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Background
The size
(more technically, the heat release
rate) of fires is limited by the flow of oxygen available to it. In
all except very rare circumstances, the flow of oxygen into a room comes
largely from open doors and open windows, and to a slight extent from
any mechanical ventilation systems and from building leakage. Once a fire
gets going, however, windows previously closed may crack and break out.
Or...they may not. The results will often be drastically different, depending
on whether the windows break or not. Thus, it becomes of significant interest
to be able to predict if, and when, glass may break out.
Here, an
important distinction needs to be made. When a window pane of ordinary
float glass is first heated, it tends to crack when the glass reaches
a temperature of about 150 - 200ºC. The first crack initiates from one
of the edges. At that point, there is a crack running through the pane
of glass, but there is no effect on the ventilation available to the fire.
For the air flows to be affected, the glass must not only crack, but a
large piece or pieces must fall out.
Understanding
the conditions under which pieces actually fall out has been of considerable
interest to many persons concerned with fire. Since the fire ventilation
openings need to be known in order for fire models to be used, glass breakage
has been of special interest to fire modelers. This has prompted a number
of theoretical and simplified studies and a few empirical ones as well.
It must be
realized that there are at least two distinct types of thermal exposure
to glass that is involved in fires:
- A window
is inside a room in which a fire is taking place. The window is being
subjected to ijavascript:MMersion heating from one side. The local gas temperature
and the radiating temperature are rather similar. There may be a gradient
of temperature and heat flux from the top down to the bottom.
- A window
is exposed to an outside fire, typically a wildland or bush fire. In
that case, there may be relatively little difference in exposure between
the top and the bottom of the window. The heating is primarily by radiation.
Local gas temperatures may be near-ambient, since flames are not directly
washing against the window and there is a convective cooling flow along
the surface.
Theoretical
and experimental studies of glass cracking in fires
Keski-Rahkonen
[1] presented the first extensive theoretical analysis of glass cracking
in fires. He identified that temperature differences between the exposed
glass surface and the glass shielded by the edge mounting play the dominant
role in controlling cracking. His theory predicted that a temperature
difference of about 80°C between the heated glass temperature and the
edge temperature is needed to initiate cracking. Pagni and Joshi [2] extended
these ideas to include more heating physics and an expanded consideration
of glass thermal properties. They predicted 58°C as the temperature difference
for crack initiation. The difference was largely due to assuming different
thermal and mechanical properties for glass. Skelly [3] conducted a series
of experiments in an unusual small-scale fire test room. One peculiarity
of his tests was that the windows were never exposed to a vertical temperature
gradient. He reported some fall-out of glass sections, but did not provide
any guidelines or tabulations to assist in determining conditions leading
to glass breaking out.
Mowrer [11]
presented the latest experimental study on the subject, heating both large-scale
and small-scale specimens with a radiant panel to simulate a wildland,
external exposure. The maximum heat fluxes, which went up to 16 kW m-2,
were sufficient to cause cracking, but not breaking out of window panes.
Cracking of single-strength glass was found to occur at 4 to 5 kW m-2.
Either black or bright insect screens raised by about 21% the heat flux
needed to cause cracking. He also found that approximately 33% of the
radiation incident on a single-strength pane of glass is transmitted through
it. This information can be of use in estimating ignitions inside a building
from external radiation.
The National
Research Council of Canada (NRCC) has had a program for developing sprinkler
protection for glazing. As part of that work, a few non-sprinklered tests
have been run where glass 6 javascript:MM thick tempered glass was exposed to simulated
room fire conditions, but without a sprinkler [9]. While such glass type
would only be cojavascript:MMon in cojavascript:MMercial buildings, the results are nonetheless
of interest. Tempered glass behaves differently, in that it shatters upon
initial cracking, but the initial cracking does not occur until the glass
reaches rather high temperatures. An exposed-surface temperature of 290-380ºC
has been found to be needed, with the unexposed surface temperatures being
about 100ºC lower. Such glass temperatures are normally not reached until
after room flashover has occurred. In a later study [10], NRCC examined
glazing using radiant heat exposure. Under such conditions, "plain" glass
of unspecified thickness was found to "break" when the exposed side reached
150-175ºC, with the unexposed side being at 75-150ºC.
