At the recent IFE national fire conference in Adelaide, several speakers highlighted the need for ‘horizon scanning’ and to question whether or not the design of buildings of mass timber construction should be examined more closely to ensure they do not follow the cladding crisis as the next major building failure mode. These included Russ Timpson, founder of the Tall Building Fire Safety Network.

Mass timber buildings are proliferating both in Australia and internationally. Their popularity is based on their sustainability advantages over steel and concrete framed buildings and being a differentiator in a crowded leasing market. These timber buildings often utilise glue-laminated timber (glulam) or Laminated Veneer Lumber (LVL) for framing and/or Cross Laminated Timber (CLT) for floors and walls. They are also increasing in height. A good number of commercial and residential timber buildings now reach 20 storeys or more.

An important benefit of timber construction is that structural elements, internal surfaces or even external facades may be left as exposed timber for aesthetic purposes.

Unlike buildings of non-combustible construction, however, mass timber is combustible and unless protected to prevent ignition, will burn if a fire takes hold.

For this reason, the proliferation of these types of buildings raises three questions:

• Have we completed sufficient research and large-scale fire testing to fully understand the behaviour of the fire and the materials in a building where the structure itself potentially contributes to the fire load and therefore the burning characteristics of the fire?
• How might fires in a mass timber building affect the life safety of occupants and fire fighters?
• Could the fire consume a sufficient volume of the timber structure to cause a degree of structural failure before the fire burns itself out or is extinguished?

Fire Research

Over the past decade, a considerable volume of research has shone some light on these questions. This includes a major 2021 fire testing program in France that used a purpose built 350 m² building which incorporated some glulam columns and an exposed CLT ceiling.

The results of this program are still being analysed. Nonetheless, preliminary findings suggest that:

• Fire does travel rapidly before the compartment becomes fully involved. Current travelling fire analysis methods for larger compartments will need to be adapted and amended.
• There is evidence of extended external flaming from windows when a compartment has exposed timber that is involved in the fire.
• No existing compartment models deliver an accurate prediction of fire time-temperature profiles. Accordingly, char depth predictions are not accurate. Current time-temperature models need to be amended.
• Thermal penetration and timber degradation continues after the peak compartment temperature has been reached. Peak timber temperatures in a timber member can sometimes occur during the fire decay period.
• Fire ventilation and the process of ventilation and impact on the burning characteristics of the fire is very important to the outcome.
• The choice of CLT adhesive is important. Should the adhesive allow for delamination of the timber glue lines, exposing fresh timber surfaces, then a fire in a decay phase may grow again to flashover.

All this suggests that fire safety engineers need to understand all issues which are relevant for safe design when working on mass timber projects. They should proceed with caution and adopt a conservative approach in both their designs and their assumptions about fire behaviour. This is particularly the case for buildings with larger compartments, where research is much more limited. Structural and fire safety engineers need to understand that exposed timber not only adds to the fuel load but also increases the fire size and slows the fire decay – an engineering challenge which can be difficult to address. Fire spread between floors externally will also be changed. The charring timber also changes the fire behaviour in ways that fire safety engineers are often not familiar.

Most designs for mass timber buildings of four storeys or more will feature an automatic sprinkler system along with other fire protection measures such as smoke detection in residential buildings. This means that where a fire occurs, it should be quickly detected and controlled or extinguished and the fire brigade will be alerted. As a result, life safety is rarely in jeopardy. Having said that, more research on fire brigade intervention is needed for buildings with larger open plan floor plans.

Sprinkler System Failure

The problem with this is what happens where the sprinkler system either fails or is overwhelmed and/or the fire brigade is not able to intervene in a timely manner. Where this happens, the fire may run out of control and any exposed timber may become involved in the fire. Charring and loss of section sizes of structural elements needs to be conservatively addressed. Considerations in this regard should take into account the issues raised above and should utilise the substantive research findings documented over the past decade. In the worst case fire scenario, any failure to understand the risk of a mass timber structure could ultimately lead to structural failure.

Some might argue that smoke detection, automatic sprinklers and early fire brigade intervention should reduce the risk of a sustained fire which could cause structural failure to such an extent as to be of no concern. Another argument could be that any collapse would occur well after occupants have been evacuated and fire brigade personnel have withdrawn. For all buildings – including mass timber structures – any measures which are needed to address the potential for building failure and collapse in an uncontrolled fire may rest partly on building code and other regulatory requirements.

