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Chief Executive's Upfront
Fire Safety - No Room For Error

Sunday, 16 December 2018  
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The concrete vs. timber vs. steel debate has been raging for decades, as representatives for each industry profess the benefits of their respective building material.

Rob Gaimster, Chief Executive

In fact, on many occasions I have used this column to stress that comprehensive life cycle assessment should be the baseline for evaluating the sustainability pros and cons of building materials.

Adopting such an evaluation technique reveals that concrete is durable, recyclable, seismically resilient; that it offers sound separation, reduced maintenance, thermal mass, flood protection and CO2 absorption capabilities.

While concrete, timber and steel interests will all agree that there are many factors that determine the most appropriate building material for a project, and that we must strive for more resilient communities; as proponents of competing materials we will no doubt continue to disagree about which is the “best”.

Yet there is one area of concern in terms of building material credentials that we cannot afford to argue about, one that needs to be considered with urgency as potential repercussions could be catastrophic - namely the fire safety risks identified for Cross Laminated Timber (CLT).

Worryingly, there is growing concern around the identified difference between the behaviour of CLT in standard fire tests as opposed to full-scale tests designed to replicate a real-life scenario.

In a recent issue of The Structural Engineer, Deeny, Lane, Hadden and Lawrence (2018) voice misgivings around what was previously thought to be CLT’s fire performance.

  • It is not possible to rely on conventional charring rate fire resistance methods for the design of CLT members.
  • Due to the delamination of the timber, CLT contributes a significant amount of fuel to the fire.
  • Unlike lightweight timber frames CLT is not normally encapsulated in fire resisting materials and/or subject to a certified required fire resistance system.
  • The failure of encapsulation systems exposing large areas of timber causes an increased risk of secondary flash (a risk to fire-fighters).
  • The additional fuel contributed by CLT was burnt externally to the test compartment with a large external fire plume, which has implications for external fire spread of CLT structures.

In summary, the authors question the current methods for fire safety design in CLT structures and call for “reliable design methods which are based on a fundamental understanding of the response of CLT.”

The authors also emphasise that this new understanding may lead to the need for limitations in the use of CLT, particularly in the Medium Density Housing space.

Regulatory authorities in New Zealand must take note and ensure that the market is aware of these fire safety developments and is accounting for them in design outcomes.

Deeny, S., Lane, B., Hadden, R., & Lawrence, A. (2018). Fire safety design in modern timber buildings. The Structural Engineer, 96 (01), 48-53