Experiments
and guidance on glass breaking out in fires
The earliest
guidance to be found in the literature on the question of when glass breaks
out in fires comes from the Russian researcher Roytman [4] who notes that
a room gas temperature of around 300°C is needed to lead to glass
breakage. The research base for this conclusion is unclear, however.
Hassani,
Shields, and Silcock [5] conducted a series of experiments in a half-scale
fire test room using 0.9 x 1.6 m single-glazed windows where they created
a natural top-to-bottom temperature gradient in the room and in the glass.
At the time the first crack occurred in 4 or 6 javascript:MM thick glass panes, gas
temperatures in the upper layer of 323 - 467°C were recorded. By the end
of their 20 min tests, gas temperatures were at ca. 500°C. Yet in only
1 of 6 tests was there any fall-out of glass. Temperature differences
between the glass exposed surface and the shielded portion ranged between
125°C to 146°C at the time of crack initiation. These temperatures were
about twice that predicted from the no-vertical-gradient theories. The
authors do not give the exact room fire temperature at which the glass
fall-out began in the one test where this occurred, but this had be higher
than 431°C (crack initiation) and lower than ca. 450°C (end of test).
One can put these data together, then, to conclude that at a room gas
temperature of around 450°C the probability is 1/6 for glass to break
out. The same authors [15] conducted further tests using a room with two
windows glazed with 6 javascript:MM thick panes. In two tests, fire temperatures
of 400 - 500°C in the vicinity of the glass resulted in no fall-out of
glass. In the third test, one pane fell out at a gas temperature of 500°C.
The gas temperature when the second window's pane fell out was also about
500°C, but some 8.5 more minutes had gone by, the temperature being relatively
steady in that time.
The
newest experimental results concerning glass exposed to a uniform hot
temperature come from the Building Research Institute (BRI) of Japan [6].
In that study, researchers used a large-scale high-temperature door-leakage
testing apparatus that resembles a large muffle furnace. Only single-glazed,
3 javascript:MM thick window glass was studied. For this type of glass, however,
enough tests were run so that a probability graph could be plotted. These
researchers' results are presented in terms of a probability of glass
breaking out, as a function of temperature rise above ambient. The figure
below shows the results.
The Gaussian
fit that can correlate this data corresponds to a mean temperature rise
of 340°C, and a standard deviation of 50°C. Thus, the BRI results are
somewhere in between the two earlier values.
The Loss
Prevention Council of the UK [12] studied room fires which were providing
fire exposure to a multi-story façade test rig. Double-glazed windows
were examined, with each pane being 6 javascript:MM thick. Using 3 MW wood crib fires,
it was found that temperatures of at least 600ºC had to be sustained for
8 - 10 min before glass started falling out sufficiently so that fire
venting would occur. When tests were repeated using a fully-furnished
office room arrangement, however, glass broke out at 5 min after the start
of fire. In that test, the temperature was also about 600ºC at the time
of failure, but occurred ijavascript:MMediately as the temperature was reached. Thus,
the findings lead to the conclusion that double-glazed windows using 6
javascript:MM thick glass will fail at ca. 600ºC and that, if the fuel load is significant,
the failure may be expected to occur essentially at the instant that 600ºC
is first reached.
Shields,
Silcock and Hassani [13] exposed two sizes of double-glazed windows to
room fires. The glass thickness was 6 javascript:MM. The room fire reached a peak
of 750ºC and no glazing fell out up to the peak. However, during the decay
part of the fire, in one of 3 tests with the larger-size window (0.8 x
1.0 m) fall-out of the inner pane occurred at 21 min, when the temperature
had dropped to 500ºC. Glass did not ever fall out from the outer pane,
nor did any fall-out occur in the smaller (0.8 x 0.5 m) window, nor did
any fall-out occur in the other two tests. The same authors later [14]
tested a room having a wall with twelve1.5 x 1.5 m double-glazed windows.
The test record is very brief, but it is indicated that total failure
of the first window occurred when the gas temperature was at 350ºC; it
is not clear what the temperatures were for the fall-out of the subsequent
windows.
For radiant
exposure, Cohen and Wilson [7] reported on an interesting series of experiments
simulating flame exposure from a wildland fire. They examined small (0.61
x 0.61 m) and large (0.91 x 1.5 m) panes, single- and double-glazed. They
also repeated the tests with tempered glass and with double-glazed windows.
For the small windows, at their lowest heat flux, 9.3 kW m2,
all windows cracked, but no glass fell out. Even at the highest flux of
17.7 kW m2 panes did not fall out. For the larger size
panes, at fluxes of 16.2 to 50.3 kW m2, at least one out
of 3 test specimens exhibited fall-out. Tempered glass, by contrast, showed
no cracking at tests up to 29.2 kW m2 in the larger size.
The authors also did tests on double-glazed windows, which showed better
performance. In experiments with large-size double-glazed windows (non-tempered),
they found that fluxes between 20 and 30 kW m2 were required
to cause fall-out in both panes.
Harada et
al. [17] tested 3 javascript:MM thick float glass (specimen size: 0.5 by 0.5 m) by
subjecting them to various heat fluxes from a test furnace. Below 8 kW
m-2, no significant fallout occurred, but for a heat flux of
9 kW m-2, in some cases 8 - 24% of the specimen area fell out.
Edge constraint did not affect the results.
Additional
data are available from the NRCC study [10], where heat-strengthened and
tempered glass (unspecified thickness) was found not to break at an irradiance
of 43 kW m-2. The latter heat flux corresponded to 350ºC on
the exposed face and 300ºC on the unexposed face. Thus, this appears to
extend Cohen's data point of 29.2 kW m-2 for non-breakage to
43 kW m-2, without actually determining the point at which
breakage and fall-out do occur.
Other
types of glass
The published
studies have dealt primarily with thin panes of annealed or tempered soda
glass. Yet, there are many other types of glass to consider. Some very
thick plate glass is used in many cojavascript:MMercial buildings. Plate glass of
6 javascript:MM thickness was found to shatter after a significant time (7 min) of
exposure to a radiant heat flux of 23 kW m-2 [15]; otherwise
information is not available on its performance. Automotive glass is another
category which has not been studied in a systematic way. Finally, there
are various fire-resistive glasses. The oldest category of the latter
is wire glass. Nowadays, several types of patented fire-resistive glasses
also exist which are not wired glass. These are usually multi-layered
structures, generally involving some polymeric inner layers. Fire-resistive
glasses will normally be accompanied by a laboratory report of the endurance
period. Such glasses can be assumed to have no ventilation flow until
after their failure time.
Effect
of window frame type
Actual fall-out
of glass from windows is also influenced by the window-frame material.
Mowrer [11] found that vinyl-frame windows tended to show a failure of
the frame (e.g., the whole assembly collapsing) before significant fall-out
of glazing. Vinyl frame failures were observed when heat fluxes came up
to the range of 8 to 16 kW m-2. By contrast, McArthur [16]
found that glass in aluminum-framed windows showed a tendency to survive
longer than did glass in conventional wood-frame windows.
Conclusions
A theory
exists for predicting the occurrence of the first crack in glazing, but
this is not directly relevant to fire ventilation. The above brief review
of the literature shows that it is, in fact, very difficult to predict
when glass will actually break enough to fall out in a real fire. The
Russian recojavascript:MMendation of 300°C appears to be a reasonable lower bound.
The new Japanese study can be taken to imply that 3 javascript:MM window glass will
break around 340°C. For thicker, 4-6 javascript:MM glass, the mean temperature of
breakage would appear to be in excess of 450°C, although the difference
between the thinner and the thicker glass results seems rather larger
than one would surmise. Double-glazed windows using 6 javascript:MM glass can be
expected to break out at about 600ºC. Tempered-glass in not likely to
break out until after room flashover has been reached.
In terms
of external fires, at a heat flux of 9 kW m2 some experimental
results on ordinary glass showed the possibility of significant fallout.
Other studies found that higher heat fluxes were needed, thus 9 kW m-2
appears to be a conservative, but realistic lower bound. Double-glazed
windows can resist approximately 25 kW m2 without fall-out.
Tempered glass is able to resist fluxes of 43 kW m2, at
least under some conditions.
Factors such
as window size, frame type, glass thickness, glass defects, and vertical
temperature gradient may all be expected to have an effect on glass fall-out.
Over-pressure due to gas explosions is an obvious glass failure mechanism.
