Stackhouse - Chalmers Lane, Devonport

Client: Stacey Sheehan
Architect: Fitzpatrick + Partners
Builder: Beardwood
Structural, Civil & Hydraulic Engineer: Collective Consulting
Key Personnel: Alan Leake (Principal Structural Engineer), Jarred Allen (Principal Civil Engineer), Trent McCarthy (BIM Modeller), Eli White (Hydraulic Draftsman and Modeller), Christopher Polis (Assistant Engineer)
Value: Approximately AUD $5 million

Three-Storey Cross-Laminated Timber Residence with Infinity Pool

Collective was recommended by retired engineer Jim Gandy, an eminent figure in Tasmania known for his contributions to structural engineering, notable steel, concrete, and timber structures. Jim was approached by James Fitzpatrick of Fitzpatrick + Partners, a highly regarded architect with a strong portfolio of major projects across Australia, particularly known for his expertise in cross-laminated timber and innovative architectural designs featuring lightweight materials.

The Stackhouse, strategically located on Chalmers Lane, Devonport, overlooking Coles Beach and the Bass Strait, proposes the replacement of a long-held family residence, a 1970s brick dwelling, with a modern three-storey cross-laminated timber structure, including a concrete foundation and a suspended negative edge infinity pool. This design aims to highlight the stunning vistas while utilising renewable materials and showcasing impressive architectural and engineering ingenuity through the innovative application of cross-laminated timber.

Engineering Challenges

• Sloping Cliff-top Site: The original structure, situated on a rocky cliff top, provided sturdy foundations suitable for the lightweight timber development planned for construction above. However, the sloping site required careful consideration of access, drainage, and foundation design to ensure stability.
• Basement Infinity Pool: A basement level incorporating an in-situ concrete infinity pool necessitated the use of concrete columns, pad footings, and walling to fulfil the client's vision. Given the sloping site, precast concrete components in manageable sizes were employed to facilitate transportation and installation in constrained conditions.
• Transfer Slab Requirements: The basement's structural framework required a transfer slab, as the floor plates did not align with the upper levels, resulting in a three-storey building with complex load transfer requirements.
• Cross-Laminated Timber Integration: The lower level comprises solid precast concrete walls and slabs, while the upper levels utilise cross-laminated timber panels, each 120mm thick and pre-fabricated in 1200mm wide segments by Cusp, a local CLT producer in Wynyard. This hybrid construction required careful detailing at the interface between concrete and timber systems.
• Stacked Timber Configuration: The upper levels were constructed using cross-laminated timber arranged in a linear formation. To enhance structural aesthetic, the architects opted for a Hampton-style aesthetic featuring large Tasmanian oak beams.
• Diaphragm Action for Lateral Loads: With the building fronting the Bass Strait, additional bracing for the glass facades was not feasible; instead, structural plywood was used to transmit lateral loads within the floor planes, facilitating diaphragm action essential in transferring loads to the bedrock below.
• Concealed Connections: Concealed connections were designed with custom solutions for lateral stability and vertical tie-downs throughout the structure, which cascaded into a garage that cantilevered over the cliff, necessitating robust two-way reinforced concrete slabs for both aesthetic and structural effect.
• Value Management Adjustments: After undergoing a value management process, adjustments to incorporate timber wall frames with deep timber studs and plywood were made to maintain the desired timber, in lieu of CLT, aesthetic while meeting budget constraints.

Structural Design Solutions

The foundation system utilised the existing rocky cliff-top conditions, with concrete columns and pad footings supporting the basement level. The transfer slab at the basement level accommodated the misalignment between basement and upper floor plates, transferring loads efficiently to the foundations.

The cross-laminated timber panels from Cusp were detailed with 120mm thickness in 1200mm wide segments, enabling efficient prefabrication and transportation to the constrained site. The stacked configuration created a distinctive architectural statement while the Hampton-style Tasmanian oak beams provided structural continuity and aesthetic warmth.

Structural plywood diaphragms distributed wind loads from the extensive glass facades to the concrete core and bedrock below, avoiding the need for visible bracing that would compromise the panoramic Bass Strait views. Custom concealed connections managed both lateral stability and vertical tie-down requirements.

The cantilevered garage over the cliff edge utilised robust two-way reinforced concrete slabs, creating a dramatic architectural statement while ensuring structural safety. This element exemplified the fusion of structural integrity and visual appeal that defines the Stackhouse project.

Manufacturing Collaboration

Collaboration between Jim Gandy, James Fitzpatrick, and representatives from Collective included an inspection of Cusp to evaluate their manufacturing capabilities for wall panels, beams, and other timber elements integral to the project's aesthetic and structural requirements. This direct engagement with the local CLT producer ensured that design intent could be realised within manufacturing constraints.

Key Outcomes

• Three-storey cross-laminated timber residence showcasing innovative timber construction.
• Suspended negative edge infinity pool integrated into cliff-top basement level.
• Hybrid concrete-timber structure optimising performance of each material.
• Diaphragm action achieving lateral stability without compromising glass façade views.
• Cantilevered garage demonstrating structural ingenuity and architectural drama.
• Local CLT manufacturing supporting Tasmanian timber industry capabilities.