In our technical blog series we will be addressing the issues that are considered barriers to the use of solid timber as a modern method of construction. We will begin with fire, as it is the most commonly asked question. Read our Technical Director Matt’s blog which discusses the issues around solid timber construction and fire.
Wood burns, so what?
One of the biggest concerns for designers, clients and end users when using solid timber for building structures is fire. Often people’s preconception of the performance of timber in fire is misconceived but perhaps this is reasonable, after all timber does burn!
To successfully implement a solid timber building you must first understand how timber burns and it is within the how that solid timber construction really comes into to its own – in terms of fire performance when compared to other traditional constructional materials and can even outperform them despite timbers relative combustibility.
There are several recent examples of catastrophic fires of timber framed construction which is particularly vulnerable during the construction phase. The significant media coverage these events have received has only served to reinforce people’s misconceptions. However, solid timber construction performs very differently to timber framed construction both during the construction phase and in the as built situation. This is primarily due to the difference in combustibility of the two construction types. But they are both made of wood and wood burns? The difference is that the combustibility of timber is directly proportional to ratio of surface area to volume.
Timber framed construction has a high surface area to volume ratio whereas solid timber construction such as CLT has a relatively low ratio. By way of a simple example try to imagine yourself lighting a pile of logs with a box of matches, its virtual impossible. Now imagine trying to light some kindling (small pieces of wood), these readily ignite and then the increased heat and fuel eventually ignites the logs. This is because the kindling has a high surface area to volume ratio whereas the logs have a low ratio, similar in fact to timber frame construction versus solid timber construction. This shows that given sufficient heat, oxygen and fuel solid timber will burn, so we must design solid timber structures in such a way to limit the opportunity of combustion and also to understand how it burns.
Key stages of a fire
- Developing fire - a) Incipient – once combustion begins, development of an incipient fire is largely dependent on the characteristics of the fire compartment and the fuel involved. b) Growth – provided there is adequate oxygen to continue fire development during this initial phase, radiant heat warms adjacent fuel and continues the process of pyrolysis.
- Flashover – flashover is the sudden transition from a growth stage to fully developed fire. When flashover occurs, there is a rapid transition to a state of total surface involvement of all combustible material within the compartment. The temperature at which flashover occurs is largely dependent on the conditions of the compartment but typically temperatures of between 500 to 600 degrees are required. Note – it is important to remember that flashover does not always occur. There must be sufficient fuel and oxygen for the fire to reach flashover.
- Fully developed fire – the compartment temperatures of a developed fire are normally in the range of 700 to 1200 degrees. It should be noted that in the UK most fires are extinguished either before or during the developed stage of a fire.
- Decay – the decay stage is characterised by a significant decrease in oxygen or fuel, putting
- an end to the fire.
How does solid timber burn?
Although timber is combustible, solid timber is not readily ignited and there are few recorded cases where timber will have been the first material to ignite. Solid timber construction will require surface temperatures well in excess of 400 degrees to ignite without a pilot flame. Even in the presence of a pilot flame the surface temperature will have to be in excess of 300 degrees for a significant time before ignition occurs. Therefore during construction careful consideration must be given to the use, and in particular the storage of, highly flammable substances that can provide the necessary heat to ignite timber.
When the surface of the solid timber does ignite it burns rapidly. The burned solid timber becomes a layer of char which loses all strength but retains a role as an insulating layer preventing excessive temperature rise in the core of the solid timber. The thermal conductivity of the char layer is about one sixth that of the virgin solid timber.
The highly insulating char layer will cause a steep thermal gradient across the char layer. Underneath the char layer, there is a layer of heated solid timber with a temperature of above 200°C, which is known as the pyrolysis zone. This part of the solid timber is undergoing irreversible chemical decomposition caused solely by a rise in temperature and accompanied by loss of weight.
The inner core of the solid is slightly temperature affected with some loss of strength and stiffness properties, mainly due to the moisture evaporation in the wood. The charring rate is more or less constant and depends on the density and moisture content of the solid timber but is typically 0.7mm/min for softwood such as Spruce which is widely used for CLT and glulam.
Fire performance of solid timber
We have seen that the fire performance of solid timber is dependent on the charring rate and the loss in strength and modulus of elasticity.
To achieve the required design fire resistance period there must be sufficient virgin solid timber remaining behind the char layer to sustain the loads applied. Therefore each solid timber element within a building structure must be designed for the required design fire resistance period and the specific loadings applied to that element.
The fire performance of solid timber can be further enhanced by the use of fire resistant claddings but the designer must be aware of the effects this approach on the solid timber as part of the overall system.
SOFIE Research Project – Fire Testing
SOFIE is a research project on sustainable buildings undertaken by IVALSA, an institute of the National Research Council of Italy, and supported by the Autonomous Province of Trento. The aim of SOFIE was to improve the construction system for multi-story wooden buildings which is characterised by low energy demand, high level of sound insulation, high durability and excellent fire and earthquake performance. The research project included fire and earthquake testing. To see how the multi-storey cross laminated timber structure performed in the fire test, watch this film provided to us by the SOFIE team.
For more information on SOFIE visit http://www.progettosofie.it/index_eng.html