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Risk assessment for development and insurance: why Engineered Wood Products are a safe solution for mid-rise construction.

7 December 2021

This article describes the typical risk assessment considerations applicable to the use of Engineered Wood Products (EWPs) in mid-rise construction, following the approach that I have learnt at the Property Council Academy, and based on the experiences from over 30 years of professional activity in this sector.

The rapid growth in the use of EWPs in Australia has been supported by the National Construction Code which, since 2016, has provided Deemed-to-Satisfy (DtS) solutions for fire-protected  timber construction to an effective height of 25 metres, and backed by WoodSolutions’ free advisory program [1]. Our definition of EWPs includes all the wood-based products where there is some form of engineering and offsite prefabrication, from stud frames, I-joists and cassettes to mass-timber components such as CLT, GLT, LVL and NLT.

While many risks are common across all building materials and systems (e.g. taxation, legal, planning…), it’s important to understand where and how EWPs may be different and what we should be aware of when assessing a project and assigning contingency sums. And besides that, some risks may be applicable just for a given product, source and/or moment in time.

If you’re looking for bullet points or Q&A, then go straight to Tables 1 and 2 at the end, while the following paragraphs provide some additional comments and references.

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Fire safety

Both during construction and in the building lifetime, fire safety is the first concern in most projects. Fully encapsulated EWPs will behave like non-combustible materials for the prescribed fire resistance levels, and the DtS provisions illustrated in WoodSolutions guides 37 & 38 [2] describe how to achieve them in different building classes, while the test certificates and assessment available from both WoodSolutions [3] and the suppliers cover a broad range of construction details.

But in many cases, it is desirable to expose some of the structural timber, and therefore a Performance Solution is needed. The approach that a professional fire engineer will apply to an exposed timber structure is fundamentally the same as for any other exposed structure; moreover, EWPs have a very predictable and well documented fire behaviour and maintain their strength and stiffness while the temperature rises. The way the timber structure burns and chars; the way it resists delamination and enhances its ability to self-extinguish; and the way a resilient structural system supports load and demonstrates structural robustness, contributes to the overall fire performance of the building. Understanding and designing for these factors is critical to demonstrating that a performance-based fire strategy will meet building code requirements and has successfully been done in several cases: in Australia, for projects up to 10 storey (built) and 15 storeys (approved), while in North America and Europe even taller building have been completed (18 storeys fully in EWPs, including the core; 24-storey in hybrid construction).

Many full-scale compartment fire tests have been performed in Europe, North America and Australia, with varying configurations of full, partial and no timber exposure. In some of them, sprinklers were not activated after the fire was initiated, and the fires were left to burn naturally for several hours, to mimic an extreme combination of sprinklers not functioning and no fire service intervention. In each case, the fire was contained within the unit of origin, the contents were consumed, there were no structural failures and, where the EWPs were exposed, they self-extinguished. WoodSolutions has a collection of test reports available to Fire Engineers and is coordinating a Fire Engineering Leadership Group that meets regularly to exchange information and comments.

Using data from 1998-2014, when approximately 10,000 units were delivered across 188 timber construction projects over 2 storeys in Sweden, a study [4] found that, of the total 49,949 building fire incidents recorded in the country during this period, only 22 happened within a timber building, a much lower rate than for the national average. In only 2 cases the fire spread beyond the fire compartment in which it started, resulting in the demolition of one structure. None of these fires resulted in fatalities.

A Canadian report [5] reviewed the type and magnitude of damages associated with a fire event in mass timber structures, the fire suppression methods utilised in its extinguishment, and effective methods for structural repairs. Based on their statistics, the majority of residential fires were small, and only localized fire damage might be expected. Even where sprinklers do not activate, test data shows that simple remediation steps can effectively return the structure to a completed state. The report also presents four case studies where fire (or fire suppression systems) have impacted a multi-residential mass timber project, and suggests a number of viable and cost-effective options for preventative and rehabilitative measures.

Finally, the precautions during construction do not differ significantly from what is applicable to other materials, and are described in [6].

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