The fire ratings can be achieved with timber in a number of ways. The three most common are:
• The timber can be protected by covering it with a good insulator such as fire-rated plasterboard. This means that the timber takes longer to get to ignition temperature. Hence, it can remain functional for a longer period while the fire is burning.
• The timber can be oversized so that allowing for loss of material charring throughout the burn period, there will still be enough timber remaining in the cross-section to give it the required strength.
• The timber can be treated with fire-retardant chemicals. This delays the initiation of combustion, and can prevent the spread of flame.
Protection with Insulators
A number of building system manufacturers have had their systems tested and a FRL awarded to the system. The implementation of the system in design is simply a matter of ensuring that the design complies with the specified building system and that all of the details at the edge of the system are able to prevent a fire from bypassing the system that has been adopted. The previous tests demonstrate the performance of the system so that it is “deemed to satisfy” the provisions of the BCA.
In the wall systems above, the insulation between the double stud wall is fire-rated. This system serves two functions as it also provides superior sound insulation characteristics; there is little solid material between the two faces of the wall to act as a “sound bridge”.
Protection by Over Sizing
Timber burns from the external surfaces only. After ignition of the timber, there will be a charred region which acts as an insulator and tends to protect the wood fibres in the centre of the beam. With time, some of the timber cross-section is lost. However, the remaining timber can and does have structural strength and stiffness.
The sizing of timber to allow sufficient residual strength after a fire can be achieved by following <AS 1720.4>. This part of the Timber Structures Code models burning as a constant char rate (based on species density). This is used to determine the size of the residual section that is required to carry the load at the time of the fire. The Code also gives help in determining the fire limit state loading, which is significantly less that the loads required to meet ultimate strength limit states.
Typical char rates are about 0.5 mm/min for hardwoods and 0.65 mm/min for softwoods. In addition to the loss of section attributed to charring, <AS 1720.4> also applies a further envelope reduction of 7.5 mm to account for heat-affected timber. Most timber structures where charring is used to achieve BCA-specified fire ratings are made from large sections. These are most commonly available as glued laminated (glulam) or LVL sections. Fire tests have shown that the glulam or LVL members can be regarded as solid timber for the application of <AS 1720.4>.