In 2017, Partridge Event were involved in the structural design of the Perth Stadium Arbour, which is a series of 43 x 10m high steel arches with a 21m span, creating a 450m long covered community arbour linking the new Stadium Station to the Swan River, an amphitheatre, children’s playgrounds, picnic areas and a boardwalk. A grid of stainless steel cables span between the steel trusses, supporting aluminium artwork panels representing Noongar Community stories, and vines planted in selected bays which will eventually cover the arbour.
The arbour flows alongside the stadium, leading fans from the bridge landing and around the stadium to the main gate’s forecourt on the east. Moving through the arches the arbour reveals an appealing and sophisticated space with perforated metal shapes suspended from the well-positioned arches allowing for an intriguing shadow play.
The arbour is constructed with 120 tonnes of structural steel, 14km of stainless steel cables, 3065 folded anodised aluminium panels, over 16,000 stainless steel clamps connecting cables panels and steelwork, and integrated LED lighting and CCTV.
Partridge was responsible for the design of the cables, all cable fittings including clamps and swages, and the aluminium artwork panels and their fixings. This scope included development of the methodology and structural certification of all the temporary works required to install the cables, and the preparation of a construction methodology report. We were also responsible for the certification of the custom cable clamps manufactured by our client.
CREATIVITY & INNOVATION:
As there is currently no Australian Standard for the design of cables, the engineers at Partridge adopted a first principles approach and used a Strand7 model to model the arbour in 3D and applied the wind loads pretension and dead loads to the cables. There is no advice in the Australian Standards for applying wind load to vines growing on a cable grid. We discussed this topic with a cable and mesh expert who had undertaken a wind tunnel test which found that given the organic nature of the cladding and the ability of the leaves to reduce their wind area able to minimise the wind load substantially. We were able to understand some parameters on the reduction in wind area we could adopt in our analysis, which depended on the species of vine used. This helped us significantly reduce the wind loads which helped to reduce the cable size, bringing down the overall tensions in the whole system.
The Australian Standard for the design of cables is the steel wire rope code which was not relevant in this application. In discussions with the cable supplier we decided to adopt the Eurocode which was more relevant in this case, together with the use of the Strand7 model and the Australian Standards for the wind load calculations. Modelling cables in Strand7 can be complex especially when trying to find a balance between pre-tensioning the cables to limit deflection, and the tension limits in the cables. We discussed the deflection tolerances with the client and it was agreed that 100mm drape was acceptable.
Once the vines have grown, they will form a canopy to provide shade for pedestrians. The structure is designed with durability as a priority and the marine grade stainless steel used for the cables and clamps to minimise potential for corrosion. We ensured the cables were prestretched before use to minimise long term stretching of the cables creating sag issues.
Architects original render
CHALLENGES & RESOLUTION:
One challenge in this project was the temporary loading of the repeated steel arches, designed with pin connections at the base and with the cable grid intended to brace them. These arches were designed by a different engineering firm. Since the arches were already designed and installed prior to the preparation of our construction methodology report, the installation methodology had to ensure the cables did not exceed the maximum allowable reaction loads at the base of the arches during the installation period. Initially Partridge prepared a construction methodology with incremental tensioning, tensioning two cables at a time per bay over the full length of the structure then repeating, installing two additional cables and tensioning over the full length of the structure and so on. However after discussing this with our client, the cable installer, they advised us that this method would create significant time delays as they only had allowance for two scissor lifts and two crews of installers. On review we found a more efficient alternative using a method incorporating temporary bracing cables to install the cables from either end of the arbour without having to return and re- tension the cables a number of times. This ensured the installers were able to complete the installation within their allowed time frame, causing no delays to the project.
During our site inspection once all cables were installed, we tested the cable tensions on site for a sample of the cables, and found that the dead load tensions were mostly within 5% of the predicted tensions which was a satisfying result and confirmed the legitimacy of the engineering approach. The installation was complete on time, all stakeholders satisfied and the result closely matched the architect’s original design intent.