While timber is a combustible material(1), in construction it has significant insulating properties and burns in a slow, predictable and measurable way. These factors see timber perform as expected when exposed to fire and provides the designer with the ability to confidently create strong, durable, fire resistant timber constructions.
When exposed to the heat of a fire, timber goes through a process of thermal breakdown into combustible gases. During this process, a layer of charcoal forms on the burning surface of the timber and it is this charred layer that is the key contributing factor in timber's fire resistance.
The layer acts as an insulator protecting the inner core of the timber, making it resist heat penetration and thus burn more slowly; while the temperature of the inner, uncharred core remains low, enabling it to continue to carry its load. Initially the rate of charring is fast but as the char depth increases it provides a stronger protective layer to the timber, slowing the overall combustion rate.
The self-protecting nature of the charring layer increases the likelihood of a timber structure surviving fire as the uncharred inner core remains unaffected, maintaining its strength and with it the structure's stability.
According to the ABCB(2), fire requires three components; oxygen, heat and fuel. Removal or depletion of one of these three will stop the combustion process. While stopping this process is important, the most important factors in fire safety design are safe evacuation of all occupants before exposure to untenable conditions; and prevention of damage to other buildings.
It is worth noting that the National Construction Code (NCC) does not facilitate comprehensive property protection. This means that buildings can burn down, provided the most important outcomes are achieved.
The NCC provides a unit of measure to which all structural building materials and systems must conform; the Fire Resistance Level (FRL) which is(3) the grading period in minutes for three criteria: structural adequacy, integrity and insulation.
- Structural adequacy means the ability of a structure to maintain its stability and loadbearing capacity;
- Integrity means the ability of a structure to resist the passage of flames and hot gases and
- Insulation means the ability of a structure to maintain a temperature below specified limits on the surface not exposed to fire.
Hence, an FRL requirement for a wall of 90/60/30 means that the wall must maintain structural adequacy for 90 minutes, integrity for 60 minutes and insulation for 30 minutes, as tested to AS 1530.4-1990: 'Methods for fire tests on building materials, components and structures – Fire-resistance tests of elements of building construction’.
Achieving the required FRL in timber construction can be achieved in several ways(4). The three most common are:
- Protecting timber by covering it with a good insulator such as fire-rated plasterboard - this means that the timber takes longer to get to ignition temperature and can remain functional for a longer period while the fire is burning.
- Using oversized timber - this will allow for loss of material charring throughout the burn period, and there will still be enough timber remaining in the cross-section to give it the required strength.
- Treating timber with fire-retardant chemicals - this delays the initiation of combustion and can prevent the spread of flame.
Char Rates and Wesbeam LVL
Char rates can be determined by one of two methods, either through calculation as per AS1720.4 which are based on radiata and mixed Pinus species using densities from AS1720.1 of 550 kg/m3; or through testing by a registered testing laboratory.
Wesbeam’s LVL has an average density of 660kg/m3. Preliminary fire testing of Wesbeam LVL has found that char rate is some 2/3 the 0.66mm/minute as nominated in the AS1720.4 calculation method. According to Exova, ‘…the relationship between density and char is not linear, it is clear that when the nominal density increases, the notional char rate decreases’(5).
Wesbeam is currently undertaking a range of fire tests and will release the findings later in 2019. In addition to this, WoodSolutions provides several fire test reports to support the use of timber products in construction which you can find here. WoodSolutions has also published a number of Technical Design Guides providing assistance with a range of considerations appropriate to good fire design found here.
Further resources relating to fire design for timber structure scan be found in WoodSolutions Technical Design Guide #38, Fire Safety Design of Mid-Rise Timber Buildings and Technical Design Guide #39, Robustness in Structures.
(1) WoodSolutions, https://www.woodsolutions.com.au/articles/fire-performance
(2) NCC Volume One, Fire Safety Resource Kit, https://www.abcb.gov.au/Resources/Publications/Education-Training/NCC-Volume-One-Fire-Safety-Resource-Kit
(3) FTNSW, http://firetrainingnswelearning.com.au/information/f-r-l-ratings
(5) Exova Warrington (2017), Assessment Report, Fire Resistance of various joints in LVL walls, roofs and floors in accordance with AS1530.4-2014, B2.2.2