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12 October, 2008
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Cracking the Eurocode
Spring 2007
Published:  02 April, 2007

Timber frame: a developing, cutting edge construction method

Student Om Harris won the Scottish Timber Trade Association’s prize for the Best Timber Related Final Year Project at the Centre for Timber Engineering for his work on designing a medium-rise timber frame structure to Eurocode 5. Anne McDonald reports

With Eurocodes set to succeed British Standards for structural design, the construction industry faces a challenge to create design procedure and best practice guides for the transfer.

Om Harris, a timber engineering student at Napier University Edinburgh’s Centre for Timber Engineering, responded by choosing for his entry for the annual Scottish Timber Trade Association competiton for final year students to design a six-storey, multi-occupancy, residential building to Eurocode 5 (EC5) and to produce a Design Guide.

So what were the reasons for Harris’ choice?

“Timber construction is a growing sector and one which has demonstrated its economic and environmental viability,” he explained. “Given the timber frame market’s potential in the UK and the fact that Eurocodes are vital to Modern Methods of Construction (MMC), it made good sense to bring the two together.”

The specific research goals of the project were:

ß to produce a design guide for the TF2000 six-storey multi-occupancy residential building constructed several years ago as timber frame test bed by the BRE and TRADA, to meet the Eurocode 5 criteria;

ß to produce detailed drawings of major members and other important structural components.

The project started with calculation of variable loads on the external structure such as wind and snow. The roof was subsequently designed as a duo-pitched truss structure spanning the lesser depth of the building, capable of taking imposed as well as permanent loads.

The external load bearing walls were then designed to support the roof and the floors designed to Eurocode 5, with loads from the latter transferred onto the internal load bearing walls.

The racking resistance was calculated to ensure resistance was greater than the wind loading.

One of the positives to emerge for Harris from the study was the strength of timber. Although, he stressed: “The strength properties of timber are complex and its strength categories need to be defined by a grading process, the main indicator being stiffness.”

One of the key challenges Harris faced was the dearth of previous research information.

“EC5 is new; so, relatively speaking, is medium-rise timber frame,” he said. “The lack of worked examples in the calculations meant a lot of work had to be done from first principles.”

Despite the difficulties, Harris is broadly a fan of EC5. Given that it is theory-based, rather than test-based like the current British Standards for structural design, it extrapolates from theory and, said Harris, allows for more variables.

“It could”, he said, “offer more economic options for just that reason.

“EC5 demands that you work out the strength of your material against the stress of the building and if, including safety margins, it is greater than the stress factors, broadly speaking you have compliance.”

Harris recommends that designers coming to EC5 should be open to its differences and exploit it.

“EC5 is an opportunity for the timber frame industry. Timber frame is a developing, cutting edge construction method. It’s got a real chance here to get ahead of the game by designing to EC5 before it has to and be ready for the change when it comes.”

The Scottish Timber Trade Association’s secretary David Sulman commented: “We are very encouraged to see timber frame manufacturers are moving swiftly to embrace EC5 and that there is generally a high level of preparedness within the UK timber frame industry. This bodes well for a smooth, effective, transition.”

•The STTA Prizes for Best Student Projects in the Use of Timber

In 2006 the Scottish Timber Trade Association (STTA) initiated and sponsored a new series of prizes for students at Napier University’s Centre for Timber Engineering (CTE). The prizes are: Best Timber Related Final Year Project; Best Design Project by a Second Year Architectural Technology Student; and Best Performance by a Timber Engineering or Timber Industry Management Masters Student.

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