Assembly Workshop, Hooke Park, Beaminster, Dorset

Introduction

A new workshop/shelter for project work at the Architectural Association’s Hooke Park campus has been built and designed by the students themselves. It provides a 500m2 sheltered workspace for construction activities and is used for full-scale prototyping, testing, pre-assembly and other research into architectural systems. Over the coming decade, its core output will be a series of student-designed-and-built campus buildings.

The workshop is the latest in the collection of innovative timber buildings at Hooke Park. The furniture maker John Makepeace originally established the site as a School for Woodland Industries. Under his ownership three pioneering timber buildings were built on the site in the 1980s, all utilising roundwood thinnings – the trunks of spruce trees which, though tall and slender, were considered to be too thin to be of use to the timber industry. Buro Happold were design engineers for all the initial buildings, with the first two designed by architect ABK with Frei Otto and the third by Cullinan Studio. The first building, a refectory, was constructed using roundwood as stiff frames supporting an innovative roof structure using timber poles in tension. The second building, a workshop, was a dome-like structure; the thinnings were secured at their bases to a circular concrete beam at the perimeter, bent over ‘in the green’ and fixed together to form the dome structure. The third building also used roundwood thinnings as structure and contains sleeping spaces for 16.

In 2001 the Architectural Association (AA) took over the ownership of Hooke Park and began to use the site and its buildings for visits by students and tutors. Since 2010 it has become part of the ‘Design and Make’, a master’s programme which runs for 16 months and is intended for post Part 2 students. The agenda of the programme is ‘design through making’, where students learn through direct engagement in the construction of a full-scale building.

The new workshop was needed because the original workshop, designed for furniture-making, was not suitable to create large timber assemblies. And like the earlier buildings, the aim was to demonstrate that low-value ‘waste’ thinnings from local woodland have structural and environmental advantages and require minimal industrial processing and material transport.

The workshop is simply a large enclosure with permanent openings, no services and no insulation. Its complex faceted envelope, supported on a series of roundwood trusses, was determined by specific site relationships; an entrance view of the campus to the north, a cantilevered canopy facing a beech forest towards the west and, towards the east, access to a large central workyard where saw milling and fabrication activities are currently carried out.

 

Choice of timber

The original buildings had aimed to develop the use of thin thinnings of spruce, an abundant tree in the 1980s and 1990s from woodland planted after World War II. But since then the Hooke Park woodland had matured and spruce was no longer available.

The timber chosen for the workshop was larch, which is now abundant, particularly due to surgical felling to prevent the spread of phytophthra ramorum (to which larch is particularly susceptible) in south west England. Some of the larch comes from Hooke Park itself, the rest from Longleat.

The thinnings needed only a small amount of finishing– the students simply stripped off the bark to prepare them.

The wall panels are clad using western red cedar boarding (typically 30mm thick and 100mm wide), sawn from about 30 trees felled at Hooke Park. The planks are carried on triangular ‘cassettes’ supported by a sawn larch substructure.

 

The structure

The building structure consists of a series of truss frames, all constructed of larch roundwood. Atelier One the structural engineer, collaborating with the Department of Architecture and Civil Engineering at Bath University, developed new techniques to demonstrate how roundwood elements could be used in complex structures without the need for major engineering processing. It demonstrates an approach to building that maximises the use of local resources and minimises reliance on industrial production of building components.

One of the most important challenges was to keep each truss working with axial forces only, in order to maintain the primary principle of truss structure. So all load transfers, including the cladding, had to occur at truss nodes, thus eliminating bending force in the truss web and chord members. To achieve this, a packer, of varying depth, is used to elevate a square-sawn backing rafter. The rafters run parallel to the top chords on which rest the joists of the prefabricated cladding cassettes. This top chord build-up maintains all forces travelling to the truss node and avoids any bending moments in any of the chords and bracing members.

The diagrams (shown top and centre page 5) show how the structure works; the trusses highlighted in red in the top illustration provide both vertical stability and act as portal frames to resist lateral wind loads. The bracing highlighted in red in the centre illustration uses the folded geometry of the building to tie the roof level to the ground and provide lateral stability in the longitudinal direction of the structure.

