, increasing the
likelihood of breakage, especially if there is localised contact (e.g.
with a fixing screw). Insufficient clearance may result from the
incorrect sizing of glass, or deviations of size.
Before glazing, the glass should be checked for size against that of the glazing opening. Glass that is wrongly-sized must not be glazed and nor should its size be reduced by nipping or grinding as this could damage the edge of the glass and the seal of glass units.
The wrong width of setting block may result in uneven support across the bottom of a glass unit or piece of laminated glass. In general, the width of setting blocks should be equal to the sum of the glass unit thickness and the back face clearance to guarantee full support across the width of the unit.
Glass breakage can also arise from setting blocks being too rigid (e.g. very hard nylon) and damaging the edge; a slightly softer compound would eliminate this risk and may also prevent relative movement between the block and glass unit. Too soft a material may cause the glass to sink into the setting block and cause localised frame contact.
Recommendations on good workmanship
in relation to glazing are given in BS 6262 and BS 8000: Part 7.
Site damage
Spatter from nearby site
welding or grinding/cutting operations can cause pitting of the glass surface
and in some cases hot metal particles can fuse into the surface, image.
The embedded metal may subsequently corrode and expand inducing a high,
local stress in the glass and causing it to crack or shatter.
Thermal stress
Glass can break as a result
of excessive thermal stress, image
. Thermally induced stress within a pane of glass results from a
temperature differential between two areas of the pane. For instance,
in hot weather, the centre of the glass warms up faster than the edge,
because the edge is within the glazing rebate and shaded from direct solar
radiation. Assuming the area of glass within the frame is insignificant
compared with that exposed to solar radiation, as the centre of the pane
expands due to the increase in temperature, the edge will be forced to
expand by a similar amount inducing a tensile stress.
The expansion in the edge of the glass is given by:
dL = a x Lx dT
where a is the coefficient of thermal expansion, L is the original length and dT is the temperature difference between the edge and centre of the pane.
The induced stress is given by:
s = ( dL / L ) x E
where E is the Young’s modulus.
The stress induced by a temperature
difference of one degree is therefore a
x E. Assuming
a
= 10 x 10-6
/ 0C and E = 70,000
MPa ( N/mm2 ) a temperature
difference of one degree results in a stress of approximately 0.7 MPa (
N/mm2 ).
The risk of thermal breakage in annealed glass becomes significant when stress levels exceed about 20MPa ( N/mm2 ). A difference of 30oC can, therefore, be sufficient to result in thermal fracture. If the glass has been poorly cut, damaged during handling or improperly glazed, breakage could occur at lower temperature differentials.
Thermal stress fractures always start from some tiny imperfection in the glass edge because this acts to concentrate the induced stress. These fractures are easily identified as they run perpendicular to the glass edge at the location of the damage. This image illustrates a typical thermal stress breakage pattern. The number of cracks depends on whether the edge of the pane of glass is damaged (single crack) or clean cut (more than one crack).
Glass should be stored on
edge, in racks and on blocks of wood or felt, in dry, ventilated conditions,
and out of direct sunlight to prevent thermal fracture.
Solar control glass
Both body tinted and coated
solar control glass will heat up more than plain glass when exposed to
solar radiation increasing the risk of thermal breakage. This effect is
increased when the solar control glass is used in insulating glazing units.
The use of low-E coatings on the inner pane of insulating glazing units
with solar control glass as the outer pane will further increase the temperature
of the solar control glass and the risk of thermal breakage.
Insulating glazing units
The risk of thermal fracture
will be increased in insulated glazing units, as the glass will heat up
faster than single glazing.
The edge spacer will also
have an effect. Glass units typically contain an aluminium edge spacer
to separate the panes of glass. The spacer acts as a thermal bridge
and in cold winter conditions will conduct heat to the edge of the outer
pane, so reducing the centre-to-edge temperature difference when the sun
is shining. ‘Warm edge’ spacers are increasingly being specified
and, although they enhance the overall insulating properties of the unit
and reduce the risk of condensation, they will increase the risk of thermal
breakage.