In Australia, the National Construction Code (NCC) sets out Performance Requirements for fire safety provisions in multi-storey buildings around the physical safety of building occupants and members of the public, prevention of fire spread to nearby buildings and facilities for fire brigade intervention (refer Section C of Volume One of the Code). Protection of physical assets is not an objective of NCC fire safety provisions. In contrast to Australia, other jurisdictions such as the US have more recognition of asset protection requirements for fire safety.

For example, under the NCC in Australia, sprinklers are not required for buildings of less than 25 metres in height (eight storeys) except for Class 2 and Class 3 buildings (apartment buildings and hotels/accommodation).

In a worst-case situation, a building of less than 25 metres constructed of any material could fail as a result of a fully developed fire and total burnout. For such low-rise buildings, some may consider this to be an acceptable level of risk. Nevertheless, the collapse of a 20-storey building of any type (including mass timber) in a serious fire would have significant political, financial, insurance and community implications even without any injury or death.

This leads to an important consideration about whether or not the defining case for structural analysis of any building, but especially for a mass timber building, should revolve around sprinklers failing or not being installed and the fire growing to flashover or full room involvement and not being able to be extinguished by firefighters. This should be addressed not only by the fire safety engineer, the structural engineer and the approval authority but also the owner and insurer. Fire safety engineers need to determine with robust analysis that, especially for high-rise buildings, the structure can survive full burnout and maintain structural stability in the worst possible fire scenario. To do this, important decisions need to be made on how much of the timber can be left exposed (not encapsulated), both internally and externally, so that stability can be maintained throughout the fire and subsequent cooling down period.

Key Design Issues

These are important matters which must be carefully considered by the fire safety engineer along with the structural engineer and the whole design team. Understanably, architects and clients may want more timber surface exposure for aesthetic reasons. To avoid structural failure, however, structural and fire safety engineers may want less of an area to be exposed. When navigating this, engineers need to look to their own competence and ethics to ensure they address all relevant engineering issues and arrive at a conservative and defensible solution. They also need to satisfy approval bodies and the local fire brigade who essentially act in the public interest.

When considering the design of a mass timber structure and the extent to which timber is left unencapsulated, the approach taken should vary according to building types and heights. This includes whether the buildings are low or high-rise as well as whether they are residential buildings or open plan offices.

For low-rise buildings, design considerations may include that:

• These buildings may be reasonably quick to evacuate
• Fire brigade intervention may occur reasonably quickly
• External firefighting is more likely
• Structural failure may be acceptable as an end point in the unlikely event that fire protection systems fail and there is no fire-fighting intervention.

For high-rise, considerations may differ. For example:

• Evacuation is likely to be slower and may be difficult in some circumstances
• Only internal firefighting will be possible
• Structural stability needs to be maintained throughout the fire, including the decay phase.

As a result, structural performance expectations will differ for high-rise buildings compared with low-rise, and for residential as opposed to offices and other larger floor plate buildings. High-rise buildings in particular require a different design approach for mass timber in terms of modelling, analysis, testing of solutions and final design criteria. Knowledge of the type of timber elements proposed, including the glue line integrity for timber structural elements, is critical. And the fire resistance of mass timber structural connections, assessing the impact of exterior flaming, resolving the impact of timber penetrations, and perimeter fire seals require experienced engineering input that must be based on the latest research and testing.

While designs may differ for individual buildings, it may well emerge that more of the mass timber structural elements may be left exposed in low- and medium- rise buildings. High-rise residential buildings, which typically consist of many small fire compartments, may also be shown to be safe with limited areas of exposed mass timber structure. On the other hand, in the case of high-rise, large floor plate office buildings, greater encapsulation may be required to ensure that structural failure and collapse does not occur under a fully developed fire scenario.

Summary

Fire safety engineers, structural engineers and other design team members should be aware of the latest fire research on mass timber buildings along with any gaps in the research or limitations in application. They need to work with owners, developers, insurers and other stakeholders to agree upon fire safety objectives and acceptable outcomes. Potential failure modes and safety factors of all fire protection measures need to be fully identified and explored. And early consultation with approval bodies and fire authorities is needed to ensure that the design meets approval requirements and facilitates fire-fighting efforts.

In summary, it is early days in the fire safety engineering design of mass timber buildings, especially high-rise structures. These buildings offer advantages in terms of sustainability and aesthetics. But there is still a lot to learn. Design should be conservative and should involve those with specialist knowledge and experience of mass timber buildings. It would be tragic if warning signs or gaps in knowledge were ignored.

As with all buildings, good design and maintenance, based on solid engineering principles and rigorous approval processes, is the key to mass timber construction.   

Acknowledgement: The contribution of David Barber and other global timber specialists in Arup to this paper are gratefully acknowledged.

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