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Wed June 19 2019

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28 May Building with structural timber could become much easier thanks to new estimating guide for engineers and designers. David Taylor reports

For at least the past two decades offsite construction – and in particular, offsite systems employing timber – has been lauded as the shape of things to come. And there’s the rub: it’s still to come and has yet to properly arrive.

Though there has been an undeniable growth in the use of timber frame for housing and a tentative adoption of timber in larger buildings, such as schools and hotels, the UK construction industry has had to be coaxed and cajoled into abandoning tried-and-tested materials in favour of structural timber.

Combustibility and vulnerability to decay and insect attack have been cited as reasons to steer clear of timber, though such fears are mostly unfounded and easily allayed. Timber structures have to meet the same standards of fire safety and durability as any other type.

According to Ian Dacre, a partner with cost consultant Rider Levett Bucknall (RLB), a far more significant obstacle to the widespread adoption of structural timber systems has been a complete lack of recognised data on the cost of building in timber.

“When you’re building in concrete, or steel, or brick – all the information is there. It’s easy to cost a design in traditional materials,” he says. “But when it comes to timber, there’s never been an accepted way of estimating costs.”

There is now, however. In 2016 Dacre, with support from the Structural Timber Association (STA), authored what he says was the first estimating guide for structural timber. He has just revised and updated the guide, largely in response to what he perceives is a rapidly growing interest in timber construction.

“In 2017 the government announced that, from 2019, it would adopt a ‘presumption in favour’ of offsite construction on all publicly-funded construction projects,” explains Dacre. This, he says, has focused people’s minds on the various offsite systems available. Timber features prominently in a great many of them.

Dacre’s interest in structural timber dates back a lot further than 2016, however. A quantity surveyor by profession, he has personal experience of the difficulty encountered when trying to cost a structure in timber rather than steel or concrete.

“Several years ago I was asked by a client to produce cost estimates for a new housing development that included 70 affordable homes, which they wanted to build in timber frame. I could cost the traditional houses, but couldn’t find any cost plan for the timber ones,” he says.

“So I asked a number of timber frame manufacturers and was surprised to find that they couldn’t help me either,” adds Dacre. Puzzled by the lack of information, Dacre – a member of RICS – asked his professional body for help. “They tried to put something together, but found that there was no benchmarking data, no technical cost data,” he says.

“So in the end I did all my cost plans for brick and block. And guess what – they were all built in brick and block.”

This, he reckons, illustrates why relatively few buildings adopt structural timber despite a willingness on both the supply and demand sides to consider it.

“The feedback from the 2016 guide has been positive, hence this update,” he says. The aim of the guide is to create a report giving brief technical information and costs for structural timber to as wide an audience as possible, ensuring that estimated costs are readily available to those in the industry trying to prepare budgets.

Dacre says that there are still differences in the way the timber industry and those preparing budgets for clients evaluate the cost of the structure. He cautions that his document is only a guide and that “the most accurate way to establish the cost for a structural timber solution, and for any supporting technical details, is to engage with an STA member for a design or a tender price”.

The guide incorporates representative worked examples for three types of building: houses, hotels and apartment blocks. These are based on actual construction information supplied by the industry. Whitbread, which owns the Premier Inn brand, supplied the data for the hotel examples while Stuart Milne supplied data for the apartments and houses, both detached and semi-detached.

“We gathered all the data together and then circulated it to STA members and asked them to price it up,” says Dacre. “We used the mean values to produce benchmark costs.”

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Dacre might be the first person to establish benchmark costs for structural timber, but he isn’t the first to try and produce a comparison with other materials. He cites a UK housing association that carried out a study for a development of affordable homes in both timber and masonry construction.

“They produced bills of quantities and asked the market for quotations,” he says. “Actually, the construction costs of timber frame and masonry were very similar. But when you factor-in time, timber wins by a mile,” he adds.

The strangest thing, Dacre admits, is that reliable cost data should be so elusive in the UK when timber construction is so widely used elsewhere. “In Scandinavia and North America it’s all timber frame. Even in Scotland, timber frame is quite common in house-building,” he observes.

One explanation might be that, with the exception of Scotland, the UK is not self-sufficient in timber, and that buildings throughout history have tended to be built out of whatever material is locally abundant.

Today, though, imported timber and engineered timber products are readily available. “As far as I’m aware the timber industry has a stable supply chain,” comments Dacre, although he adds that some UK timber frame manufacturers are now including a Brexit clause in their quotations.

Dacre believes that the use of structural timber in the UK will only continue to grow. Its suitability for offsite manufacture is a major incentive, but so too are its environmental profile and its versatility.

Dacre is now turning his attention to specific types of timber product, such as glulam, structural insulated panels (SIPs) and cross-laminated timber (CLT). Cost guides for the two latter product types are currently in production and are scheduled for publication this autumn.

“Plyscraper” hailed as the world’s tallest timber building

The Mjösa Tower, an 18-storey, 85.4m-high mixed-use building in Brumunddal, Norway, is believed to be the world’s tallest timber-framed building.

Designed by Trondheim-based Voll Arkitekter for developer AB Invest, the
Mjösa Tower (pictured here and on the previous page) is constructed from a combination of glue-laminated timber (glulam) and cross-laminated timber (CLT) from Norwegian supplier Moelven, and Kerto laminated veneer lumber (LVL) supplied by Finnish timber producer
Metsä Wood.

The tower – already nicknamed the “Plyscraper” in the architectural press – is said to be similar in structure to a conventional building except that the dimensions of the structural elements are much larger than if they were made from steel or concrete.

Both the frame and the façade of the building are made of wood, with glulam columns, beams and diagonals used for the frame. Kerto LVL was used to make the floor cassettes for the first 10 storeys; concrete was used for the decks on the upper floors to stabilise the building and stop it swaying in the wind.

At 85.4m in height, the Mjösa Tower replaces the 53m-high Brock Commons building in Vancouver as the world’s tallest timber building. It’s also taller than the 84m HoHo tower, currently under construction in Vienna.

This article was first published in the May 2019 issue of The Construction Index magazine

UK readers can have their own copy of the magazine, in real paper, posted through their letterbox each month by taking out an annual subscription for just £50 a year. Click for details.

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