Can low emissivity glazing be utilized for preventing fire spread into buildings?

Andrew Oxley, Dr Weng Poh

Umow Lai 10 Yarra Street, South Yarra, Victoria, 3141, Australia

Nathan White, Dr Bill Tiganis

CSIRO 37 Graham Road, Highett, Victoria, 3190, Australia

ABSTRACT The benefits of low-emissivity glazing in building façades are well recognized with regard to energy efficient design of buildings. These glazing systems are designed to limit the transmission of solar radiation into the buildings and hence reduce the solar heat gain and energy required for mechanical cooling of the buildings. To the best of the authors’ knowledge, there have been no studies conducted to explore the potential of such glazing for use in preventing fire spread through radiation into buildings. In particular, whether unprotected low-emissivity glazing can be utilized to prevent fire spread through radiant heat into a building from a significant fire in the adjacent building. In order to explore this potential, 3 small-scale specimens of low-emissivity glazing were tested at the CSIRO laboratory in Melbourne. Activation of external sprinklers exposed to radiant heat was also examined. The specimens were representative of the options of façade systems specified for an actual high-rise residential building in Melbourne. They were double-glazed with low-emissivity glass in the inner glass panes. The glazing was supported in aluminum frames. The testing was conducted by subjecting each specimen to incident radiation flux of at least 40kW/m2 emitted from a 1.5 m × 1.5 m gas fired radiant heat panel for a period of up to 60 minutes. The results show that the low-emissivity glazing tested significantly reduced the transmission of radiation through the glass. In all tests the levels of radiation recorded on the inner side of the glazing were well below that required to cause fire spread. In using the glazing system for preventing fire spread into the building, consideration must be given to ensure the glazing remains in place. For the testing provided, Specimens 1 and 2 dislodged from the frames or fractured prior to the intended exposure period. Test Specimen 3 remained in place for the entire test period of 60 minutes and passed the criteria for preventing fire spread.

INTRODUCTION One of the objectives of fire safety design of buildings is to prevent fire spread between buildings. This includes preventing fire spread between buildings through glazed openings in the external walls. Such fire spread may occur when the radiant heat from the adjacent building of fire origin transmitted through the glazing exceeds the critical heat flux to cause ignition of the materials in the adjacent building.

Figure 1 Radiant heat from fire in adjacent building

To prevent fire spread, one of the solutions prescribed by the Building Code of Australia (BCA) [1] is to install external wall-wetting sprinklers to protect the glazing. The question is, without the protection of the external wall-wetting sprinklers, can the glazing over the openings be relied on or designed to prevent fire spread - particularly using low- emissivity double glazing? Low emissivity, or simply called low-e, glazing is increasingly used in external windows or façades of buildings to reduce solar heat gain, thereby increasing the energy efficiency of the buildings. The low emissivity of the glazing is achieved by treatment to one surface of the glazing to provide a transparent coating, which allows visible light to penetrate, but reflects a significant portion of infra-red light (heat energy).

Figure 2 Transmission of Solar Radiation Through Low-e Glazing

Radiant heat from fire is similarly infra-red radiation, albeit with significantly larger wavelength than those within the solar spectrum. Based on the information on the radiation attenuation for solar radiation, it is expected that the glazing would also be able to reduce the passage of radiant heat from fire. However, to the best of the authors’ knowledge, there have not been such studies conducted. In order to explore the potential of such glazing for use in preventing fire spread through radiation into buildings, a test program was formulated to investigate whether unprotected low-emissivity glazing can be utilized to prevent fire spread by radiant heat into a building from a significant fire in the adjacent building.

RADIATION TESTING The investigation comprised testing of 3 small-scale specimens at the CSIRO laboratory in Melbourne. The tests were conducted by subjecting the specimens to radiation emitted from a gas fired radiant heat panel to represent the exposure to a fire occurring in an adjacent building. Test Specimens The test specimens, each measured 1 m × 1 m in glass pane area, were manufactured to suit the test set up and to represent the glazing options being considered for the curtain wall façade of a high-rise residential building in Melbourne. Each of the glazing systems was designed to satisfy the safety glass, energy efficient requirements of the BCA and architectural requirements for the building.

Figure 3 Test Specimens

The specimens had the same outer glass panes and varied in the inner panes. The glass panes varied with respect to thickness, manufacturing process and heat treatment which include:

• Annealed - This glass is cooled gradually during manufacture to reduce residual stresses and strains which can be produced during cooling. At failure, it tends to break into large jagged shards.

