Alfriston School, Beaconsfield, Buckinghamshire

Introduction

Alfriston school, a secondary-age day and boarding school in Beaconsfield, Buckinghamshire, caters for 120 girls with a wide range of special educational needs and disabilities. Its new swimmming pool building, designed by Duggan Morris Architects, is enclosed by a beautiful multi-faceted timber structure, its innovative shape a direct yet imaginative response to the requirements of the brief.

Although its grounds are large and surrounded by woodland, the main school buildings cluster together in a corner near the entrance while the rest of the site is given over to school playing fields. The new building with its 16.5 metre long pool is linked to the existing gym, connecting it back to the main buildings; it extends on a sloping site into the school playing field area.

In concept the design was envisaged as a lightweight timber structure floating over the water. Poised on a series of steel columns, three pitched and tapered roofs span the entire pool and turn down at the edges in a series of fractured triangular gables, like a complex origami ‘hat’. This dramatic and innovative form was designed as a contemporary expression of the local pitched roof vernacular, linking it to the traditional school buildings alongside.

On the outside the roof and gables of the pool building are clad with timber boards, reflecting the nearby mature woodland setting. On the inside the timber structure is exposed and acts as an acoustic baffle, reducing sound reverberation to create a quiet and peaceful pool space. This was an important requirement of the brief; many children with autism or learning disabilities are disturbed by noisy surroundings and strong colours, and respond better to environments in which visual clutter is reduced.

Timber was the the logical and most economical choice for the structure of the new pool for its qualities of natural sound absorbtion and its ability to cope with the humid and aggressive atmosphere of a pool environment. It was also the ideal material for off-site prefabrication of the structure.

As part of the project and with a view to opening the pool to public use to generate revenue for the school, the architect has also refurbished the existing gym and consolidated the sports facilities. The new pool building is linked to the gym by a smaller building, now the main entrance to the sports department, with a suite of new changing rooms shared by gym and pool, fitness suite, offices and plant room. Access ways are wide enough to accommodate disabilities and there are large storage areas for mobile hoist equipment.

The new pool building extends into the school playing fields which in turn are surrounded by mature woodland. To accommodate the sloping site, it is set on a solid monolithic plinth which is faced in a sober dark grey render; the adjacent entrance/changing room building is faced in similar render. The timber structure is supported on a series of one-metre high column stubs; they run in a zig-zag formation at the perimeter of the pool with glass strips between, giving swimmers clear views of the playing fields and woodland from within the pool itself while offering a certain amount of privacy. The glazed perimeter also gives the impression that the roof is suspended above the pool.

 

The timber structure

The timber structure consists of a series of stressed skin timber panels which span the full width and length of the pool and are folded and pitched to create an inherently stiff and stable structure. They are supported on the perimeter columns which lift the enclosure clear of the pool.

The twelve roof panels, twelve wall panels and two gable panels were all prefabricated and manufactured off site by Cowley Timberwork. Each panel consists of an internally visible frame; glulam beams at all the edges with a series of intermediate Scandinavian whitewood rafters between, were glued and screwed to a top covering of 21mm thick cross-laminated timber to create a composite stressed skin panel. This panel system allowed the creation of large, clear spans with economically sized components. Cross-laminated timber was used instead of ply as it is available in large lengths, so avoiding joints between ribs.

The stressed skin roof panels provide diaphragm action and transfer lateral loads to the wall panels by means of concealed steel connectors. The wall panels provide diaphragm action and transfer lateral loads to the steel columns. Due to the zig-zag arrangement of columns and the diaphragm action of the panels, pairs of columns act together to create a ‘push-pull’ action.

Cowley Timberwork used 3D modelling in combination with 5-axis CNC milling to manufacture the panel components with great accuracy so that they could be assembled together with small dimensional tolerances: mock-ups were used to test individual joints.

Each panel was designed with glulam beams running at all edges; once on site these panel beams were bolted to the adjacent beams to form pairs. The bolted connections are deliberately concealed. Bespoke and recessed steel plates were used to make connections between components, providing tight flush fitting between panels. Bolts were countersunk and concealed with glued timber plugs, sanded and painted to match the adjacent timber. The panels were fitted onto the circular columns with steel connector plates.

The panels were delivered with a weather tight membrane to protect them from water damage during assembly and, when the roof structure was complete, to allow the insulated cladding system and timber rainscreen installation to follow on immediately.

Above the timber roof structure is a layer of insulation, topped with a standing seam roof system with integrated cladding clips which support a rainscreen of open-jointed timber boards on battens and counter-battens. The entire pool roof and perimeter walls are clad in the same timber boards, with precise spacing and alignment. The timber boards, Platowood heat-treated Fraké African hardwood, are fixed to battens with 10mm spaces between; this dimension is crisply maintained across the sloping and vertical folding plates of the roof and the gables. All rainwater gutters and downpipes run outside the structure and insulated fabric of the roof, concealed behind the timber rainscreen roof cladding. As a result, roof and walls are visually integrated into a single homogenous form.

 

The interior

The glazed strip is achieved using a self-draining shuffle glazing system with minimal framing components. The sills follow the zig-zag geometry of the columns and the facades above and are powder coated matt black to match the masonry finish.

The interior faces of the timber panels were factory finished in three coats of breathable translucent light white stain. The interior finishes palette is minimal but offers a vivid contrast with the colours of the trees outside when viewed from the low level glazing; the surfaces of the roof panels are enlivened by watery reflections from the pool below.

The pristine and simple lines of the structure are enhanced by other details: pressurised air is introduced via a finger-width gap running at the edge of the glazing; pendant lamps provide a gentle wash of lighting. The water level matches the level of the surrounding floor to assist disabled students: a mobile lift is also available to help them. The overriding interior quality is warm, clear and silent.

The entire assembly of the frame took place over an eight week period. The prefabricated panelised system produced a precise, high quality structure and finish with no need for temporary internal scaffolding, as well as cost and programme benefits. The overall ambition was achieved; to reveal the structural logic and, through the simple execution of detail, selection of fittings and finishes, retain a practical appearance, free of decoration and unnecessary maintenance.

Completion Date:

July 2014

Year Published:

February 2015

Building Type:

School

Location:

Beaconsfield, Buckinghamshire

Client:

Alfriston School

Architect:

Duggan Morris Architects

Structural Engineer:

Elliot Wood Partnership

Environmental Engineer:

Skelly and Couch

Main Contractor:

Feltham Construction

Timber Prefabricated Structure:

Cowley Timberwork

Timber Cladding:

Pratley Carpentry

Timber Suppliers:

Cowley Timberwork, Platowood

Timber Elements:

Glulam and CLT wall and roof structure, thermo-treated external cladding

Timber Species:

European and Scandinavian whitewood, Fraké African hardwood

Awards:

Wood Award 2014: Structural Award Winner. MIPIM AR Future Projects Award Winner 2014.

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