Some countries, such as Norway, already use timber in the construction of houses and low buildings. Nevertheless, it’s still the norm for higher buildings (usually in the range of 3 floors and above) to be constructed from concrete or steel. This is due to the structural integrity of those construction types and their relatively small size to floor area ratio. Mass Timber Buildings are starting to become the sought-after solution to the problem of sustainable construction, while also providing a great option of structural stability, even for high rise buildings.
Mass timber in higher buildings is a relatively newer type of construction process and building material itself, and it is promising to revolutionize the industry with its performance, sustainable and environmental attributes, and aesthetics. It is composed of several solid wood panels that are connected together with glue or nails/dowels, hence providing strength and stability to each other. It’s being advertised as the strong and low carbon alternative to concrete and steel. Moreover, in countries that have an abundance of trees, such as Norway, it is also a cheaper substitute.
But, what about fire safety?
Lots wonder if building with mass timber, or any type of timber for that matter, is safe. Well, in short, it is. Timber elements used in construction are often oversized than what the structural integrity of the building requires, hence their bulky nature. Mass timber construction specifically addresses that aspect, leading to columns that can sometimes become architectural features of the design due to their cross- section areas. This additional layer of wood is there to protect the wood itself in case of a fire. As wood is one of the most predictable elements in terms of fire, scientists tend to know almost exactly how it reacts, and the amount of charring it undergoes. That charred layer acts as a protective cover to the uncharred clear wood behind it, thus allowing it to retain its full structural capabilities for quite extended periods of time. Moreover, timber is usually covered by a protective layer that reduce the exposure of its surfaces to fire, acting like a fire retardant. That layer is consumed first, and then the timber itself gets in contact with fire. Nevertheless, cases of exposed timber do exist, and are quite common in lower buildings (usually up to 8-9 storeys high).
Considering that mass timber in higher buildings can rather be considered a novelty, strict regulations are still surrounding it, which is definitely a good thing. The enforcement of these regulations comprises several mitigation measures, considering the combustible nature of wood itself. These include but are not limited to: the usage of non-combustible materials, requiring sprinkler protection, and some other strict performance based testing to ensure that fire would not spread to other compartments or even surrounding buildings.
Generally speaking, high rise buildings (above 18m or 6 storeys in the United Kingdom) are required to be protected by a suppression system, most commonly sprinklers. Sprinklers in that case are there for life safety purposes. They provide enough tenable times for occupants to be able to safely evacuate before the fire gets a chance to develop further. They are not extinguishing systems meant to extinguish the fire but rather meant to control it for enough time, as a first line of defence.
The recently amended International Building Codes (IBC) now allow the erection of a maximum of 9 storeys of exposed mass timber construction for residential and business occupancies. These are required to have sprinkler protection. The higher a building gets, the higher the requirements for suppression or extinguishment rather than control becomes. Systems become required to fully extinguish the fire, and that’s mostly due to the protection of structural integrity of timber. Here comes the introduction of water mist systems as a replacement to standard sprinklers. They can provide better efficiency using minimal amount of water and also promise less post fire water damage and easy clean-ups, and essential element in lots of cases. This is due to major concerns of the effect of water damage on mass timber structural elements.
Exposed mass timber construction
This type is mostly used in lower rise buildings that do not usually require additional protection to the timber elements. That said protection can be boarding, intumescent paint, or fire-retardant coverage. When the elements are exposed, it means they react faster to fire. They would need to be treated as special cases as the fire load can be significantly higher by basically adding a very combustible product into it. That also means, however, that they are much more affected by water damage, and everyone who has a wooden door at home knows what water can do to it if left unprotected.
According to the requirements of FM Global (insurers) and BS 8458, only fire control and suppression rather than extinguishment are required in residential buildings, as the main focus is life safety. Also according to BS 8458, “in special circumstances with great fire loads or hazards, enhanced performance, reliability and resilience arrangement should be provided”. As exposed timber structures are considered as higher fire loads, and hence higher hazards, extra measures should be taken.
Water mist systems have shown through various studies that they can provide better levels of fire protection while using less amounts of water, and that’s basically due to smaller water droplets that allow for better temperature reduction. This is especially important for exposed timber structures as the fire can be supressed quicker while exposing the timber elements to less water damage. Studies that researched suppression systems effect on mass timber construction (such as the ones explained in this presentation here – full source below) have shown a high level of damage due to water. Timber, especially when exposed, can react highly to water, mainly through absorbing it into its outer layers. That absorption can lead to mould when left for longer periods of time, and it can also affect the structural integrity of the timber elements. Worst case scenario, an exposed timber structure that has been through a fire that was extinguished can lose its structural ability and might need to be renovated or even teared down.
As mass timber construction is not going anywhere any time soon, and luckily so due to its sustainable attributes, further studies should be taken on the effect of water on timber elements. Without those, we are basically risking the loss of those buildings at any time, ad that’s exactly the opposite of sustainable development.
Study conducted by Yoon Ko, Eric Gibbs, Nour Elsagan from the National Research Council Canada: “Effectiveness of Water Mist System for the Protection of Mass Timber Buildings: Test Results” presented in the 20th International Water Mist Conference