Mass timber’s next challenge is engineering confidence

Engineers have spent more than a century learning how to make steel and concrete buildings behave under pressure, and they have got very good at it. But the adoption of a new material – mass timber – requires a rethink, as well as more convincing of its utility.

Cross-laminated timber (CLT), which is made by bonding layers of wood at right angles, can form large structural walls and floor plates. Another wood-based product, glued laminated timber or glulam, can take the place of beams and columns. Both are comparatively light, can be prefabricated off-site and assembled quickly, and use a renewable material in place of some of the steel and concrete that make construction so carbon-intensive.

“A lot of the challenges regarding these materials aren’t so technical anymore,” says Dr Martyn McLaggan, lecturer in low carbon design at the University of Sheffield. Specialists know how to use them, he says, but distributing that expertise out to the wider engineering profession is a challenge.

Key documentation can help with that: the second generation of Eurocode 5, the European standard governing timber structures, includes a fully codified reference to CLT for the first time, while structural fire design and connection rules have also been substantially revised to account for the material. Until now, McLaggan says, engineers designing CLT have had to lean more heavily on professional judgement and guidance gathered from elsewhere than official documentation. “When I’m teaching the undergrad students, they’re astounded to learn that there was no CLT in Eurocode 5,” he says of the first-generation rules.

That matters because, while timber’s limitations are not usually critical failures, they are no less annoying. “If you’re in an office and your desk is rattling up and down, then you won’t be very happy with it,” McLaggan says. Glulam beams can span 10 or 12 metres and, in some designs, as much as 16 metres, he says, while CLT slabs are more likely to span five to eight metres.

Mass timber’s relatively low weight is one of its attractions, but mass is also useful for stopping sound. Bare CLT floors can perform poorly at blocking impact and airborne noise, meaning designers may have to add topping layers such as acoustic mats. Each addition undermines the mass-saving benefit of switching from concrete.

The need to add small fixes to bring timber up to parity with the products it’s meant to replace is one reason McLaggan remains unconvinced by the trend to build ever taller timber towers. He reckons instead that timber is best used for medium-rise buildings or vertical extensions, where it won’t overload a building’s foundations. This use in lower-slung buildings also addresses another key concern about any kind of timber: fire risk.

“The fact that the building now is fuel really does change the game,” says Luke Bisby, chair of fire and structures at the University of Edinburgh. Traditional fire engineering has developed around the assumption that the contents of a room may burn, but the primary structure itself remains standing and does not become more fuel for the flames.

Mass timber, of course, does not behave like that. “It is certainly possible to design and construct a large, higher-consequence timber building,” he says. “It’s just that the level of care and the level of understanding required of the designers and approvers of those designs is at a much higher level.”

The issue has prompted significant work across the industry. A UK Roadmap for Fire Safe Mass Timber, produced by Built Environment – Smarter Transformation with researchers from the University of Edinburgh, argues that one problem isn’t a lack of knowledge, but differing interpretations of existing guidance and a need to build a shared understanding among all those who would be involved in the construction of such buildings.

Water is also anathema to wood, with timber able to absorb moisture during construction, resulting in defects that compromise the building over its lifetime. The Structural Timber Association recommends a moisture management strategy, running from initial design through to the construction and eventual use of a building, which goes beyond just trusting labourers on a site to keep things dry.

Overall, though, the opportunity exists to use mass timber in major construction projects once these issues are taken into account and overcome. “The specialists have the knowledge to do it,” says McLaggan, “and it’s ready for the slightly more generalists to start adopting it more often.”

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