City of London Freemen's School Swimming Pool, Ashtead, Surrey

Introduction

The architect Hawkins\Brown has used timber to create an elegant cathedral-like structure for a new swimming pool, a structure which is also corrosion-resistant, resilient and carbon-neutral. The new six-lane, 25 metre competition pool building stands in verdant parkland, surrounded by ancient woods, on the campus of City of London Freemen’s School, a co-educational private school for about 800 day and boarding pupils, near Ashtead Park in Surrey.

In response to its sensitive location within the Green Belt, near ancient woodland and close to the school’s Grade II* listed Main House, the architect has created an exterior with minimal external impact, a simple, low-lying form, clad with dark copper-coloured standing seam zinc panels to respond to historic features in the main house.

Its low form has been achieved by partially submerging the pool to a lower ground floor level, which moulds the building into its surroundings, and helped to preserve a large number of the existing trees.

The building has a gently pitched roof with the highest point indicating a glazed main entrance. Once inside, in contrast to the modest exterior of the building, the reception area gives visitors a wide view beyond to the pool enclosure, a light and spacious hall of white-stained exposed glulam frames and cross-laminated (CLT) panels, its tranquillity heightened by beautiful views to the woodland outside.

The two long sides of the pool are level with the ground and the large glazed panels between the columns go down to ground level, with direct views from the water out to the trees, giving the impression of swimming in nature.

The pool itself, a waterproof concrete slab, is set into the ground at semi-basement level and the sloping ground allowed the east end of the building to accommodate two floors, a lower level of changing rooms and toilets which leads directly to the pool, and a steel-framed upper ground floor level which aligns with the main entrance and houses a flexible teaching space and event space, plus a large plant room.

The swimming pool (which replaces one destroyed by fire) is the second phase of a four-stage masterplan by Hawkins\Brown to improve the quality of the school’s listed campus setting. It was designed with a range of users in mind and is fully accessible. Outside school hours it can be used by local residents and swimming clubs.

The use of timber

The pool is enclosed by a glulam portal frame structure braced with CLT roof and wall panels. The glulam portal frames which run at the long sides of the pool are consistently at 2.5 metre centres but each ridge is offset from the other, creating a subtle variation of roofscape above the pool. A further row of glulam columns runs along the west end, the ‘deep end’ of the pool. The glulam beams and columns are butt-jointed and all connections are concealed; there are no exposed bolts.

Large glazed panels are set between the glulam columns, with the CLT wall panels at high level above them. Each glulam column is fitted at low level with a viewing bench, fabricated from CLT, with another set of CLT benches closer to the pool.

The glulam portal frame and the CLT roof are exposed internally and coated with a surface spread of flame system with a white translucent finish, helping to create a soft, warm environment to swim in.

The timber structure had a number of advantages in dealing with the challenges of a pool environment; timber is resilient, a thermal insulator and corrosion-resistant. The PEFC-certified spruce was pre-cut, drilled and machined in the high tech factory of Wiehag in Austria, and all metalwork was factory-fitted, enabling a fast, safe installation which minimised disruption to the school as well as producing little waste or dust. The programme for timber production was seven weeks (including shipping) and on site the erection of the glulam portal frame and CLT walls and roof took just over three weeks, meaning that from detailed design to full construction the building was completed within a year. 

The building was designed in Revit to BIM Level 2 to model the complex forms of the structure and this allowed the process to be closely coordinated by the design team, including the services engineer. The same BIM model was used by the contractors to develop construction information.

The timber structure

Arda Ozel, structural engineer at Eckersley O’Callaghan, describes the structure:

‘The glulam portal frames are formed of a continuous section profile for the stanchions and rafters of 1020mm deep by 220mm wide. The depth of these sections was selected to translate to the natural fabrication size of the timbers, which are formed of a series of 41mm thick laminas bonded together to form the overall desired depth. In an earlier design the portal frames were set further apart and the greater span required a stronger, less readily available grade of timber. During the design development process it was suggested that the overall building size be reduced; however the architects considered that if the frames, at the same sizes, were set closer together, they would create a stronger visual presence in the pool hall. This had the added benefit of overall cost savings, as it allowed a lower grade of timber to be used and haunch restraint struts and steel cross bracing to be omitted.

The timber frames are braced by the CLT skin which wraps around the building roof and walls, acting as a rigid diaphragm, tying the frames together in both directions. Overall stability is provided by moment connections between the stanchions and rafters and at the ridge. In the long direction, the CLT fixings between stanchions provide a series of moment frames to distribute lateral loads between all the frames.

To save costs and to provide additional space, the upper ground floor has a steel frame structure. To transition the laterally less rigid timber frame with the steel frame, the steels next to the timber were designed as moment frames to allow a similar degree of lateral movement. Steel cross bracing was provided to give increased stiffness at the main entrance where there are large areas of glazing.

The rafters are formed of three sections, with joints designed and positioned to be parallel and offset from the ridge. They were fabricated in sections and fixed together on site. Some rafters were fixed prior to lifting into place, and the final connection between rafter and stanchions was made using a pair of mobile elevating work platforms, working in tandem.’

Sustainability

The architect states: ‘We knew from early in the design process that an engineered timber structure was the right approach. It is thermally insulating, carbon neutral, corrosion resistant, and requires very little ongoing maintenance’. The timber, supplied by Wiehag, is from sustainable PEFC-certified forestry which helps to embody carbon throughout its growing period. The CLT roof and wall panels are clad in cellular glass insulation with an exterior cladding of either standing seam zinc or fibre cement board, all chosen for their low environmental impact and durability. Because of the high temperatures within the pool hall and changing areas, a simple rectangular form was chosen which limited the thermal envelope, and glazing is focused in areas where it has the most impact, to limit the potential for heat loss.

Photo-voltaic panels provide ten per cent of the pool’s energy. The ventilation strategy within the pool hall creates a cushion of still air over the pool’s surface to reduce the potential for heat and evaporative losses. Air is introduced at low level under the glazing to combat condensation. A pressurised plenum below ground level distributes air in a concealed way so that the pool hall is relatively free of visible services. Electrical services are concealed in a timber channel running at a datum level above the glazing, which locates cabling away from the ‘wet-zone’ below.

The building achieved a BREEAM Very Good rating.

Awards

RIBA National Award 2018

Structural Timber Awards, Education Project of the Year 2018

RIBA South East Award 2018

RIBA South East Sustainability Award 2018

RIBA South East Client of the Year 2018

Completion Date:

September 2017

Building Type:

Swimming pool

Location:

Ashtead, Surrey

Architect:

Hawkins\Brown

Structural Engineer:

Eckersley O’Callaghan

M&E Consultant:

Skelly and Couch

Main Contractor:

Gilbert Ash

Structural Timber Subcontractor, Supplier and Installer:

Wiehag

Timber Elements:

Roof and wall structure and panels, benches

Timber Species:

PEFC-certified Austrian spruce

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