External View of the Tumbalong Green Sound Shell

#2863 Tumbalong Green Amphitheatre

2024 LSAA Design Award Entry #2863 Tumbalong Green Amphitheatre

ID NUMBER	2863
ENTRY CATEGORY	2 Medium Structures
ENTRANT ROLE	Engineer / Install
PROJECT NAME:	Tumbalong Green Amphitheatre Roof
APPLICATION:	Public Amphitheatre roof covering

PROJECT DESCRIPTION:

Creating a showpiece architectural structure in Darling Harbour, the Tumbalong Green Amphitheatre is a remarkable marriage of beautiful elements and engineering expertise. Exemplifying the stunning possibilities of lightweight structure design, the soundshell curves up and over the stage, appearing to almost float over the performance space and encourages sound to wash over the audience like a breaking wave.

An example of architectural design meets functionality, the combination of Cross Laminated Timber (CLT) structural elements supporting a 1000 square metre PVC membrane have created a lightweight, and visually appealing public amphitheatre roof.

Award Medium Tumbalong

THE PROJECT

Creating a showpiece architectural structure for Darling Harbour, the project architect for the Tumbalong Green Amphitheatre designed a stunning shell-shaped amphitheatre to upgrade and modernise the existing bat-wing style tensile membrane stage roof at Tumbalong Park. The enormous sound shell (looking remarkably similar to a natural shell you might find at one of Sydney’s many beaches) curves up and over the performers, allowing sound to wash out over the audience like a breaking wave.

The Tumbalong Green Amphitheatre Roof is a marriage of architectural elements and engineering expertise, exemplifying the aspirations of lightweight structures. The unique and visually stunning curved form of the amphitheatre roof appears to float above the stage. An example where architectural design meets functionality, one of the key elements of the amphitheatre roof structure is the combination of Cross Laminated Timber (CLT) and tensile membrane to create the lightweight and visually appealing stage roof.

Our scope on the project was to design and engineer the architectural fabric cladding that makes up the outer layer of the Tumbalong Green Amphitheatre Roof. This included the three-dimensional form-finding and patterning of the fabric to achieve the desired multi-plane curvature, as well designing an extruded aluminium fixing system to interface with the steel and CLT elements of the roof.

STRUCTURAL SYSTEMS

Critical to the success of this gravity-defying structure is the use of lightweight PVC membrane tensioned over the structural roofing elements. The low weight of the tensile membrane roof has allowed for a minimisation of secondary supporting materials for the roof cladding, helping to reduce the overall load bearing requirements of the soundshell’s three load bearing contact points.

A small, but visually prominent part of the entire project, the design and form-finding process for the tensile membrane element of the Tumbalong Green Amphitheatre demanded extensive communication with stakeholders and considerable patience as the project advanced.

Designing membranes with multi-plane arched curvature requires experienced engineers and sophisticated drafting software to determine the optimal cutting pattern and pre-stress loads necessary to achieve the desired final form while, while also ensuring proper tension distribution.

Using a parametric 3D model of the structure, we were able analyse the loading of the membrane alongside the other structural elements, providing real-time feedback to the head contractor and specialist CLT contractor. The live parametric model also enabled us to design and engineer the fixing elements required to attach and tension the fabric onto the structure. An example of this was the cleat system to fix the aluminium extrusions to the outer steel ring beam. We needed to supply detailed specification and technical drawings back to the head contractor to include these fixing elements in their steel fabrication plans.

MATERIALS

The architect had specified Serge Ferrari Xtrem TX30-II for this project prior to our involvement. TX30 is an architectural PVC renowned for its high performance in tensile architecture applications such as roofs. With its crosslink technology, TX30-II boasts exceptional durability, ensuring a prolonged lifespan, making it ideal for permanent outdoor structures like amphitheatre roofs. Thanks to Serge Ferrari’s Precontraint technology, TX30-II maintains its shape effectively, enduring wind and minor temperature fluctuations with ease. This dimensional stability is important for a large multi-curved roof structure such as this.

In the early stages of the project, we helped address a minor design question related to material reflectance. The council raised concerns that the bright white fabric might cause excessive glare and affect nearby buildings. To address this, we worked with the architect, head contractor, and fabric manufacturer, Serge Ferrari. Serge Ferrari commissioned an official Light Reflectance Value (LRV) report for their TX30-II material. The report confirmed that the LRV met approval requirements, allowing the project to proceed with the TX30-II PVC membrane.

FABRICATION

Following the approval of the initial designs and the development of membrane cutting/fabrication plans, our biggest challenge was navigating the waiting period as the project installation progressed through various phases. Despite the membrane canopy being fully designed and ready to send to our fabrication partners in Brisbane, achieving the most accurate patterning necessitated waiting until the structure was fully erected to perform a final detailed site check-measure prior to fabrication.

Unexpected project delays resulted in a 14-month gap between the final approval of our designs and gaining access to site fort final surveys. The final survey site measurement involved additional complexities due to limited site access. Even after our final site visit, portions of the CLT roof were remotely resurveyed after de-propping, with the design team still needing to estimate some elements of the final fabric shape for patterning.

Once we had issued the plotting book to the fabricator, the entire fabric roof was created into a single fan-shaped membrane. The single roof piece was constructed by plotting, cutting, and welding together nearly 280 separate fabric panels (with an internal seam length of more than a kilometre). The fabricated roof fabric spans 56 metres at its widest point and 32 metres from front to back.

CONSTRUCTION

Once onsite, the folded fabric was craned into place in the centre of the CLT roof. The single fabric panel was then unfolded and affixed to the extruded aluminium connections. The 130-metre perimeter length features a 12mm rope edge welded into place for the extrusions to grip.

Ultimately, after months of waiting for our turn to mobilise, we completed our installation phase quickly and efficiently. Crews were on-site for only five days for the main installation, with a final visit to complete flashings and a final fabric cleaning once other trades had finished their respective sections.

As the specialist membrane contractor, our role focused on membrane expertise, collaborating within the larger project team. While some design decisions involved compromise, one particular choice impacted the final aesthetic. The project design team value-engineered the project, removing the cable tray conduits on top of the rafters, which resulted in minor ridges in certain areas of the fabric where lighting cables now run between the membrane and the rafters. Ideally, we would have advocated more strongly for keeping the cable tray conduits below the rafters to achieve a smoother, more visually appealing finish.

However, it's important to highlight that the membrane fabrication and installation itself was completed efficiently, on time, and within budget. Overall, we're pleased with our contribution to the project.