The low rise and mid rise floors of the 42 level office tower at 259 George Street in the heart of Sydney’s CBD have recently been the subject of a full refurbishment by Bovis Lend Lease.
The renovations were complimented with improvements to the ground floor lobby and the construction of a child care facility on level 4, but by far the most striking of the changes can be seen adjacent to the new food court located on the ground floor.
Food @ 259, as the establishment is now known, is an indoor/outdoor eatery protected from the elements by a lightweight suspended glass awning. Conceived by architects, PTW, the glazed awning is a feature of the site and is installed over the main eastern eatery entrance.
The design consists of glass roof panels supported on light steel outriggers that protrude 6m from the façade at 1500mm centres across the 16.5m wide structure. Tied to the building façade at one end, the outriggers are fully supported approximately ¾ of the way along their length by a stainless steel cable and rod truss designed by structural engineers, Robert Bird and Partners with input from Ronstan Tensile Architecture.
The truss itself consists of two ACS2 36mm diameter stainless steel cables – one located above and the other below the awning. At each outrigger a pair of 8mm diameter stainless steel hanger rods attach the outrigger to the main cables. The upper cable with its rod hangers provides support for the mass of the structure and downward wind loads, while the lower cable/rod combination restricts uplift in the opposite case.
Ronstan Tensile Architecture designed custom stainless steel cable clamps to meet specific clamping forces and positively locate the hangers on the main cables. A third cable truss member runs through the middle of the outriggers across the structure at the truss location. Cable clamps were designed and installed on the cable each side of every outrigger to provide the lateral support necesary to locate the outrigger in position.
During the development phase, Ronstan was approached by the engineers to undertake the detailed design of the truss cable, its components, and to determine its interaction with the surrounding structure.
Ronstan Tensile Architecture, with partner, Pfeifer Cable Structures, then engineered cables, connections and rods in accordance with the predicted design loads and cable properties. At execution Ronstan co-ordinated with the builder, architect and steelworker, gathering inputs for Robert Bird to accurately define the geometry of the system and the loads applied to it. This information was used to prepare shop details and manufacture components, before the final geometry of the glass awning was modelled.
With the geometry and loads of the structure known, cables could be manufactured to the exact length apparent at the defined load and geometry. The cables were pre-stretched 5 times during manufacture to alleviate constructional stretch, before being cut and swaged under load at a constant temperature. Clamp positions were marked while the cable was in a loaded state to replicate their exact location once the cable had been placed into the pre-stressed structure – all critical considerations to ensure the required lengths and geometry were achieved.
During the installation and commissioning of the structure, Ronstan Tensile Architecture supervised the riggers and ensured methods were followed. The tension in the cables was closely monitored to ensure the system was installed in accordance with the structural engineer’s requirements.
The entire project was completed in close consultation with Pfeifer Cable Structures, arguably the worlds leading cable structures partner.