Haxtead Garden House – An engineering case study
The project involved a new concrete and steel-framed pavilion in coastal rural NSW. The material combination was chosen to offer protection from the harsh elements while engaging with views of the surrounding landscape and Sapphire Coast.
The structure is 40m long x 8.4m wide which incorporates an impressive 43m x 2m wide long off-form concrete roof gutter, and off-form concrete columns that accommodate roof drainage and cast-in rebates for windows to the south elevation. The main roof is a thin profile light weight roof to the north. The northern windows were bespoke steel window frames designed to be integrated with the structural steel support for the roof.
There was also extensive external works required which involved a new driveway and parking area, garage structure, 30m long external terrace and pergola structure, and a new in-ground swimming pool.
Partridge commenced initial concept design work in 2014 and the project commenced construction work in 2015. The construction work was completed and handed over to the client in early 2017.
The existing site was largely a greenfield site with an old horse stables occupying the eastern area where the new residence was constructed. The project brief was to design a carefully crafted residence where the structure would be celebrated in the architecture and that the structural elements were to be integral to and exposed in the façades throughout the building. The south façade and roof gutter were to be Class 1 off-form concrete and have integrated window suites with a maximum exposed roof edge of 400mm at the south elevation. The northern façade was to be exposed structural steel framing and integral with the bespoke framed steel windows. The main roof profile was to showcase a thin expression around the perimeter with a 1.8m eave to the north, east and west elevations with a maximum edge profile of just 75mm deep, tapering up to a maximum of 150mm at the perimeter walls of the building.
Design and Construction Process
The design and construction process involved close collaboration between Partridge and the Architect during the concept and preliminary design development. Once the preliminary design and documentation had been developed, a builder was selected. The builder then engaged with the design team to discuss their preferred methods of construction. Partridge engineers collaborated with the architect and the builder during the design finalisation and construction documentation phase to detail the expressed structural steel framing which would achieve the architect’s design intent. Partridge worked with the builder to develop the builder’s construction sequencing and tolerances, giving due consideration to the waterproofing of the northern façade, and expected thermal expansion and contraction movements of the custom framing.
Creativity and innovation
This residence is not of typical house construction. Creative solutions were required to support the concrete roof gutter and detailing of the reinforcement to allow for the rebates and unusual architectural details, and also to prevent cracking to the Class 1 off-form concrete. In addition to the finishing and jointing requirements, there was a large clear span of 12.6 m along one edge of the gutter – a very challenging distance for the thin-profiled concrete to achieve.
The northern façade and eaves overhang was to be constructed of expressed structural steel framing and required careful detailing of the connections and arrangement of the beams, columns and window frames in order to be able to achieve the desired aesthetic in a buildable and waterproof manner that was able to accommodate the thermal movements of the façade. Partridge carried out significant expansion and contraction calculations to establish the likely thermal movement of the steel and other interfacing materials to ensure that adequate allowances were made for the detailing and joints accordingly.
The house was designed with sustainability initiatives in mind, such as orienting the building to maximise the solar efficiency with maximising glazing to the north façade and reducing the glazing to the south façade. The north façade incorporates high level operable windows to assist in passive thermal control and reduce heating and cooling costs. External louvres are also incorporated with the northern façade window framing. The steel framing was hot dip galvanised to HDG900 and the detailing was prepared to avoid site welding. The concrete was designed and detailed for the coastal environment to ensure a long maintenance-free lifecycle.
Built environment and heritage
As a new build domestic residence there are no heritage requirements for the project. The built environment is also minimal given the rural coastal site. The architectural design adopted a modern architectural approach to respond to a classic Australian landscape. The outcome is a finely crafted structure that is respectful of the natural beauty of the site in which it occupies.
The challenges and resolutions
The concrete gutter was to be approximately 2m wide and 43m long overall with a maximum expressed depth of 400mm to the south façade. The southern edge and soffit was to be off-form concrete supported by off-form concrete columns. The glazing suites were to be recessed into the concrete to achieve a flush finish to the frames. In one area of the residence, the northern edge of the concrete gutter was required to have a clear span of 12.6m. Additionally, no joints were permitted in the gutter and no downpipes were to be visible.
The gutter design was achieved using concrete upstand beams either side of the gutter; considered detailing of the steel reinforcement for flexural and shrinkage stresses; low shrinkage concrete mix design; and continuous water curing of the concrete to minimise the risk of shrinkage cracking. The drainage provisions were achieved using cast-in HDPE downpipes to the gutter slab and supporting concrete columns. (This required further special detailing to ensure the cast-in elements didn’t induce cracking or movement due to discontinuities in the concrete). The 12.6m span was supported using a combination of the continuity of the concrete edge beam and cantilevering the gutter slab from the column at the southern edge.
The concrete blade columns are 220mm wide and incorporated the downpipes and rebates for the windows frames. Careful detailing of the reinforcement was required to ensure the downpipes and rebates could be accommodated within the column dimensions, while still achieving the requisite cover for durability.
Between the concrete columns along the southern façade there are off-form concrete spandrel panels that form the single skin external wall. The structural design required careful detailing of the reinforcement and joints to avoid cracking to the off-form concrete walls and water ingress to the building.
Partridge engineers worked with the builder to develop and recommend the construction sequence for the concrete pours, giving consideration to sequencing to avoid cracking, the construction program and buildability of the concrete elements.
The construction was successfully executed and all the Class 1 off-form concrete is performing as designed with no cracking to any of the off-form concrete elements.
Concrete gutter and columns
Expressed steel framing
The exposed steel framing posed many challenges including the detailing of connections to meet the architect’s intended aesthetic; alignment and arrangement of steel members to achieve the desired architectural profiles and depths; and then achieving all this while maintaining permissible tolerances for buildability and thermal movements.
Concrete gutter and columns
The architectural design gave a maximum structural depth for the northern roof of 150mm overall. All the structural elements thus needed to be sized within this limit, thus allowing for cladding and finishes so that the 150mm deep plane would not be exceeded. This meant the support beams, cantilever beams and double cantilever beams all needed to be aligned in the same plane causing many beam/column intersections and structural plane “clashes”. Furthermore, the fascia beams were to be a maximum 75mm deep, which was particularly adventurous and challenging for the double eaves cantilevers at the corners of the building. The eaves design relies, in part, on cantilever beams supported by custom-fabricated rectangular hollow sections acting as torsion beams, which also act as the window head beam to the custom steel framed windows. This was all innovative and custom design, as no “off the shelf” products or solutions could work within the allowed parameters.
The steel columns, window frames and eaves framing were all visibly expressed as part of the architecture of the building. This presented challenges for designing the connections of the structural members, the location of the splices and joints, and the methods of connection. Site welding was considered and dismissed in the design due to durability concerns. All connections were shop welded where possible before hot dip galvanising. All site connections are thus cleverly concealed bolted connections, which at some locations required careful detailing of column cap plates and local removal of the bottom flange of the roof beams.
Given the building is 40m long and the structural steel framing to the north façade is integral with the structure and provides structural support to the roof, careful consideration needed to be given to construction tolerances, construction sequencing, and thermal movements to ensure the buildability of the structure and the subsequent waterproofing details. Partridge worked closely with the builder and architect to ensure the architect’s vision was able to be realised with the appropriate construction quality from the builder, and tolerances to allow for the predicted thermal movements. This was achieved by providing a 3mm construction tolerance at each side of the window frame panels which then provided up to 4mm tolerance at each window frame to accommodate thermal movements. The builder was able to erect all the framing without any need to make any modifications on site. The steel framing assemblies “fitted together like a watch” according to the builder and the waterproofing of all the joints has proven to be successful.