Yet, normal fires do show pressure variations and these could potentially
affect the failure of glass panes. All of these factors deserve some more
study to obtain useful, quantitative guidance.
The above
review has dealt only with the role of glass breakage in fire ventilation.
A number of other aspects of glass breakage are important to fire investigators;
these have been well presented by Schudel [8].
References
[1] Keski-Rahkonen,
O., Breaking of Window Glass Close to Fire, Fire and Materials
12, 61-69 (1988).
[2] Pagni,
P. J., and Joshi, A. A., Glass Breaking in Fires, pp. 791-802 in Fire
Safety Science-Proc. Third Intl. Symp., Elsevier Applied Science,
London (1991).
[3] Skelly,
M. J., Roby, R. J., and Beyler, C. L., An Experimental Investigation of
Glass Breakage in Compartment Fires, J. Fire Protection Engineering
3, 25-34 (1991).
[4] Roytman,
M. Ya., Principles of Fire Safety Standards for Building Construction,
Construction Literature Publishing House, Moscow (1969). English translation
(TT 71-580002) from National Technical Information Service (1975).
[5] Hassani,
S. K., Shields, T. J., and Silcock, G. W., An Experimental Investigation
into the Behaviour of Glazing in Enclosure Fire, J. Applied Fire Science
4, 303-323 (1994/5).
[6] Tanaka,
T., et al., Performance-Based Fire Safety Design of a High-rise Office
Building, to be published (1998).
[7] Cohen,
J. D., and Wilson, P., Current Results from Structure Ignition Assessment
Model (SIAM) Research, presented in Fire Management in the Wildland/Urban
Interface: Sharing Solutions, Kananaskis, Alberta, Canada (2-5 October
1994).
[8] Schudel,
D., Glass Fracture Analysis for Fire Investigators, Fire and Arson
Investigator 46, 28-35 (March 1996).
[9] Richardson,
J. K., and Oleszkiewicz, I., Fire Tests on Window Assemblies Protected
by Automatic Sprinklers, Fire Technology 23, 115-132 (1987).
[10] Kim,
A. K., and Lougheed, G. D., The Protection of Glazing Systems with Dedicated
Sprinklers, J. Fire Protection Engineering 2, 49-59 (1990).
[11] Mowrer,
F. W., Window Breakage Induced by Exterior Fires, pp. 404-415 in Proc.
2nd Intl. Conf. on Fire Research and Engineering,
Society of Fire Protection Engineers, Bethesda, MD (1998). Also: Mowrer,
F. W., Window Breakage Induced by Exterior Fires (NIST-GCR-98-751), Natl.
Inst. Stand. and Technol., Gaithersburg MD (1998).
[12] Fire
Spread in Multi-Storey Buildings with Glazed Curtain Wall Facades (LPR
11: 1999), Loss Prevention Council, Borehamwood, England (1999).
[13] Shields,
T. J., Silcock, G. W. H., and Hassani, S. K. S., The Behavior of Double
Glazing in an Enclosure Fire, J. Applied Fire Science 7,
267-286 (1997/98).
[14] Shields,
T. J., Silcock, G. W. H., and Hassani, S. K. S., The Behavior of Glazing
in a Large Simulated Office Block in a Multi-Story Building, J. Applied
Fire Science 7, 333-352 (1997/98).
[14] Shields,
T. J., Silcock, G. W. H., and Hassani, S. K. S., The Behavior of Single
Glazing in an Enclosure Fire, J. Applied Fire Science 7,
145-163 (1997/98).
[15] Moulen,
A. W., and Grubits, W. J., Water-Curtains to Shield Glass from Radiant
Heat from Building Fires (Technical Record TR 44/153/422), Experimental
Building Station, Dept. of Housing and Construction, North Ryde, Australia
(1975).
[16] McArthur, N. A., The Performance of Aluminum Building Products in
Bushfires, Fire and Materials 15, 117-125 (1991).
[17] Harada,
K., Enomoto, A., Uede, K., and Wakamatsu, T., An Experimental Study on
Glass Cracking and Fallout by Radiant Heat Exposure, pp. 1063-1074 in
Fire Safety Science--Proc. 6th Intl. Symp., Intl. Assn. for Fire
Safety Science (2000).
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