LSAA 2018 Design Awards HIGH COMMENDATION - Category 6 Collaboration
APPLICATION OF PROJECT:
Mixed fabric PTFE and EPTFE Tensile Membrane Roof over the 60,000 seat Perth Optus Stadium.
The crowning cap of the New Perth Stadium is the “halo” roof that appears to float above the main seating bowl and façade. The use of architectural membrane materials created an ultra lightweight roof, enabling a lightweight steel frame to be constructed that compliments the minimal form of the roof.
The roof structure is a continuous fabric roof free from movement joints and integrated with lighting, PA and other systems. The translucent roof was enhanced by the use of 2 fabrics – a PTFE and an EPTFE fabric – with contrasting light transmissions creating a visually exciting texture to the Halo internally.
Cantilever truss frames support the halo roof with an innovative fabric gutter system reticulating all roof water drainage to syphonically drained sumps hidden within the trusses. The primary roof trusses are supported by feature columns and back stays that are all exposed above the level 5 concourse. Design details of all connections were undertaken to a high level architecturally exposed structural steelwork specification to minimize visual impact and to maximise finish quality. Extensive use of tapered tubes accentuates the high quality design, fabrication and construction of the roof.
The resulting design was made possible through strong collaboration between the project team including architects Cox & Hassell, engineers Arup, Maffeis & MakMax and the fabricators & installers, Civmec and MakMax.
DESIGN / FABRICATION / INSTALLATION BRIEF
The key design brief from the client was for the roof to be designed for Perth’s climate conditions, easy to construct and install. The architectural concept underlying this was to create a unique design, recognizable as Western Australia across Australia and worldwide, with the roof the capping keynote feature of this project.
The architectural intent for the roof in the concept stage was to create an illusion of a ﬂoating ‘halo’. This concept was the inspiration for the choice of material used, form and detailing. To achieve this lightweight ‘halo’, the engineering team has developed efficient fabric forms that minimised the forces on the supporting roof structure. The main roof panel fabric between roof trusses is a conventional barrel vault supported by three arches. The main feature of the fabric form is where the PTFE fabric surface sweeps up to the outer ring of the roof with all primary steelwork being above the cladding. Between the bottom chords of the roof the fabric form is ﬂat, which was achievable due to their small spans and allowed for drainage due to its fall to the sump gutter.
To minimize the impact of secondary steelwork and gutters an innovative fabric gutter detail was proposed at an early stage. This detail needed to join two non-compatible materials together and maintain a weather proof seal as well as having the capacity to reticulate the large volume of roof drainage water expected.
Reducing working at height was a key focus for the team from a safety in design perspective and a construction planning focus. Arup’s steel design drove the tri-chord trusses into fully welded 44m long sections which enabled fabric cladding to steelwork to be completed at ground level prior to lifting the trusses.
The roof was designated as one of the key stadium features by the project architectural team and an extremely strong focus was maintained throughout the project on design detailing and finishes. Multiple design workshops, modelling renders, sample mock-ups and BIM modelling were undertaken throughout the project to ensure that the finished project quality would not be compromised. A comprehensive approvals system was employed to ensure that agreed details at the design stage were ultimately realized on site.
The roof structure consists of fifty (50), approximately 35m triangular truss cantilevers (~44m in total length). The bottom chords of the trusses aren’t triangulated by bracing but rely on Vierendeel design action. This Vierendeel action is significant for the design of the robustness load case of the roof involving a loss of a panel of fabric which results in large out of balance in-plane loads on the roof truss.
The scheme for the stability system was that pairs of trusses are stable due the connecting chevron bracing above the forestays that are located on every second bay. With the in-plane torsion being resolved by the rod bracing at the rear of the roof.
In order to avoid unsightly joints in the roof, the roof structure was designed as a continuous element, straddling the movement joints in the stadium bowl. This was achieved by taking advantage of the relative flexibility of the fabric-clad roof steel structure and incorporating joints and discrete details with inherent flexibility to avoid concentration of thermal strains. A continuous fabric roof, free from movement joints and integrated with lighting, PA and other systems was not just possible, but quick and safe to build.
Two types of fabrics were used to make a feature of the lighting, including a PTFE for the main panel with the truss fabric panels being a higher translucency ePTFE fabric. The PTFE fabric was Chukoh FGT800 (Japan) material and the ePTFE membrane was Sefar Tenara 4T40HF (US/Swiss) fabric.
The key architectural brief was to create visual texture in the roof through a mixed use of translucency. The specification demanded approximately 25% differential in translucency between the main panels and the truss infills leading to the selection of a standard PTFE coated glass material for the main panels and the approximately 40% translucent Tenara material for the truss infill panels.
