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Fire safety design for tall timber construction

In many jurisdictions, fire safety regulations are a key challenge to delivering tall timber construction. Traditional fire design methodologies assume use of non-combustible construction materials only. To enable the construction of modern mass timber at increasing storey heights, this project aimed to modify traditional design methods to extend their applicability to timber construction. Through this project, we defined the principle questions needed to prioritise the focus of current and future research activities. We presented this work to the UK fire safety community and international industry group (COST Action FP1404) to explore consensus.

The research focussed on three areas: (1) self-extinguishment in combustible construction; (2) fire severity in combustible construction; and (3) critical failure modes of mass structural timber.  Through joint Arup and UK EPSRC Impact Acceleration grants we developed a design method and undertook full scale fire experiments to generate the necessary data for items (1) and (2). For item (3), we have established the limitations of Eurocode 5 methods to predicting cross-laminated timber (CLT) capacity in fire.

Experience from real fires and testing shows that lightweight timber burns relatively easily and quickly and therefore has very little inherent fire resistance. The typical strategy for fire protection is to encapsulate this structural form in fire resisting construction, typically with plaster board or fireline board.

The solid structural timber elements are likely to be ‘oversized’ from a fire perspective and the relatively low area of exposed surface means that the burning wood itself will not contribute much to the fire. It will char, and will not continue to burn once the fire has burned out.

The timber structure will contribute to the behaviour and severity of the fire itself. Even if the fire in a room has burnt out, but the timber construction surrounding a room is smouldering, the fire can transition back to a flashover fire event.

There is the issue of charring rates in real fires, as opposed to charring rates derived from small-scale tests upon which we currently rely. Test evidence shows the rates are altered as a function of the real fire severity.

For laminated timber elements, the glue layer can fail in a fire, and so layers can detach and, interestingly, can change the charring rate and can also contribute more fuel to the fire.

There is also the matter of whole-frame performance of the structure in a fire rather than just the performance of a single structural element.

The design method for determining the conditions for self-extinguishment in mass timber construction can be used by fire engineers to advise architects on the safe exposure of mass timber: currently architects are required to encapsulate the timber thus hiding the construction from the architectural design.

The design tools for determining fire severity in mass timber construction can be used to determine fire severity in development of the holistic fire strategy. This includes considering the impact of fire severity for internal compartmentation, external fire spread hazards, structural fire performance and means of escape.

The outputs of these structural fire investigations can be used by both fire engineers and structural engineers to define the inherent fire resistance of exposed CLT (cross-laminated timber).

This project enabled us to define research objectives for the development of a fire safe design methodology appropriate for timber in tall building construction. We defined a design methodology for assessing the conditions required for self-extinguishment in exposed mass timber compartments and are currently obtaining the data necessary to support the methodology.

The current design approaches within Eurocode 5 for fire performance are of limited applicability to CLT (cross-laminated timber) structures.  We defined a fundamental research project using novel fire testing methods to develop an appropriate design methodology.

In many jurisdictions the use of timber for tall building construction is explicitly prohibited.  Rational design methods underpinned by fundamental fire safety research is required to challenge this and define the safe use of timber in tall building construction.