Apart from stripping off the bark, the roundwood thinnings for the structure were used without being sawn or planed; the natural fibre and grain of the timber were undisturbed and its structural integrity preserved. But the natural variations in the timber had to be matched to differing structural performance requirements within the structure. By mapping the engineer’s analysis-derived forces onto the structure, the natural variations in diameter, taper, straightness and quality (measured by the number and size of knots) were taken into account so that each tree is optimally used in the building.


Use of timber connections

One of the aims of the project was to minimise the use of steel in the form of flitch plates, the conventional way of connecting timber truss members. Some of the more complex truss connections are made by flitch plates, but as a general solution, particularly as it relies on off-site fabrication, it seemed inappropriate for construction by a relatively low-skilled student team. The solution was found in a new type of structural timber screw from the German company Heco. As timber alone has limited structural capacity in compression perpendicular to the grain, the majority of the strength in the Heco joint is through interaction of the screw thread with the timber. The Heco Topix Combi Connect screws, up to 400mm long, have two separate threads with subtly different pitches which pull the timber members together, which in turn maximises the capacity of the joint. A set of these screws, used at cross angles through the joint, could achieve a connection of sufficient strength.

A laboratory and on-site regime to test the application of Heco screws in larch roundwood was set up. The laboratory testing, undertaken at Bath University’s Department of Architecture and Civil Engineering, determined the Heco screw’s compressive and tension strength at typical web-to-chord connections by loading and pull-out-tests. To anticipate shrinkage, over time, of the wet, green wood which might result in exposing the screws between chord and web, a joint mock-up was built with a gap between web and chord. This made it possible to test screw strength only, without relying on its timber bearing capacity.

Mock-up joints were built on site to monitor the condition of screws and timber through time and to test the effect of potential corrosion of the screws in wet roundwood and the weight reduction of the timber as its moisture content lowered. A final post-construction test to assess the stiffness of the completed structure was done by applying a point load to the roof at the position of a truss joint node.

Another constructional/technical challenge was to maintain the primary principle of truss structure; in order to keep the truss working along axial forces only, all load transfers need to occur at truss nodes, which eliminates bending forces in the truss web and chord members The design problem was to design a cladding build-up whose load would transfer to the truss nodes only. The solution was a pair of raised backing rafters; at each truss node, a varying depth of packers elevates a square-sewn backing rafter. These run in parallel to the top chord on which rests the joists of the prefabricated cladding cassettes. This top chord build-up maintains all forces travelling to the truss node and avoids bending moments in any of the chords and bracing members. The gap between the backing rafter and the top chord also creates a zone that normalises any irregularities in diameter of the top chords.

 

Fabrication

The fabrication of the trusses was carried out by a team of students from the Design & Make course, together with summer volunteers on the AA’s ‘SummerBuild’ programme at Hooke Park, who worked together with experienced timber professionals led by Charley Brentnall and by Piers Taylor, executive architect, of the Mitchell Taylor Workshop. Each of the planar trusses was fabricated and assembled horizontally, before being lifted into position on the building’s concrete slab foundation. Likewise the cladding cassettes were assembled flat and then raised into place.

Completion Date:

April 2012

Year Published:

July 2013

Building type:

Workshop

Location:

Hooke Park, Beaminster, Dorset

Owner / Client:

Architectural Association

Executive Architect:

Piers Taylor, Mitchell Taylor Workshop

Design Team:

AA Design and Make (MA Arch course) and Diploma 19

Structural Engineer:

Atelier One

Timber Testing:

Department of Architecture and Civil Engineering, University of Bath

Main Contractor / Joiner:

Charley Brentnall

Timber Supplier:

Hooke Park, Vastern Timber Company, S H Somerscales Ltd, Euroforest Ltd, Wessex Woodland

Timber Element(s):

Structure, cladding

Timber Species:

English larch, English western red cedar

Awards:

Wood Awards shortlist 2012

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