The high tensile strength and 40+ year lifespan of PTFE coated glass was an essential requirement for the main panels with the stadium having a design life criteria of 50 years in the project brief. For the truss panels high light transmission was deemed the most important criteria and a reduced design life with a potential mid-life replacement was determined as an acceptable performance solution.
To ensure the manufacturers specified performance criteria were delivered through the project a comprehensive evaluation and testing regime was undertaken on all delivered materials. From the standard tensile and tear tests, through to wicking, water absorption and age accelerated weathering testing, each batch of material to be used on the project was comprehensively tested prior to acceptance for use.
Full material traceability was, of course, an essential part of this high focus on quality management. The quality reporting ensured that each panel on site can be traced back to its original production batch and the testing and QA that had been undertaken at the time of material delivery.
The truss infill panels were penetrated by the tri-chord truss webs, creating a waterproofing risk at this junction. A clamp ring around the web was utilized, and with the relatively small geometry of the truss panels, fabrication tolerances and accuracy were critical to ensure fitting on site and to avoid water leakage issues.
To speed up fabrication and to ensure accuracy, a series of custom welding bars were fabricated to suit the horseshoe shaped clamps. Multiple stages of dimension checking were used and enforced to ensure that both the plotted sub-panels and the final assemblies matched the design. Check measurements of the steelwork were taken by the construction team to ensure that the fabricated steel also met design.
The pitch infield was the main area for pre-assembly and lifting of materials to the roof and as is typical for stadium constructions, available space was limited. Fabrication works for all components including steelwork, catwalks and fabric panels was required to be undertaken in strict accordance with the construction program and delivered to site following the just-in-time methodology to minimize congestion on site.
COLLABORATION, CONSTRUCTION AND MAINTENANCE
Projects of this size and nature require a significant amount of collaboration between the contractor, designers, fabricators and installers. To ensure that the project would realise high quality designs and finishes with tight project budgets and timeframes this collaboration was a strong focus throughout the project with many stakeholders actively participating.
Throughout design, fabrication and construction, key team members from the architectural team, engineering team, shop detailers and the steel and membrane fabricator/installers participated in design workshops, fabrication workshops, construction planning sessions, mock-up and prototyping and sample review workshops that all formed part of a comprehensive management and approval system overseen by the head contractor and the State Government.
A strong focus through collaboration was to develop design and fabrication details together as a group prior to submitting for review. This process dramatically reduced review and approval times as most stakeholders were involved in development of the project prior to the submission of formal drawings.
Advanced modelling techniques were used to show design details in 3D through the concept development stage, these 3D models eventually became part of the overall BIM model for the stadium which was another major coordination and collaboration tool.
A comprehensive BIM strategy was implemented at the project level for the full life-cycle, with a focus on improving collaboration between subcontractors and reduction in on site re-work. The team developed a streamlined delivery process - from Analysis to Design to Database to Model to Drawings, enabling for architectural and structural refinements to occur in parallel without losing element-to element connectivity or resulting in the separation of modelling parts which avoided timely re-work at each design update. As an outcome of the digital workflow, complete coordination and alignment of design between architect, engineer and sub-contractors was maintained throughout. The client had the requirement to use BIM to inform the construction phase as well as for operational maintenance once completed. It was imperative that the systems and processes were set up to capture the holistic BIM requirements, and for client to confidently move from phase to phase.
To expedite roof construction, the advantages of off-site construction were exploited, fitting out as many ancillary items to the steel trusses as possible in the workshop. By utilising custom bogeys, each roof truss became a semi-trailer, readily transportable to site. Once on site, the bogeys doubled up as stillages to facilitate a safe stage 2 truss fit-out at ground level, with truss infill membrane panels, cable trays and other miscellaneous fittings prior to being lifted into position.
This decision to lift each roof truss in its entirety was taken to minimise the working at heights requirement on the project, ensuring that a large amount of fit out works of other trades could occur in a safer method and with easier and quicker installation methodologies.
Construction workshops focused on optimizing the construction schedule to maximise productivities between overlapping trades. Works at heights took priority, so careful planning of exclusion zones was required to ensure that the bowl fit-out works could also proceed in a timely fashion.
Maintenance of the structure was planned by the roof access safety consultant. Incorporation of catwalks to access the roof and to support the sports lighting ensured that lighting maintenance could be done simply and easily, with the added benefit of ease of access to the roof for membrane inspections. The structural engineering team designed anchor points to be contained in the truss webs for a permanent roof access line to be installed to each truss, providing a safe access way to the sump locations to allow for maintenance access to the syphonic sumps.
Entrant: MakMax Australia
Role played by Entrant: Designer, Engineer, Architect, Fabricator, Installer
Location: Perth, WA
Completed: December 2017
Client: Multiplex for WA Government
Architect: Hassell / Cox / HKS
Specialists: MakMax / Maffeis (membrane roof designers)