• Heat strengthened – This glass is strengthened by heat treatment to induce residual stresses. At failure, it tends to fracture into large fragments similar to annealed glass.

• Toughened (or tempered) – This glass is strengthened by heat treatment and rapid cooling so that the residual stresses are relatively high (greater than those for heat-strengthened glass). This provides increased strength in the order of four to six times the strength of annealed glass. At failure, it tends to shatter into small pieces.

• Heat soaked toughened (or tempered) – This includes further heat treatment of the toughened glass by heating it for a period of about two hours and then slow cooling. This provides better stability in the glass thereby reducing the chance of imperfection in the glass causing failure.

• Laminated - two or more sheets of glass permanently bonded together by a plastic interlayer material. At failure of the glass, the glass will tend to adhere to the plastic interlayer instead of falling apart.

The three specimens had the same coatings — a reflective coating on surface 2 (outer pane) and a double silver low-e coating on surface 3 (inner pane). The internal air gap varied from 9 mm to 16 mm for the 3 different specimens. The double-glaze assemblies were mounted within identical aluminum frames. The glazing was attached to the frames by means of structural silicone, in the same way that they would in the façade system of the building considered. It is noted that, after witnessing the failure of the structural silicone in the first test on Specimen 1, additional aluminum angles were also screw fixed to the top and bottom edges of the frame of Specimens 2 and 3 in an attempt prevent the outer glass panes from falling out of the frame.

Test Set-up In order to test the specimens, each of the specimens was first mounted on a movable platform, which was towed towards a gas fired radiant heat panel to simulate increasing radiation exposure of the specimen.

Figure 4 General Test Set-up

In order to gather some activation data on the BCA prescribed wall-wetting sprinkler solution, three sprinklers were attached to the Specimen 1. The sprinklers were representative of three different types and mounting arrangements that comply with the Australian Standard 2118.1 [4]. These include:

1. 93°C quick response sprinkler (Tyco model TY3431) mounted in pendant orientation 2. 93°C quick response pendent sprinkler (Tyco model TY3231) mounted in horizontal

orientation 3. 93°C quick response sidewall sprinkler (Tyco model TY3331) mounted in pendant

orientation

Figure 5 Test Specimen 1 (with air charged sprinklers) prior to testing

The sprinklers were connected to a section of 25 diameter black steel pipe and attached to the top of the Test Specimen 1. The sprinklers and the pipes were not charged with water. They were however charged with compressed air to help identify time of activation during the test. Test instrumentation Three heat flux meters were set up for measuring the heat flux received at the top, centre and bottom of the specimen (see figure below).

Figure 6 Test Instrumentation

The position of the Heat Flux Meter 1 on the inside 365 mm from the glazing was based on the set-up specified in AS 1530.8.1 [6].

Radiation heating The test set up was aimed at subjecting each test specimen to a thermal radiation of at least 40 kW/m2 for a duration of 60 minutes. The radiation exposure was selected with reference to verification method CV1 of the BCA which requires an opening to withstand 40 kW/m2 when it is located 1 m from the allotment boundary. The exposure period of 60 minutes was selected to correlate to the period of fire resistance required by the BCA for construction required for protection against fire spread from an adjacent building. No inference can be made from the BCA on the rate of rise of the thermal radiation at the start of the tests. Hence guidance was sought from AS 1530.4 [6] where a period of 10 minutes was specified to achieve the target heat flux for radiation heating. For the sake of simplicity, it was decided to move the specimen at the lowest speed allowed by the testing apparatus to the location where an exposure of at least 40 kW/m2 is expected. This took 500 s, which is within the 10 minutes specified by AS 1530.4 The location of the specimen to achieve a minimum exposure of 40 kW/m2 to the specimen was determined by calibration tests conducted without a specimen in place. It is also noted that the incident heat flux during the actual tests was greater than the calibration tests due to the presence of a specimen being located in close proximity to the radiant panel. This caused re-radiation between the specimen and the radiant heat panel. This resulted in increased panel temperature and hence increased incident heat flux to the specimen. The calibration tests indicated that heat flux at the centre (Heat Flux Meter 1 without the presence of specimen) was approximately 10 kW/m2 greater than at the top or bottom of the specimen location (Heat Flux Meters 2 and 3). This is due to the configuration factor of the radiant heat panel. During the actual tests the radiant heat flux of up to 50 kW/m2 was recorded at the bottom of the specimens (Heat Flux Meter 2). This means that an incident heat flux of up to 60 kW/m2 may have occurred at the centre of the specimens. Hence, the specimens were exposed to radiant heat significantly higher than the target value of 40 kW/m2. Test criteria Fire spread into the non-exposed side was considered to have been prevented if, for the duration radiant heat exposure of 60 minutes:

• the radiation measured on the non-exposed side remained less than 20 kW/m2 • no flaming of any of materials on the non-exposed side

The 20 kW/m2 limit above is a level generally accepted as the limiting level for non-piloted ignition of cellulosic materials in the design of buildings [3].

