1 July 2022 · By Geoff Wilkinson

Geoff Wilkinson looks at fire safety concerns surrounding structural CLT

More and more non-residential projects built with cross-laminated timber (CLT) are emerging. It’s great to see, as CLT has many positive attributes, being sustainable, aesthetically pleasing, and capable of providing occupants with physical and psychological benefits.

But questions still hang over the performance of CLT in a fire; some readers will recall the total loss within a few hours of a building that was 70 per cent complete in a blaze at Nottingham University in 2014. So should architects be adopting CLT more widely, or do we still need to be treating such designs with caution?

On the one hand, timber has a longstanding proven history and is the common structural construction material of choice in many countries, including the USA. On the other, there is no escaping the fact that timber is combustible and therefore its use is often limited by codes to buildings fitted with sprinklers and only to restricted heights.

Traditional timber construction relies heavily on the char rate and the residual structural integrity of the mass timber element. But CLT doesn’t have the same structural mass as traditional timber, so you cannot rely on previous assumptions and rules of thumb about structural fire performance.

In addition, where timber forms large parts of the surface area exposed to fire, it can contribute fuel to the fire. So a fire in a CLT building, compared with one where non-combustible materials are used, can lead to greater heat release, increased compartment gas temperatures, higher heat fluxes to structural elements, and prolonged fire duration.

Advocates of CLT are keen to point out that, unlike steel frames, much of its fire-resistance is inherent and relies less on the subsequent application of a fire-rated layer (typically plasterboard or intumescent coating) and so it remains structurally stable when subjected to high temperatures. Let’s not forget that steel frame fires at London’s Minster Court and Broadgate Phase 8 accounted for £138.5 million of fire losses in the early 1990s.

The extent to which CLT can be exposed to fire and remain safe is still being researched in the UK. Recently tests have been carried out on office enclosures to examine the charring and self-extinction characteristics of large, exposed CLT ceiling configurations compared with encapsulated ones, and in particular the contribution of different types of adhesives.

The experiments exposed four different ceiling configurations to a fire generated by an array of propane gas burners. Measurements were made of the radiant flux to ceiling and floor, in-depth CLT temperatures, timber mass loss rate, final char depth and slab deflections. The calculations indicated that the initial involvement of exposed CLT slabs delivered a peak heat release rate three times greater than when the ceiling slabs were fully encapsulated in plasterboard.

This peak heat release remains a major concern, especially to firefighters. Additionally, local adhesive failure and char fall-off were observed in the case of exposed CLT slabs using standard polyurethane adhesive. However, in all three exposed CLT experiments, the ceilings auto-extinguished once the burners were turned off.

Testing of residential-scale compartments is also currently in progress. The aim of this research is to determine how many layers of encapsulation material to walls and ceilings are necessary to get CLT self-extinction after fire has consumed the room contents. Results so far have shown that limiting the exposed element to just the CLT on the ceiling allows self-extinction to occur. Where both the ceiling and one wall are left exposed, then the CLT continues to burn.

These tests raise significant concerns about the safety of exposed CLT in wall construction and continue to support the government’s ban on the use of CLT in external walls (and, indeed, they raise questions over its use elsewhere in the structural walls of buildings). Further work is planned to examine the performance of joints and junctions, as this is considered to be a major cause for concern in terms of workmanship and tolerances in real-world applications.

Experts broadly agree that there is nothing intrinsically unsafe about timber-frame buildings, but that it is essential we continue to test the systems before we move to widespread adoption or push the boundaries of use further.

Architects should consider the fire safety design of mass timber buildings a specialised area, requiring specific knowledge – and such expertise is not widely held, even within the fire safety profession. Where such schemes are proposed, it is essential that both the design team and the authorities having jurisdiction (building control and the fire service) have the necessary competence and experience in the field of fire safety and mass timber structures.

As a first step, anyone thinking of designing with CLT should make themselves familiar with the Structural Timber Association’s guide to fire safety, Volume 6, Mass timber structures – Building Regulation compliance B3(1)

Geoff Wilkinson is managing director of approved inspectors Wilkinson Construction Consultants. www.thebuildinginspector.org

Tags Cross-laminated timber fire safety The regs

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