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Aquatic Centre – Diocesan School for Girls

The Diocesan Aquatic Centre an indoor swimming pool facility at Diocesan School for Girls in Epsom, Auckland.

It incorporates an eight-lane 20m long main swimming pool, with variable depth floor system, spectator seating for 400 people over a junior pool, and adjoining fitness centre, which includes a classroom and sports staff offices. The main pool is enclosed with a curved roof which spans 26m in two directions. It faces directly onto a heritage precinct and significant lawn area at the heart of the 100 year-old school campus. The building was required to fit within tight maximum height limits and be sympathetic in scale to nearby residential buildings.

The use of a timber roof structure was preferred to create a warm visual ambience for the interior and avoid the industrial look so prevalent in public pools and sports halls. However it was also important to avoid the rather heavy, chunky proportions associated with traditional glulam timber frames, and to somehow achieve a relatively light, delicate appearance. Timber was also considered to be the most environmentally sustainable option being a local, renewable resource with low-embodied energy.

Durability was another essential consideration in a humid and chemically-aggressive pool environment. The initial design concept was based on stainless steel trusses to form the roof over the main pool, but it was discovered that stainless steel in heated pool environments is known to corrode. However, given appropriate treatments, stable humidity and adequate ventilation of all surfaces, timber is known to be very durable.

Post-tensioned box-beams met all the critical requirements, for durability, strength, insulation and ease of construction. One metre deep post-tensioned rafters, spaced at 3.0m centres across the roof, fabricated using Laminated Veneer Lumber (LVL) chords and plywood webs, all of Radiata Pine, have been used to support the roof panels. The rafter spacing was determined by the maximum span of the Kingspan sandwich-type roof panels and eliminated the need for an additional purlin system. It was originally intended that the plywood and LVL elements of the box beams be glued together, unfortunately this proved chemically and physically impractical and the beams were fully nailed together. Ducts for the post-tensioning cables were installed during fabrication. Rafters are supported by primary box beams, which have a top profile matching the curve of the roof and are fabricated in a similar fashion to the rafter beams, although with increased depth and three times the post-tensioning requirement. Steel end connections were also designed for ease of fixing during erection.

The primary beams also form an integral part pool enclosure; the cavity was filled with insulation and the inner face fully sealed against internal moisture to create an ideal envelope with minimal thermal bridging. All beams were factory fabricated, delivered to site and erected in place over a scaffold.

The rafter beams were joined to the primary beams using coach screws. Timber cross braces provide lateral restraint and also transfer wind loads out to lines of support. The bracing members were prefabricated and bolted in place between rafters prior to the beams being post-tensioned. Acoustic ceiling panels are suspended just below the roof bracing and designed to permit free movement of air to all of the timber structure. This is vital to prevent the risk of mould-growth caused by any build-up of moisture. When the box beams were to be glued together, the intention was to have the structural elements of the beams left exposed as the finished surface. But to conceal the extensive nailing a 12mm “skin” of decorative Hoop Pine ply was applied over the exposed portions of all beams with a clear polyurethane finish.

The design and construction of this roof has been a true collaboration between the architects, engineers, suppliers, manufacturers and contractors to achieve what we believe to be a first for New Zealand, a complete roof built from post-tensioned timber box beams.

To complement the roof, timber finishes were also chosen for some of the key wall and ceiling areas, particularly where acoustic absorption was required. Extensive use has been made of slotted Hoop Pine plywood to ceiling and staircase walls. The main pool hall features a whole wall of horizontal Western Red Cedar slats with acoustic material behind, stained and a clear fire-retardant finish applied. Moreover, the glazed walls each end of the main pool are supported by tapering Radiata Pine Glulam columns with secret dowel connections.

This use of timber on the Diocesan Aquatic Centre adds real warmth and richness to the interior, fulfilling structural, acoustic and durability criteria, while creating an impression of a truly well-crafted building.

Winner – Commercial Engineering Excellence
NZ Wood Timber Design Awards 2009