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Resting in the sand dunes of Bilgola Beach, this 873-square-metre house overlooks the beach and the ocean. The property’s entrance leads to a self-contained and vast residence that can be opened up to the guest wings. Partridge collaborated on this project with internationally renowned Seattle-based firm, Olson Kundig as their first project completed in Australia.
Bilgola Beach house was an intricate project in engineering and construction. It involved excavation depths of 4m across two residential lots which were joined into one property, the design of shoring for excavation support, basement concrete slab and pile design to resist coastal erosion and wave loading, expressed concrete walls, exposed architectural steel framing, operable façade elements hanging off a 4m double cantilever, timber framed floors and timber framed insulated green roofs, and bespoke steel stairs hung from the floors above and hung floor plate.
Photo: Nic Lehoux
Partridge was engaged to act as the structural, civil, and hydraulic engineers to deliver the analysis, design, documentation, and site supervision to support the architect’s design and provide a dwelling of impeccable finishes where the family is able to connect with the natural environment.
The design process was not our typical process as the architect was, based in Seattle, USA; the client based in London, UK; and all project co-ordination and collaboration occurred remotely, noting this was in a pre-Covid environment. The project was designed to minimise the visual impact from the beach and settle into the mildly undulating sand dunes. The board-formed concrete walls were coloured to match the tones of the surrounding sand, and the grasses planted on the green roof, relate the architecture to the site and help it merge with the natural condition of the beach and headland as it weathers overtime.
The environment is known for its extremities from flooding and storms to summer heat, so the foundations of the building needed to be designed to resist the coastal loading to ensure the building is robust and durable. Due to a prior erosion event in 1974 (pictured right) a rock revetment was built as part of the emergency response works which meant the pile foundations were likely to encounter the revetment.
The architectural design required large spans supported on slender steel columns, operable façade elements, insulated green roofs, concrete walls with the building set over the coastal erosion zone. The architectural design proposed a floor zone at the first-floor level of 380mm including finishes, building services and architectural items such as recessed blind pelmets and door tracks.
Once on-site, Partridge worked closely and collaboratively with the builder to assist and support their desired construction methods. The building was designed using timber framing wherever possible to minimise its carbon footprint and keep the structure as light as possible. The green roof was externally insulated and the perimeter off form concrete walls had cavity insulation provided to assist with the thermal properties.
Photo: Nic Lehoux
Photo: Giles Davies
The operable façade screens doubled as shading elements and generous awnings when the full height doors were open. The ventilation strategy keeps the building cool in summer without relying on air-conditioning, and in winter the heating is provided using hydronic heating pipes cast into the concrete floor slabs. The central courtyard further assists with the natural ventilation and the courtyard water feature and green roofs also helps control the local climate and cool the air.
Photo: Rory Gardiner
The first-floor level needed a 4m double cantilever at the north eastern corner of the main living wing. To further complicate the cantilever there were top hung glazing doors underneath the cantilever with minimal deflection tolerance and the architect wanted the external expressed steel columns to be continuous from ground floor to roof level effectively creating pin joint between the columns and first floor edge beams. To achieve the architectural design, Partridge proposed and designed the steel framing to partially cantilever at first floor level with a beam cantilevering north off the fireplace to support a column at first floor which supported a beam in the roof framing. The roof beam cantilevers east over the support column and then has a hanging column to suspend the first floor at the cantilevered corners giving the building a delightful floating effect at night.