TEST RESULTS AND OUTCOME Test 1 – Specimen 1 The heat flux on the inside remained below 3 kW/m2 during the test while the low-e glazing (inner pane) was in place.

Received Radiation (bottom of specimen)

Transmitted Radiation (top and centre of specimen)

The air-charged sprinklers activated after 5 and 7.5 minutes of exposure. The outer glass pane dislodged and fell out as a complete panel after approximately 11.5 minutes. The test was stopped and re-started; and the inner glass pane cracked and fell out after approximately 32 minutes.

Time 7:30 - Window seals at edge frame start smoking Time 11:26 - External glass pane falls out of the support frame

Time 18:30 - Exposed surfaces of seals ignite and starts flaming (no flaming on the inside)

Time 32:00 - The inner glass pane cracks and larger sections of glass gradually fall away

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Test 2 – Specimen 2 The heat flux on the inside remained below 7 kW/m2 while the low-e glazing (inner pane) was in place.

Received Radiation (bottom of specimen)

Transmitted Radiation (top and centre of specimen)

The outer glass pane cracked and fell out after approximately 8 and 10 minutes, respectively. The inner pane shattered after approximately 31 minutes.

Time 8:14 - Exterior glass pane cracks Time 10:06 - Exterior glass pane falls out of frame

Time 18:21 - Flaming along external seals at all edges (no flaming on the inside)

Time 31:42 - Inner pane suddenly shatters into small granular pieces

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Test 3 – Specimen 3 The heat flux on the inside remained below 6 kW/m2 for more than 60 minutes.

Received Radiation (bottom of specimen)

Transmitted Radiation (top and centre of specimen)

The outer glass pane cracked and fell out after approximately 8 and 10 minutes, respectively. The inner pane remained in place for more than 60 minutes and the test was terminated. The inner plane cracked and fell out after the test shortly after the heating panel was turned off.

Time 8:00 - Outer glass pane cracks Time 9:14 - Pieces of outer glass pane begin to gradually fall away

Time 16:40 - Flaming on external seals (no flaming on the inside)

Time 68:00 - Inner Glass pane still intact

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TEST SUMMARY Test Specimen Time from start of test (min:sec)

and observation Assessment against test criteria criteria

passed

1 1 Outer pane: 8 mm heat strengthened Inner pane: 13.25 mm laminated clear annealed

5:08 Sprinkler 3 activated 6:09 Sprinkler 1 activated 7:33 Sprinkler 2 activated 11:26 Outer glass pane fell out of

the frame. (Test halted for 6 minutes.

Debris removed and test restarted)

32:00 Inner pane cracked, laminating material melted and pane fell out.

Heat flux measured on the inside < 3 kW/m2 (significantly less than 20 kW/m2 required for fire spread) However both the outer and inner and outer glass panes fell out of the frame before 60 minutes.

2 2 Outer pane: 8 mm heat strengthened Inner pane: 6 mm clear tempered heat soak (toughened)

8:50 Outer glass pane cracked 10:06 Outer glass fell out 31:42 Inner glass pane (6 mm clear

tempered and heat soaked) shattered into small granulated pieces after significant bowing of the pane.

Heat flux measured on the inside < 7 kW/m2 (significantly less than 20 kW/m2 required for fire spread) However both the outer and inner and outer glass panes fell out of the frame before 60 minutes.

3 3 Outer pane: 8 mm heat strengthened Inner pane: 10 mm clear tempered heat soak (toughened)

8:00 Outer glass pane cracked 9:14 Outer glass pane fell out 68:00 Test terminated (The inner glass pane

remained in place. It bowed during the test but amount of bowing observed was less than that for test 2).

Heat flux measured on the inside < 6 kW/m2 (which is significantly less than 20 kW/m2 required for fire spread) No flaming of material on the inside observed. The outer glass panes fell out of the frame but the inner pane remained within the frame beyond the test period of 60 minutes.

