Performance of Glazing in Bushfire Prone Areas

Over the years, glazing has grown in popularity in residential and commercial buildings. A highly recyclable and cost-effective material, glass offers uniqueness and an extraordinary level of transparency that can brighten up a building's space. In areas prone to bushfires, however, it is crucial to ensure that buildings incorporating glazing maintain adequate fire hazard properties to withstand the intense radiant heat and ember attack.

Glazing and Bushfire Resistance

Typically, glass is installed on a steel or timber frame. Once the glass is exposed to radiant heat, the exposed side heats up, leading to a thermal gradient across the thickness of the pane. The thermal gradient imparts thermal stress on the material, which is primarily distributed in the central region of the pane.

The frame somewhat shields the edges and the rims and remains relatively cooler. This means the central area expands while the edges stay in place. This shift in the thermal gradient causes the glass to break. Glass can also crack at the edges due to a large thermal gradient between the embedded glass in the frame and the exposed glass.

In addition to this, failure can also occur due to other glazing components. Several studies have shown that variables such as glass type, frame material and fixings are vital in bushfire performance. For example, insulated and laminated glass are likely to offer superior bushfire performance compared to single, non-insulated glass.

Key Performance Factors in Glass

Glass is versatile and manufactured to meet specific applications within construction.

• Laminated glass is comprised of multiple thin layers, making it more resistant to impact.

• Toughened glass undergoes a special manufacturing process and contains inert components, giving it superior strength.

• Floating glass is produced in various colours to enhance a building’s aesthetic appeal.

• Tinted glass is designed to absorb UV rays and provide privacy.

Key performance factors in insulated glass are reported to be the air gap and fireside pane size. In the case of laminated glass, the interlayer gel is reported to be crucial in limiting shattering and gap formation. Thicker glass and frames generally perform better than their thinner counterparts.

Apart from thickness, the construction of the frame is also crucial. As more combustibles are introduced (e.g., seals and gaskets), the structure will be less likely to perform well under a bushfire attack. The nature of the fixing has also been observed to influence the glazing's performance. For example, point fixing introduces weak points that are prone to premature failure.

Assessing Bushfire Attack Levels

Standards Australia AS 3959:2018 stipulates the construction requirements for buildings in bushfire-prone areas, which are categorised based on the expected radiant heat exposure level.¹

• BAL LOW
• BAL 12.5
• BAL 19
• BAL 29
• BAL 40
• BAL FZ

Each Bushfire Attack Level (BAL) represents a predicted level of bushfire attack. As specified in section 2 of AS 3959:2018, you must assess the applicable BAL for any building construction. Sections 5.5, 6.5, 7.5, 8.5 and 9.5 of AS 3959 stipulate the construction requirements for external glazing in BAL 12.5, BAL 19, BAL 29, BAL 40 and BAL FZ exposures. These stipulations relate to additional screens or shutters, frame material, hardware, glazing, seals and weather strips.

As the BAL exposure increases, the requirements become more stringent. For example, under BAL 12.5, bushfire shutters protecting windows and doors can be made of:

• Non-combustible material.
• A timber species specified in Paragraph E1 of Appendix E.
• Bushfire-resisting timber.
• Any combination of the above.

In contrast, under BAL 40, the shutters can only be made of non-combustible materials. Likewise, the frame must strictly be constructed of metal under BAL 40, while the frame material for windows and sidelights under BAL 12.5 can be made of:

• Bushfire-resisting timber.
• A timber species specified in Paragraph E1 of Appendix E.
• Metal or metal-reinforced uPVC.

In addition to the requirements mentioned above, any novel glazing system that satisfies the test criteria of AS 1530.8.1:2018 (from BAL 12.5 to BAL 40) or AS 1530.8.2:2018 (BAL FZ) can be used.^2,3 This satisfaction of the test criteria makes the guideline in AS 3959:2018 dynamic. As construction is not rigidly restricted to the standard's requirements, new systems can be introduced if they demonstrate satisfactory performance.

What's Next for the Glazing Industry and Bushfire Requirements?

Given the popularity of glazing, the industry is adapting to bushfire requirements by offering products that strictly adhere to AS 3959:2018. New and innovative products are also taking advantage of the pathway through AS 1530.8.1:2018 or AS 1530.8.2:2018. Some of these innovations include reinforcing frames, incorporating additional protection such as seals, gaskets, intumescent materials or mineral wools within the frames, and enhancing the strength of the glass by introducing features like air gaps, inert gases or intumescent layers.

Through careful planning, durable and robust constructions can be developed that significantly reduce the risk of house loss during a bushfire while maintaining an aesthetically pleasing design.

References

1. Standards Australia. (2018). Construction of buildings in bushfire-prone areas (AS 3959:2018).

2. Standards Australia. (2018). 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 (AS 1530.8.1:2018).

3. Standards Australia. (2018). Methods for fire tests on building materials, components and structures – Part 8.2: Tests on elements of construction for buildings exposed to simulated bushfire attack - Large flaming sources (AS 1530.8.2:2018).