27 April 2021
Compliance road map for the structural fire safety design of mass timber buildings
Timber is combustible and where it forms large parts of a fire compartment’s surface area in which it can contribute as a source of fuel, it can affect the fire dynamics within. Relative to using non-combustible materials this may lead to: higher heat release rates, increased compartment gas temperatures, higher incident heat fluxes to structural elements, prolonged fire duration, and more severe external flaming, etc.
These fire dynamics implications can undermine assumptions underpinning fire resistance paradigms for cases where the structure must survive burn-out and the structure is not prevented from contributing as a source of fuel. This challenges the designer to consider how regulations pertaining to structural performance in the event of fire can be satisfied, and what design evidence must be produced.
A compliance road map has been developed to guide designers towards the most appropriate route for compliance with English regulatory requirements concerning structural performance in the event of fire.
Guidance- and performance-based routes to compliance
Concerning the performance expected of the structure in the event of fire, the Building Regulations in England set out the minimum expectations under a life safety purview, with Regulation B3(1) stating: ‘The building shall be designed and constructed so that, in the event of fire, its stability will be maintained for a reasonable period.’
For most common and straightforward building situations, Regulation B3(1) is addressed through the adoption of fire-resistance ratings in, for example, Approved Document B (ADB) or similar codes. Subsequently, elements are either designed to inherently achieve, or are protected to achieve, the recommended fire-resistance rating.
Following ADB or similar is not the only means of satisfying the relevant requirements of the Building Regulations. Alternative routes exist and these are discussed in BS 7974 and the associated suite of Published Documents.
For some more complex situations, such as those falling outside of the scope of guidance, alternative fire engineering approaches may be the only means of demonstrating compliance with the Building Regulations.
Mass timber and the route to structural fire safety compliance
The Structural Timber Association’s compliance road map proposes that the relevance of a guidance-based route to compliance depends on the structural fire performance objectives. Structural elements are routinely afforded fire resistance and this fire-resistance rating generally varies in function of building height and use. However, the objective underpinning the structure being afforded fire resistance can range from achieving sufficient time for occupants to escape and the fire service to intervene where the failure consequences are ‘low’, through to the structure having a reasonable likelihood of surviving burn-out as failure consequences increase.
Surviving burn-out requires that the structure can be prevented from contributing as a source of fuel by fully encapsulating it with a non-combustible lining material such as gypsum plasterboard. Alternatively, if there is contribution, then it must be shown that any combustion of the structure will self-extinguish while the applied loads are supported during and after the fire.
A consequence-based design tool for compliance
Failure consequences due to fire drive the structural performance objectives, and are differentiated in guidance addressing general structural design (Approved Document A (ADA)) and fire safety design (ADB) through the use of consequence classes and trigger heights, respectively. Considering failure consequences as the primary factor (and trigger heights as a relevant factor), a general design tool has been developed to assist designers in identifying the most appropriate route to compliance for a mass timber building project.
The compliance road map enables those involved in the design of mass timber structures in England to assess the most appropriate compliance pathway to meet Building Regulation B3(1) in function of the failure consequences and the preferred design solution. It will ensure consistency in how mass timber building design is approached and will assist approval authorities in the scrutiny of designs.
It should be stressed that other hazards exist when building with mass timber, in particular those concerning internal and external fire spread, as discussed in more detail elsewhere. These should be considered as part of a holistic fire strategy for any given project.
The authors wish to acknowledge the STA cross-laminated timber special interest group, with funding made available by Stora Enso, Binderholz and KLH to realise the compliance road map.
About the authors
Prof Danny Hopkin, Technical Director, OFR Consultants
Dr Michael Spearpoint, Research Leader, OFR Consultants
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