COMMENTS ON SYSTEM PERFORMANCE Radiation performance

• The radiation received behind all three glazing systems remained below 7 kW/m2 while the low-e glazing (the inner panes) remained in place. This is well below the criterion of 20 kW/m2 for fire spread.

• The low-e glazing appeared to have prevented more than 85% of the incident radiation heat from passing through – i.e. reducing from more than 50 kW/m2 to below 7 kW/m2.

• The radiation performance does not appear to be affected by the presence of the outer glass pane – i.e. there was no significant increase in the heat flux received on the inside when the outer glass panes fell off.

Combustibility performance • The frames and glazing are generally non-combustible and did not burn. • Other components of the glazing system however did burn and flame. These include

the structural silicone that supports the glazing and the spacers. The material in the laminated glass (Specimen 1 also melted, burned and flamed).

• In the test, the flaming of the above material occurred on the outside. No flaming was observed on the inside.

Outer structural silicone flaming Spacers or silicone sealant flaming Laminating materials melted and

flaming Structural performance

• The outer glass panes of all three specimens failed relatively early during the tests (at approximately 10 minutes). The mechanism of failure varied:

o Specimen 1 – fell off as a complete panel (without cracking) due to the failure of structural silicone that supports the glass pane.

o Specimen 2 – cracked and fell off. o Specimen 3 – shattered and fell off.

• The failure mechanism of the inner panes also varied: o Specimen 1 – the laminated glass stayed in place until the laminate materials

melted. o Specimen 2 – the thinner toughened glass bowed outwards significantly before

it cracked and fell off o Specimen 3 – the thicker toughened glass also bowed outwards but remained

in place for more than 60 minutes. It failed after the test during cooling. • Significant outwards bowing of the glazing was observed in all the tests.

Figure 7 Outward bowing of glazing

CONCLUSIONS The radiant heat testing conducted on 3 different small-scale specimens shows the low-e glazing tested blocked a significant portion of radiant heat from transmitting through the glazing. When exposed to an outside radiant heat of more than 40 kW/m2, the heat flux received inside was less than 7 kW/m2.

While the low-e glazing of all three specimens tested reduced the transmission of radiant heat, the glazing of one specimen remained in place for more than 60 minutes, those of the other two specimens failed and fell of the frame after approximately 30 minutes. This highlighted that, for the low-e glazing to be successfully utilised to prevent fire spread into the building, the glazing must be able to remain in place for the desired duration of exposure.

Observations during the tests indicated that there are many factors could lead to failure of the glazing. These include:

• Failure of the supports including the frame or the structural silicone which could deteriorate or burn away.

• Failure of the glazing (cracking or shattering) that could be related to: o glass manufacturing process and heat treatment that induces in plane stress

within the glazing o edge treatment, or lack of, that could contain imperfections that promote crack

propagations o uneven heating or exposure of the glazing due to partial shielding of the glass,

flame impingement etc o excessive bowing of the glazing particularly for large glazing panes o thermal shock caused by sprinkler water spray.

• Flaming spread due to burning of support of laminating materials. The above factors have not been thoroughly investigated in the testing presented in this paper. Behaviour of full-scale glazing system could vary the test outcome of the small-scale tests. Further investigations and testing, including testing of full-scale specimens, may be required in order to develop a general solution or guidelines on the solution approach.

Nevertheless, the testing conducted thus far shows that low-e glazing can be utilized for preventing fire spread into buildings. REFERENCES

[1] “Building Code of Australia”, Volume 1—Class 2 to 9, Australian Building Codes Board, 2014.

[2] “Glazed Facade System Radiant Heat Exposure Tests”, CSIRO Report No. EP142918, May 2014.

[3] Bennetts, I.D., Poh, K.W., and Thomas, I.R. “A Framework for Fire-Engineering Design”, Australian Journal of Structural Engineering, IEAust., 3(1-2), 9-22, 2000.

[4] “AS 1288 Glass in buildings - Selection and installation”, Standards Australia, 2006. [5] “AS 2118.1 Automatic fire sprinkler systems. Part 1: General requirements”, Standards

Australia, 1999. [6] “AS 1530.4 Methods for fire tests on building materials, components and structures,

Part 4: Fire-resistance test of elements of construction”, Standards Australia, 2005. [7] “AS 1503.8.1 Methods for fire tests on building materials, components and structures

Part 8.1: Tests on elements of construction for buildings exposed to simulated bushfire attack—Radiant heat and small flaming sources”, Standards Australia, 2007.