Framing design
The thermal safety of glass
is also influenced by frame design. The important factors are heat loss
from the frame to the surrounding structure and the response of the frame
to solar radiation.
The risk of thermal breakage will be increased if the frame is in good thermal contact with a substantial heat sink thus keeping the edge of the glass cool. The most severe example is where the glass is glazed directly into concrete or brickwork.
Frame material, colour and
design will affect the temperature rise of the frame and hence the edge
of the glass in response to solar radiation. The risk reduces as the frame
is changed from wood to metal and then plastic and dark frames give a lower
risk than light ones.
Internal features
Conditions and features
on the room side of the glass affect the amount of heat absorbed and lost
by the glass. Internal blinds may reflect radiation and reduce heat loss
by restricting air movement, structural elements may reflect solar radiation
back to the glass and radiant heaters can direct additional heat towards
the glass.
External features
External features, which
affect the risk of thermal fracture, are shadows, which may cause areas
of cooler glass, and reflections, which may increase the temperature of
the glass.
Shadows may be created from
overhanging balconies or walkways, nearby buildings or trees. Projecting
mullions or columns will produce slowly moving shadows and give rise to
large temperature differentials. The shadow from a cleaning cradle
parked for any length of time in front of a glass panel can induce thermal
fracture.
Handling/glazing
Correct handling and glazing
of the glass is essential to avoid edge damage and minimise the risks of
thermal and mechanical stress fracture. The glass should be properly
supported and have adequate clearance to allow thermal and structural movements
to occur without causing glass to metal contact, especially localised contact.
Window and curtain wall glazing gaskets should be compatible with the service
temperature.
Checking for thermal
safety
Project-specific stress
analysis using suitable software should be undertaken and any glazed areas
found to be at risk should be thermally treated by heat-strengthening or
toughening (and heat soaked). The safety check should take account
of temporary conditions such as scaffolding and cleaning cradles and future
conditions such as trees planted in close proximity to a glazed wall.
Toughened glass
Uniform load, impact, weld
spatter, inadequate clearance and edge damage may also cause breakage of
toughened glass, although the risk of breakage is lower since higher stress
levels are tolerable.
Thermal breakage of toughened
glass with clean cut edges will not occur in normal circumstances because
this requires a centre-edge temperature difference of about 90 to 150oC.
Stress concentrations such as edge damage can cause thermal breakage of
toughened glass.
Nickel Sulfide inclusions
Glass may contain nickel
sulfide impurities, in the form of small crystals, image.
As glass is heated during the toughening process these impurities change
state. The high temperature a-state
of the impurities may be frozen when the glass is quenched, and recovery
to the low temperature b-state
may then take several years. Spontaneous breakage of the glass may
follow, as the low temperature b-state
of the nickel sulfide impurities occupies a slightly greater volume and
so generates a local stress concentration. However, only the largest
impurities will cause failure, and only then if they are in the core tensile
zone of the glass.
One large reputable manufacturer reckons that only 1 critical inclusion occurs in every 13 tonnes of glass that they produce, and that 90% of the inclusions will cause premature breakage during their heat soaking process. This means that only one nickel sulfide inclusion will ‘get through’ for each 12,700 m2 of 4 mm heat soaked toughened glass, or for each 8,470 m2 of 6 mm heat soaked toughened glass.
Nickel sulfide failures can
start to occur any time after production, with experience suggesting a
peak failure rate typically four to five years after production.
Diagnosing NiS failure
Nickel sulphide breakage
of glass exhibits a characteristic fracture pattern. However, it
is not unusual for toughened glass to fall out of its frame when broken,
thereby preventing examination of the fracture pattern and origin.
Assuming that the fractured pane remains in place, at the origin or epicentre of the fracture there should be located two fragments which are larger than the rest and which form a ‘figure of 8’ or ‘butterfly’, image. In the centre would be located the inclusion which is a small, round, shiny, yellow-black particle which may be as small as 0.05mm in diameter.
The risk of nickel sulfide
failure can be reduced by heat soaking as described in Section
08.02.
Section 08.06:Version
1.00.aa
© Centre for Window
and Cladding Technology - 2000, 2001