Whether at the domestic level or in larger-scale building projects, the use of green (unseasoned) timber has been making something of a comeback in recent years, so much so that TRADA has recently published a book on contemporary green oak construction [Green oak in construction, by Peter Ross, Christopher Mettem and Andrew Holloway].

The value of this initiative should not be underestimated: many construction industry professionals still regard the use of green timber as the stuff of carpentry and nothing whatsoever to do with the complexities of modern architecture and structural engineering. While the book does much to debunk this view and provides valuable advice on how traditional timber buildings can inform the design and detailing of new – and often hybrid – post and beam frame systems, it is only the continual pushing of structural frontiers in real projects that will ultimately convince those still sceptical of the 21st century relevance of this very specialised area of timber engineering.

Perhaps, unsurprisingly, Scotland, with its extensively forested areas, has quite a number of good recent examples of green timber construction, although, with no great reserves of useable European oak at present, these have more often been formed from species such as Douglas fir, Scots pine and European larch. The latest and largest of these green timber framed structures is Carrochan, the new headquarters of the Loch Lomond & The Trossachs National Park Authority.

Sited at Balloch on the southern-most boundary of this internationally recognised area of outstanding natural beauty, the new building’s adjacency to a traffic roundabout can hardly be described as either obvious or auspicious. Nevertheless, Page\Park Architects has used the unprepossessing location to advantage by designing a building that mediates between its semi-urban position and the landscape beyond.

Key to this transition – and to the project’s sustainable credentials – is the use of natural materials, with the proposition to use a green timber frame accepted by the client at the outset.

And it is this frame that establishes the form of what at first appears to be a very simple building – traditionally, timber frames are set out on a grid sized to suit the available material, but here the building’s plan has been established on a series of lines radiating from the centre of the roundabout. Some may see this as an abrogation of the architect’s design role – the project’s siting and geometry established by the prior arrival of the road engineer – but, in what architect David Page describes as a “non-place”, this unquestionably anchors the structure to its location. More importantly, it sets out the grid in a series of non-parallel lines, a clear break with traditional green timber techniques, but one in which the resulting shallow ‘S’ curve of the building demands innovative engineering solutions to ensure it is braced effectively.

From the outset, Page\Park Architects worked with the Glasgow office of structural engineer Buro Happold, but the decision to go with a green timber frame required specialist experience and contractor Carpenter Oak & Woodland Co Ltd was brought into the design team at an early stage. It, in turn, employed structural engineer SKM Anthony Hunt to develop the frame connectors with the éminence grise of timber engineering in the UK, Gordon Cowley, also involved in the design of the building’s unique prefabricated wall panel system. 

The result of all this top-flight engineering input is a building that appears remarkably simple but which takes the design of green timber structures to a new level. Conceptually, the project is made up of two parallel, pitched roof barn-like forms tweaked into an unusual ‘S’ shape that clearly distinguishes it from its agricultural forebears.

The building’s 20m cross-section is made up of 2x8m frame structures separated by a 4m central ‘street’ that is glazed overhead. Breaking down the plan and section in this way permits the accommodation to be laid out on 8m floor plates, ideal for ventilated, single-sided office space. There are 26 of these post and beam frames in the 78m-long building, the vertical elements of which are massive 6.5m-high Douglas fir columns 300x650cm in section. The full height of these single element posts can be seen in the double height café and staff library areas, their scale magnified by the requirement to double up their structural size to allow for charring should a fire occur.

Eight hundred tonnes of timber, sourced from Scotland and the Borders, was used in the building’s construction

The frame, despite the size of the individual elements, is no simple post and beam affair, but – because of the building’s upper level open plan spaces – is, instead, a cutting-edge hybrid timber structure that also embraces highly-engineered, prefabricated floor ribs and wall panels. Diagonal bracing members are traditionally used in post and beam systems to combat wind loads, but were not compatible with the concept for this particular building.

In order to achieve the required stiffness, the engineering team instead developed a system, unique in the UK, that makes use of prefabricated wall panels with a central vertical ply sheet for the web with timber top and bottom flanges (essentially a very tall I-beam) and vertical ribs at regular intervals to resist buckling. From here, the loadings are transferred to ground via the massive Douglas fir buttressing columns along the rear elevation of the building.

The upper floor system is equally innovative, being an I-box floor beam and skin made up of prefabricated ribs, themselves formed from two vertical OSB webs with top and bottom flanges of kiln-dried spruce. These create a series of stiff, strong but small-sectioned boxes capable of spanning large distances and, with the addition of a site-applied plywood deck, effectively form a stiff diaphragm floor.

There is much to this building to be admired but it is its claim to be the largest contemporary green timber framed building in the UK that merits emulation. With 160m3 of locally-sourced timber weighing in at 800 tonnes, Carrochan ably demonstrates the potential of large-scale green timber structures in this country.

Peter Wilson is the author of the recently published New Timber Architecture in Scotland

The timber technology

STRUCTURE FRAME: 78x20m in plan area, the structure uses 26 post and beam frames. The columns are green Douglas fir 6.5m high and 300x650cm in cross section. Douglas fir glulam beams to support the floor system span laterally between the frames. For reasons of durability, the column structure at the building’s entrance is formed from European oak posts.

FLOORS: the first floor is an ‘I-box’ system made up of prefabricated ribs formed from two vertical OSB webs with wide top and bottom flanges of kiln-dried spruce. The webs are not continuous, making the beams light and easy to handle on site as well as permitting the throughput of services in the resulting floor void. The addition of a ply deck turns the I-beam system into a very stiff diaphragm floor.

WALLS: the rear wall to the building is made up of prefabricated panels set between the structural frames. All of the panels are straight, but stepped to form the curvature of the building. The panels are effectively vertical I-beams – a vertical plywood sheet acts as the web, with timber top and bottom flanges. Regularly-spaced vertical timber ribs prevent the web from buckling. 
Externally, the rear wall and end gables have vertically fixed Scotlarch, Russwood’s trademarked Scottish-sourced European larch cladding. The elevation facing onto the roundabout is a drystone wall of Burlington slate.

CONNECTORS: structural engineer SKM Anthony Hunt developed steel flitch plates to connect the Douglas fir elements. These are set within slots in the timber, thereby protecting them from fire.

The building’s 20m cross-section is made up of 2x8m frame structures separated by a 4m central ‘street’ that is glazed overhead

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