Managing fire risk in timber frame construction the Tenos way

In order to properly discharge their duties under CDM 2015 and The Fire Safety Order, a contractor and lead designer erecting new buildings must implement suitable and sufficient physical and procedural fire safety measures to manage the fire risk to which relevant persons will be exposed. Relevant persons include persons working on or visiting the construction site as well as the public at large.

Evidenced by well-publicised fires such as Collindale, Peckham and Nottingham University, timber frame buildings present a particular risk during construction. This is due to the large volume and surface area of combustible timber surfaces that may be exposed to an accidental or, potentially, deliberate ignition event before the fire resisting protection to the timber frame is complete. It is the potential for these fires to grow extremely rapidly to involve the full structure which must be considered in the assessment of ‘on site’ fire risk (i.e. persons on site) and ‘off site’ fire risk (i.e. public at large).

The HSE is the statutory body with the role of enforcing fire safety on construction sites and has been very pro-active in raising the issues with project delivery teams and has taken enforcement action on sites where risk mitigation measures were not considered adequate.

With the assistance of HSE, The Structural Timber Association (STA) has produced a guide to assist designers with the timber frame specification to control the fire risk during construction. The guide allows for the assessment of simple geometries and site boundary conditions and points the user at various options e.g. fire retardant treated timber or panels pre-fabricated with fire resisting boards to address situations where the distance to existing buildings is a risk. It is important to note that, where the contractor or lead designer uses this guide then they remain reliable for the interpretation of the guidance and the adequacy of the measures implemented as a result.

The guide is based on data from fire testing and acceptance criteria which can be used with computational fire engineering techniques to further optimise the solutions that are possible and which extends the methodology to cater for more complex building geometries and boundary conditions for which the guide’s tabular approach is inadequate. Also, for some sites there may be situations where the ‘on site’ or ‘off site’ fire risk may include issues relating to the protection of escape routes for which the tabular approach in the guide is also not suitable.

Tenos has found that the best solution generally results from a workshop meeting (normally requiring no more than half a day) where the fire engineer can sit down with the contractor and go through the various options including phasing sequences and construction materials. The model for the fire engineering analysis can be set up on a notebook computer and re-run multiple times during the course of the workshop to develop and agree the fire risk mitigation strategy. This can then be written up as a short report after the meeting and providing the contractor with certainty in terms of progressing with the project.

The following graphic shows the results of a typical analysis carried out at one of these workshops to develop a protection strategy: