Fire Safety Engineering.
References: AS3745 Bushfire standard lists Australian hardwoods and how quickly they burn.
This is the second article in a series on innovative use of timber in commercial and multi residential buildings around the world.This information was collated from presentations at the WOOD SOLUTIONS 2011 SEMINAR SEPTEMBER 6TH, attended by John Scandurra from Scandurra Architects.
Fire Safety Engineering, presented by David Barber, Principal, Australasia Fire Leader, ARUP (Melbourne).
1. Understanding of the fire safety requirements within the National Construction Code (Building Code of Australia).
2. Understanding the fire properties of timber.
3. Understanding the process for gaining approval of a fire engineered approach to a timber building.
Fire Properties of Timber-Concepts.
Concept 1. Can fire performance be predicted?Solid wood has a highly predictable performance in fire. There is little variability from country to country with regard to species type; therefore we are able to use the same data from different countries.
Concept 2. Not all wood is the same. Different types of fire affect different scales of timber elements differently, especially large members, eg. 150 x 900 glulam beam.
Concept 3. Char. This is timber’s own natural protection. Presence of char allows for continued but diminishing strength during a fire.
Concept 4. Expected fire performance. National Construction Code, Performance Requirement CP1. Structural wood needs to provide a level of performance equal to concrete or steel.
Materials’ inherent protection from fire.
Concrete. In a substantial fire, over 600 degrees, concrete will spall, i.e. exposure of the steel reinforcement. Bending will occur. Failure is very rare. Replacement of concrete members is necessary after a substantial fire.
Steel relies on inherent protection, and applied protection, against fire. It will deflect at temperatures over 550 degrees. It is unlikely to endure a long duration fire, but is unlikely to collapse. Again, it needs replacement, following a fire.
Wood’s inherent protection is provided through its charring. In a substantial fire, it will deflect, but is unlikely to collapse. It also needs replacement, following a fire.
National Construction Code provisions for design of fire resistance in wood.
Type A construction is required for buildings of:
Class 5 in 4 storeys or more;
Class 2 or 3 for 3 or more storeys.
NCC is generic in regard to fire ratings, i.e. wood solutions can be employed, but submission of a fire engineer’s approval is required.
Once this principle is accepted, it opens the way for the design of engineered timber facades, as well as engineered internal elements, such as stairways.
5 storey building. NCC required 120 FRL to the structure, with fire separation. Use of sprinklers, with a trade off in FRL, yielded reduction in requirement to 60FRL. Wood elements were pre-assembled for speed; wood was assumed to have no FR plasterboard surrounds (no maintenance). Compare to steel which would have required spray protection.
Challenging preconceptions about the properties of timber in fire.
Architects’ understanding of timber’s properties under fire needs to be expanded in order to allow for its correct use in the design of buildings.
1. Like all natural materials, timber is characteristically unstable; presumably, therefore, its behaviour in fire cannot be controlled.
2. All timber burns in the same manner. In fact, the way timber burns can be measured relative to size of elements.
3. Charring is a characteristic of timber’s structural weakness, relative to steel. Char is a useful concept unique to timber and cannot be compared in this way to other materials. It is not a function of strength or weakness but of self protection under fire.
4. Concrete, steel and timber all provide inherent protection from fire. After a substantial fire, it is not only timber that needs replacement: concrete and steel also need replacement. Substantial fire ratings can be achieved from wood, comparable to steel and concrete, if the system is designed correctly.
In a fully developed fire, wood can perform equivalent or better than steel.
Compliance with NCC through engagement of a Fire Engineer.
It is possible to achieve a timber structure which complies with NCC performance. The way to achieve this is by engaging a fire engineer who understands the process of approval and design implementation. This includes design of connections between floors, and structural elements. Architects need to source a fire engineer who is capable of designing those types of details.
Therefore, design of timber structures for multi storey buildings is already an accessible technology in today’s terms: it becomes a question of sourcing the right fire consultant. The structural detailing of steel & wood are in fact similar in methodology.
Other design details to be taken into consideration with fire engineer.
1. Effectiveness of fire resistance relies on design of connection details, i.e. floor & structural connections, edge details, etc.
2. Services penetrations for need specific detailing.
3. Façade interface with wood edge beams are problematic details.
4. The fire resistant properties of wood vary according to whether it is used for floor, walls or ceiling elements.