A Real Opportunity for the Timber Frame Industry
Anne McDonald talks to Om Harris, winner of the Scottish Timber Trade Association’s Prize for the Best Timber Related Final Year Project at the Centre for Timber Engineering , Napier University, Edinburgh about his winning project on Designing TF 2000 to Eurocode 5.
With Eurocodes set to succeed the use of British Standards for structural design throughout the UK, the industry faces the challenge of creating design procedure and best practice guides to facilitate the transfer.
One response to this challenge came from Om Harris, a final year timber engineering student at Napier University Edinburgh’s Centre for Timber Engineering. For his final year Honours project in 2006, (for which he was awarded a First Class degree), he undertook to design a six-storey multi-occupancy residential building to Eurocode 5 and to produce a Design Guide.
He chose as his exemplar building TF 2000, the iconic 6 storey timber frame structure erected in 2000 as an industry–funded research project by Building Research Establishment at their test facility in Cardington. (This unique test facility is a remarkable structure from another era – it was one of two hanger-like sheds built at the start of the last century to house airships.)
Asked about his reasons for choosing to design a multi-storey building in timber to Eurocode 5, Om gave as his drivers the fact that it had never been done before; that medium rise timber frame was a growing market; and that he was personally interested in sustainability. He explained:
“Timber is part of a growing sector and one which has clearly demonstrated its economic and environmental viability. Given the timber frame market’s potential in the UK and the fact that Eurocodes are vital to Modern Methods of Construction, it made good sense to me to try and bring the two together. It also made sense to do something which will potentially increase the use of timber by engineers and architects. The environmental advantages of timber makes it essential that construction professionals are supported and facilitated in their use of timber.”
The Scottish Timber Trade Association’s Secretary David Sulman commented:
“As an Association which works to increase the understanding and use of timber in construction and other sectors, we were delighted to recognise and reward this practical and timely piece of research. It aims to prove the capacity of timber frame construction to meet the demands of the latest European design code and most importantly, it aims to simplify compliance with Eurocode 5 for engineers and architects.”
The specific research goals of the project were:
The project started with the calculation of the variable loads on the external structure such as wind and snow. With these actions the roof was designed as a duo-pitched truss roof spanning the lesser depth of the building capable of taking the imposed loads as well as the permanent loads.
With the roof load calculated, the external load bearing walls could be designed to support the roof. The floors were then designed to Eurocode 5. The loads from the floors were to be transferred onto the internal load bearing walls.
The racking resistance was then calculated ensuring that the resistance was greater than the wind loading.
The swiftness of this description speaks of a simple process. Clearly, this was not entirely the case.
One of the first challenges Om faced was the dearth of previous research information in the area. He comments:
“There wasn’t much data out there; EC5 is new; so, relatively speaking, is medium rise timber frame. The lack of worked examples in the calculations meant a lot of work had to be done from first principles.”
Designing the roof trusses was also a key challenge. With EC5 being theory derived rather than test derived (limit state design) it meant that up to 10 different loadings needed to be calculated to find the most critical. “Lots of tedious calculations” says Om.
Suitable software is however already being developed to address this difficulty. The STTA’s David Sulman explains:
“Industry based software is currently widely used in roof truss design and major manufacturers are already developing software suitable for use with EC5. In future, EC5 compliant calculations will be much more simply achieved.”
Calculating wind loading and deflection also proved quite complex and while Om’s work showed that all standards could be met, he felt that these areas might be suitable for further study.
One of the key “positives” to emerge for Om from the study was the strength of timber. He explains:
“Since timber is an organic material it is grown rather than manufactured. The strength properties of timber are therefore complex and its strength categories need to be defined by a grading process, the main indicator being the stiffness of the timber.
Values for strength and stiffness can be obtained in respect to the grain and load direction and are given through the testing of timber members. From these values design checks can be made against the actual applied load to ensure structural stability. In the beginning I was regularly surprised and impressed with how strong and stable the timber member I was testing proved to be.”
The Eurocodes were born from an action programme in the field of construction based on Article 95 of the Treaty of Rome. The main objectives of the programme were for the unification of standards across Europe and for the removal of technical obstacles to trade. Which is all very well in theory, but just how useable did Om find the Eurocode for timber design, Eurocode 5? And what advice would he give to other timber designers using the Code?
While agreeing that EC5 is more complicated to use than the comparable British Standards for structural design, Om defends it as being “more comprehensive” and “giving a better understanding of how the structures work”. Given that EC5 is theory based, rather than test based like the British Standards for structural design, it extrapolates from theory and therefore, says Om, it allows for more variables than the British Standards. “It could” he posits “offer more economic options for just that reason”.
This theory based approach, or limit state design is, explains Om “a different kind of design”. Rather than seeking to meet test-derived data as in the BS codes, it demands that you work out the strength of your material against the stress of the building and if the strength of the material -including safety margins-is greater than the stress factors, broadly speaking you have compliance. Because it is so different its adoption may be slow: it is however only a matter of time until it is here and Om recommends that other designers coming new to EC5 should be open to its difference and , if necessary, seek help. He says:
“I see Eurocode 5 as an opportunity for the timber frame industry. Timber frame is a developing, cutting edge construction method, growing in market share and evolving. It’s got a real opportunity here to get ahead of the game by designing to Eurocode 5 before it has to and be ready for the change when it comes.”
David Sulman adds:
“We in the industry are very encouraged to see that 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 to Eurocode 5 in due course.”