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Safety vs sustainability: how fire engineers can approach the net zero challenge

Sustainability By Calum Smith, Senior Fire Engineer – 21 September 2022

Two mini houses. The one on the left is made of cement and wood and the the other on the right is made our of pure timber.

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Calum Smith

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It’s quite typical for the phrase “life safety trumps sustainability” to be thrown around when it comes to fire safety design, but with the increasing urgency of the climate crisis, that kind of thinking should be seen as a cop out. There are also schools of thought that would argue that buildings which are more fire safe are also inherently more sustainable as they can reduce the damage and environmental impact that a building fire may cause. This also fails to address the impact of the embodied carbon that goes into the original construction. We need to think instead about how to both maximise fire safety and sustainability as this does not have to be a zero-sum game.

As fire engineers, we obviously don’t want to put lives at risk, but just because there are challenges around designing for sustainability in our discipline, this doesn’t mean we shouldn’t do it. We are all well aware of the role our industry plays in contributing to global emissions, and it is clear that all of us - developers, contractors, architects and engineers - need to play our part in finding solutions to the problem.

To date, there has not been a huge wealth of information on how to create a sustainable design that is also fire safe. This is because it hasn’t yet been given the attention it deserves. It’s true there are challenges in fire engineering that are not encountered by other disciplines when they consider sustainable design, but it is not impossible. We need to take the time to explore the solutions that will work best for us without compromising on safety.

One obvious solution to the challenge of sustainable building design is timber. As a building material, the benefits and drawbacks of timber are hotly debated. As my colleague, Qian Li, recently wrote in his article When is timber the right choice?, some argue that it is a low carbon, all-encompassing solution, while others argue that its “effectiveness as a renewable material depends on our ability to plant and grow sustainable forests.”

What is not up for debate is that timber is becoming an increasingly popular material for structural design these days, and this is clearly in conflict with making your building fire safe, because while timber burns, concrete and steel do not.

There’s a misconception when it comes to designing a timber structure for fire resistance, that it’s enough to simply oversize your timber so that a char layer forms as the wood burns and retains a core of unburnt wood that can still support the load. However, this sacrificial design assumption using timber has two flaws.

One is that structural fire resistance periods that we specify as fire engineers are based on a building being able to survive a fire until all the fuel is spent and burn-out occurs. These fire resistance periods are based on the structure not contributing to the fire load, but of course, wood does burn. Second, even once your fire has gone out, the centre of your structure continues to heat up and therefore continues to lose its structural stability, an effect that is much more pronounced for timber structures than it is for concrete

So, when it comes to fire safety, why use wood instead of concrete or steel? And what can we do to make timber structures fire safe? One easy option is to encase the wood in protective plasterboard, the same way you would for steel – ensuring that the timber does not contribute to the fire load. The problem is finding the products that are tested and certified to be right for the job.

A more complicated solution is to attempt to answer the question of whether our timber structure will survive burn-out of the fire when it is exposed. To do this, we would first need to solve the ‘energy equation’ of this fire: once the fuel within a compartment is spent and the only burning fuel item left is our timber, will we then be able to lose heat from the compartment quickly enough for the fire to no longer be self-sustaining and thus achieve burn-out? And will the structure still be able to support the load as the core continues to heat up after the fire is extinguished? If you can remove enough of the heat energy generated by the fire, via ventilation or heat radiation through walls, you may be able to reach the point where full burn-out occurs. Unfortunately to date, this is something there is no standard answer for. Ultimately if you have a room that’s on fire and it’s made of exposed timber, the wood will keep burning while the heat is retained in the room. However, if you can remove enough of the heat energy generated by the fire, via ventilation or heat radiation through walls, you can reach a point where there’s no longer enough energy for the fire to stay self-sustaining. Ultimately you have removed enough heat to achieve burnout.

Factoring these solutions into your design can greatly reduce the safety risk caused by fire in a timber structure, but all the additional elements required to remove heat and achieve burnout in the event of a fire can also increase the embodied carbon of your building.

In British standards, like Approved Document B, BS 9999 and BS 9991, there are certain benefits that you can design into your building that assist in reducing the embodied carbon requirements of your building. These include reducing the structural fire resistance elements with either sprinklers or a ventilation strategy.

For example, in a concrete building, you can reduce the amount of concrete that you need by taking advantage of features, like sprinklers, that reduce your fire resistance period. But is the reduction in embodied carbon from a lower concrete specification enough to offset the increased embodied carbon that a sprinkler system might have? And does this reduction actually reduce the amount of concrete required or will there be no effect due to the concrete required to support the building loads itself?

The answer is that we don’t know, because to date there has been very little, if any, research into how we design sustainable buildings that are also fire safe. Currently there is no embodied carbon information for fire safety systems like sprinklers, and that needs to change. It is now up to everyone in the industry to start asking the questions, gathering the data and sharing it with our clients and peers. Hopefully that will drive manufacturers to think about the embodied carbon of the products they make and help us solve some of the challenges we face around sustainability and fire safety.

Calum Smith is an Advocate for Zero Carbon Design 2030 based in London.

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