Modern timber frame structures have superb environment credentials, are quick and easy to build and durable, but managing fire risks can be complex.
Following a spate of incidents in the UK, the Health & Safety Executive has written an open letter to all those involved in the design, specification, procurement and construction of timber frame structures in relation to the management of fire risks.
The letter has been produced in cooperation with the Structural Timber Association (STA), which represents the industry’s manufacturers and suppliers. STA and HSE are working together to promote a better understanding of fire risk management throughout all parts of the supply chain.
With structural timber the fastest growing construction method in the UK, increasing awareness and understanding is essential.
Modern timber frame structures are precision-engineered, strong and durable. The build method relies on timber frame as a basic means of structural support; carrying the loads imposed by the floors and roofs, before transmitting them to the foundations.
As with all forms of structural timber, timber frame has superb environment credentials, and it is quick and easy to construct.
Although it is a combustible material, timber is a good insulator and burns in a predictable fashion. However, evidence from recent HSE inspections has indicated that the risk of harm to occupants of neighbouring buildings from fire during the construction phase is not always effectively managed.
From April 2009 to March 2012, Fire and Rescue Services in England attended 2,485 fires in buildings that were not under construction that were recorded as being of timber frame construction.
Fires in timber framed dwellings under construction had on average larger areas of damage compared to dwellings of no special construction. Out of total fires in timber framed dwellings under construction, 24 per cent of these resulted in damage of an area of more than 100 square metres compared to four per cent for dwellings of no special construction.
“All those making design and procurement decisions that significantly affect fire risk should consider and reduce the risk and consequences of fire during the construction phase through DESIGN,” states the HSE letter.
“This duty is imposed on anyone who makes decisions affecting the design, including, for example, architects, structural engineers, clients, suppliers, principal and other contractors and even those involved in the planning approval process where they specify particular construction methods or products.”
The STA has produced a publication, Design guide to separating distances during construction, which provides guidance on assessing the off-site fire risk from timber frame structures under construction. Despite its title, it is intended to be used at the design and procurement stages of a project.
Three generic categories of timber frame have been evolved with increasing resistance to fire spread and associated reduction in emitted heat to neighbouring properties. The user of this Guidance can select either a Category A, B or C frame type depending on the site conditions and distances to surrounding buildings. The critical point is that a recommended timber frame solution can be specified to suit the individual site location and conditions, no matter how challenging, even in densely populated inner city developments.
“If fire safety considerations are left to a late stage in the design development of the project, there is an increased potential that the solutions will be inconsistent in approach and less effective if there is a fire, and more time consuming and expensive to implement. Appropriate mitigation methods should always be developed before construction work starts on site,” the HSE letter said.
“Over the past 15 years, research and testing has been performed to better understand the fire performance of timber structures. However, opportunities for further research remain,” said David Barber and Robert Gerard, fire performance engineers with Arup. “Research and testing could lead to performance benchmarks and design tools that would allow a designer to characterise fire performance, engineer fire protection strategies, and demonstrate safe design.”
Barber and Gerard note that previous fire testing has shown that exposed timber has the potential to contribute to the fuel load in compartment fires. Fire tests have shown that delamination can occur in exposed CLT panels. This delamination may result in an increased burning rate for a limited period of time and also can result in an increased char rate for the exposed solid wood.
“Design solutions must be balanced by structural-efficiency, cost-effectiveness, aesthetics and, importantly, fire performance,” said Barber and Gerard.
They add that understanding penetration behaviour is critical to demonstrating that compartmentalisation is achieved for fire safety in timber buildings. The combustible nature of fire-rated structural elements presents unique challenges compared to noncombustible structures. Charring behaviour must be considered for appropriate fire-stopping solutions to be developed to meet the fire performance requirements.
“Two possibilities to solve this challenge include the development of fire test standards that account for combustible bases or substrates, and proprietary products for walls and floors in timber buildings” they suggested.
Another trend challenging fire performance engineers is the growing global precedent for tall timber buildings. A significant amount of ongoing research is aimed at filling the gaps in knowledge and leading to a better understanding of the fire safety challenges for this type of structure.
“Currently a high burden of proof is required that relies on a comprehensive understanding of timber fire performance, an engineered fire protection strategy, and possibly fire testing, to justify safe design,” said Barber and Gerrard.
Given the hotter, drier climate in Australia, the risks of fire are even greater.
Fire ratings can be achieved with timber in a number of ways. 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.
A number of building system manufacturers have had their systems tested and a fire resistance level (FRL) awarded to the system so that it is 'deemed to satisfy' the provisions of the Building Code of Australia (BCA).