Mass timber refers to a class of engineered wood products made up of laminated pieces to form larger structural members such as beams, columns, and panels. Gaining popularity in recent years as a sustainable and aesthetically pleasing alternative to traditional construction materials, mass timber structures have many advantages such as being cheaper, faster and safer to build with. However, there are concerns about their fire safety, more specifically, that mass timber structures may be more susceptible to fires than traditional construction materials like concrete and steel.
Research suggests that mass timber structures can be designed to meet or exceed the fire safety requirements of traditional construction materials. In this blog post, we will explore the fire safety of mass timber construction, including its fire resistance and performance, design considerations for enhancing fire safety, fire protection and prevention measures as well as the pathways for compliance.
Fire resistance is the ability of a material or structure to resist, withstand, or slow down the spread of fire or the penetration of heat. It is measured by the length of time that a material or structure can withstand fire exposure without losing its structural adequacy (load-bearing capacity and/or structural stability) and maintain adequate insulation and integrity performance.
Fire resistance is a crucial property for building materials and structures, as it can help prevent or reduce the spread of fires as well as protect human lives and property. Due to wood’s combustible nature and its potential to fuel fires, it is important to prioritise fire resistance in mass timber buildings. By incorporating fire-resistant design and construction measures, mass timber buildings can be made highly resilient and safe from fire hazards.
Mass timber can perform better in a fire compared to steel and concrete due to its inherent charring behaviour. When exposed to fire, the outer layer of mass timber will char, which creates a protective layer that insulates the underlying wood and slows down the spread of fire. This is known as self-charring. The unaffected wood behind the sacrificial char maintains structural integrity.
In contrast, steel and concrete can lose their strength and structural integrity when exposed to high temperatures. Steel can weaken and buckle when heated, while concrete can spall and crack due to the expansion of water trapped inside the material. This can lead to building collapse or extensive damage during a fire. Mass timber has a lower thermal conductivity compared to steel and concrete, meaning it does not conduct heat as readily. Aside from being a natural insulator, this can help prevent the spread of fire to adjacent materials and slow down the overall progress of the fire.
MASSLAM, ASH’s range of mass timber components, passed Australia’s first 120 minute ‘loaded’ beam fire test. The glue-laminated members were subjected to the Fire Resistance Test (FRT) and Fire Resistance Level (FRL), meeting the guidelines specified in the Australian Standards 1530.4:2014. Despite being exposed to temperatures exceeding 1,000 degrees Celsius, the MASSLAM members remained structurally sound and capable of bearing the expected load of a commercial building.
While it’s important to note that mass timber is not completely fireproof, its inherent charring behaviour can make it a viable and safe option for construction when appropriate fire protection measures are in place, even on the assumption that other fire protection measures (such as fire brigade intervention and sprinklers, etc.) fail. Utilising sprinklers and mass timber protection measures together, generate a safe outcome. Sprinklered buildings are significantly safer than non-sprinklered buildings, regardless of structure and outcomes for occupant health and mortality are dramatically improved. A positive consequence of mass timber construction is that sprinklers are typically used. FRL’s come into play upon the unlikely scenario that sprinklers have failed.
When designing a mass timber structure for fire safety, if not pursuing a Deemed to Satisfy pathway (DtS), you should consult your fire engineer. However, several principles should be considered to ensure the safety and resilience of the building. This includes:
Performance of glue types in mass timber systems influence fire performance. Not all glues are created equal. Some glues, if not meeting the prescriptions of AS1720.4, need to be validated through compliance to AS1530.4 at NATA accredited laboratories. This is due to the variability in glue performance under fire from company to company.
ASH use PUR glues due to their improved health and safety factors, VOC’s and suitability for volume manufacturing. Some glue types from other companies may plasticise at low temperatures, others at >300°C (which is the temperature that timber chars. Meaning that the wood fibre is the limitation, not the glue).
Of course, in the case of non-edge laminated CLT, for example, if the first layer is <50mm from the face, temperatures of >300°C will reach the glue line within the nominated FRL period and there is a risk that it may de-laminate, if not using a high temperature glue and/or validated. These CLT producers advertise char rates that incorporate the specific char performance of their make-up. MASSLAM, however, is face and edge glued, in thicker sections, and of denser hardwoods (which char much slower). The closest a glue line will be to the bottom edge is sometimes as much as 140mm. The sacrificial char required to meet 90min FRL in MASSLAM 45 = 50mm.
MASSLAM has been tested in numerous NATA accredited tests for char, char under load, column to beam connections and composite flooring systems for 90 & 120 FRL requirements, each under load. Each test validates the ability of the member to maintain the load for our advised sacrificial char rate.
Regardless, the Australian Standards provide a pathway. AS1720.4 presents a prescriptive method to calculate a notional char rate if you have not tested to AS1530.4 but only if you are using a phenolic glue. If not using a phenolic, it states to test at a NATA accredited laboratory to AS1530.4.
AS1720.4 – part 3.2.2 states the following:
Despite this pathway existing, the following tests and assessment re-validate this pathway in Australian Standards.
For more information about fire performance and NATA accredited fire tests, contact your MASSLAM specialist.
Mass timber is a sustainable and aesthetically pleasing alternative to traditional building materials. When carefully designed and constructed, mass timber can exceed the fire safety performance of other building materials and meet building codes and regulations.