Engineered Timber CLT and GLULAM - Some Firefighting Concerns.

Firefighting in Engineered Timber Buildings

The timber industry has been enjoying significant growth in the last decade, primarily due to the increase in mass timber products such as cross-laminated timber, glulam and laminated veneer lumber, as well as many board products such as OSB that make use of smaller offcuts. There has been much research into the fire resistance of engineered timber from a design perspective, although there are two conflicting trains of thought that the research to date is either proof enough, or incomplete, in demonstrating that a compartment predominantly consisting of exposed timber surfaces can survive a ‘burnout’.

However, there has been only a small amount of research directed at the problems likely to be faced by firefighters. It’s factual that the research so far has demonstrated that the fire dynamics associated with either encapsulated or exposed timber fire compartments, will present a range of conditions that can generate far more severe fires. Although not a complete guide to design, here are some important aspects of what we know may impact on firefighting:

1.   CLT: cross-laminated timber comprises layers of timber boards, also known as dimensional lumber, arranged perpendicular to one another and glued together, forming a single structural member. The perpendicular layers provide additional strength and stability.

2.   Glulam: glued laminated timber comprises layers of timber arranged along the same grain and glued together to produce a single structural member.

3.   LVL: laminated veneer lumber comprises multiple layers of wooden veneer bonded with glues.

4.   PSL: parallel strand lumber comprises multiple layers of thin wooden veneer strips, bonded with resin.

5.   Composite buildings, with elements of engineered timber, steel and concrete combined.

Firefighting Considerations

Such buildings may be protected by encapsulation of structural elements, in part or totally. Compartmentalisation may reduce open-plan compartment sizes and active suppression systems may well exist. All such design features may lessen the impact of fire in most cases. However, research and past experience demonstrates that no amount of protection is guaranteed to prevent, mitigate or control every type of fire scenario and there must be some expectation of fire spread that is difficult to access, where high quantities of pyrolysis gases are being produced with very intense burning and the potential for structural collapse. These points are key when pre-designing and constructing but also where pre-planning strategy and tactics for such fires. Note some some evidence based expert views -

• Globally, there is no design methodology for quantifying the effect of exposed mass timber on the fire dynamics which has been thoroughly validated, even for small compartments that have been studied extensively from the 1960s. The largest compartment fire tests carried out to date with exposed mass timber surfaces are only 84m2 in floor area and are primarily representative of rooms in residential and hotel uses. There is no large-scale open plan test or experimental data available at present that is representative of typical office configurations. As a result, it is difficult to draw conclusions with respect to the fire dynamics of high-rise mass timber structures where open plan spaces are being proposed. (Rackauskaite; Kotsovinos and Barber 2020).
• Open-plan compartment floor areas (1000 to 4600 m2) for currently proposed mass timber buildings are up to to 55 times higher than timber compartments for which experimental data is available at this time. Computer modelling of fire growth and decay in such compartments has also failed to deliver reliable data at this time.
• At the same time, steel starts losing its strength at ~400oC and would still maintain ~50% of its strength at 550oC. In addition, its material properties are largely recoverable. However, for exposed mass timber structures, the timber, if the material reaches 100oC, would have lost 35% and 75% of its strength in tension and compression respectively (Law A, Hadden R (2020).

Wind Driven Fires are of Utmost Concern

• There has been little or no consideration to date in international research of how a wind-impacted fire might weaken a heavily engineered timber building. The impact of wind driven fires on exposed timber will create extremely intense burning that may develop increased risks of delamination of engineered timber, leading to multiple flashover events, backdrafts and other forms of rapid fire phenomena not commonly experienced in traditional constructions.
• The increase in heat release from such intense burning will likely place the exterior fa?ade under greater heat exposure as external flames lengthen, compared to a more code compliant traditional fire load. If a wind is preventing the release of such intense burning to the exterior and redirecting the burning fire gases back into the structure, the resulting fire will continue to spread further into the building and create an even larger fire for firefighters to deal with.
• This fire development may be so intense that the energy release (MW) may surpass any suppressive capacity (L/min) that can realistically be deployed by firefighters. Unlike traditional construction with non-combustible elements, such a fire may create a thermal runaway until collapse of the elements becomes inevitable.

Whilst it is clear that engineered timber is being considered a modern, sustainable and rapid form of construction, we do seriously need to place greater emphasis on ‘worst-case’ scenarios that are not currently considered for construction methods where non-combustible elements and materials are used.