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Timber structures Fire safety

This alternative solution path creates the opportunity for elements of the performance-based approach to be applied. In the UK, the design needs to meet the performance requirements, for example a certain fire resistance. The path taken to satisfy the ‘functional objectives’ is of lesser importance. However, in the aftermath of Grenfell Tower fire and at the time of writing, the Building Regulations have recently changed and introduced a ban that prohibits the use of materials other than those classified A1 or A2-s0,d0 to BS EN 135011:2007+A1:2009 in the external walls of buildings with a storey above 18m.3 Cross-laminated timber (CLT) does not meet this requirement and it is currently uncertain if CLT will be eventually be exempt from the ban, although there is a great amount of discussion within the industry on this matter.

The reason for this behaviour is charring, a process characteristic of thermosetting polymers and most solid organic compounds. It is a process of an incomplete combustion, leaving a porous residue composed primarily of carbon. In heavy timber members subject to fire, a char layer insulates the virgin material underneath. The protection of timber can be further increased with an application of fire-retardant coatings or impregnated treatments for smaller timber elements. Currently available semi-transparent products allow for an increased fire resistance, while retaining the natural beauty of CLT.

Nevertheless, high-rise timber buildings must also feature conventional fire protection systems. This includes, but is not limited to: • sprinklers • detection and alarm systems • compartmentation • smoke extraction. In the design of mass timber buildings, it is good practice to incorporate passive and active fire protection systems exceeding the regulatory requirements to further demonstrate fire safety.

Demonstrating performance The achieved level of safety can be demonstrated quantitatively with a variety of tools: from simple analytical relationships, through numerical models to full-scale laboratory testing.

Will it burn? Combustibility of Figure 2: T20 – timber high-rise residential tower. Feasibility study for UK and Canada. Source: Entuitive timber depends on many factors, with one being the most important: size. Anyone who has prepared a campfire knows it intuitively – one cannot start a fire with a tree-trunk size log. No matter how long we try to ignite it, the log will not sustain burning once the applied heat is removed. However, if exposured to heat for long enough, eventually even the largest logs will ignite.

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Timber 2019 Industry Yearbook

Figure 3: Charring of timber. Source: Entuitive

Architects and clients often wish to expose large areas of structural timber in bespoke projects. The exposed timber surfaces may continue to burn after the entire fuel load (furniture, stacked paper etc) has been consumed.

Fire behaviour A typical fire can be divided into three phases: • growth • developed fire • decay. Each phase presents a unique challenge to be considered in design.

Figure 4: Three phases of fire development. Source: Entuitive

Profile for BM TRADA

Timber 2019 Industry Yearbook  

The annual publication of the Timber Research and Development Association (TRADA) includes topical and technical features on all aspects of...

Timber 2019 Industry Yearbook  

The annual publication of the Timber Research and Development Association (TRADA) includes topical and technical features on all aspects of...

Profile for trada