Sir Frank Ledger Breakfast 2016
Speaker: Ilario Spagnolo, Project Director, Main Roads WA
This annual Perth Branch Event commemorates the influence of Sir Frank Ledger in promoting the technical society functions now carried on by MA, in sharing and disseminating knowledge in the engineering and manufacturing industries in WA. Branch President Geoff Williams acknowledged Sir Frank’s contributions and welcomed Mike and Tim Ledger, his grandson and great grandson respectively. Mike responded, and expressed his best wishes to the Institute on its success over the past 70 years
After time for socialising over the breakfast, Ilario Spagnolo addressed the audience of over 50 people on the topic of the Swan Rive Pedestrian Bridge. This seems destined to become a major landmark and a tourist destination in its own right.
The bridge was conceived as an integral part of traffic management system for the new 70,000 seat Perth Stadium which is currently under construction. The stadium traffic flow is designed around public transport as private vehicle access would be impractical for the site on the Burswood Peninsula. The new stadium has a train station and bus port, but there is a need to move some 14,000 patrons to Elizabeth Quay to catch trains on the Mandura and Joondalup line and buses to southern suburbs.
The solution is the new footbridge, from Burswood to near Gloucester Park. It is designed for safe passage of 14,000 people in an hour, directing them to new shuttle bus services and away from residential streets in East Perth. To handle this volume the pavement is nine metres wide (three normal traffic lanes). Since it will only have this volume when events are held, the brief called for a design that wold be attractive in its own right, so it is laid out like a linear park, with trees gardens and seating in widened areas that will not obstruct peak pedestrian traffic flow.
The concept was put to competitive tender, with freedom to come up with a design that met the comprehensive selection criteria set by the WA Government. The more unusual criteria include “Identity”, “Engagement” and “Valuing heritage”. The one that caused most controversy was “Value for money”. Despite The WA Government preference for local content, the winning bid has meant that the bridge is assembled from modules constructed in Malaysia. Ilario was challenged on this, and had to respond by pointing out that a specified process had been transparently applied, and ultimately it is up to governments to determine the rues for expenditure of public funds.
The was however, no doubt that the winning design, by the York, Rizzani di Eccher joint venture, is quite outstanding. Essentially it is a piece of functional architecture, with the complex engineering necessary to make it work largely hidden from view. Technically, it is a 400 m cable stayed steel truss structure, supported on four concrete piers, two of these in the river, supporting a 160 m central span. The assembled steel truss structures are in four pieces, with the highest rising 65 m above the river.
The steel structure is completely covered with Teflon-coated fibre glass membrane; the same material used for the stadium roof. The trusses are hidden so that by day the structure appears to fabricated from solid material. At night, the membrane skin will be lit internally with LED lighting, with capacity for changing colours. The shape is open to interpretation, with some seeing a swan, others the Wagyl river spirit, or a dolphin, or just sinuous waves.
The construction of the $54 million bridge is quite complicated, both in fabrication and in assembly. The trusses ae made from circular sections with particularly complex welded nodes. These are built using temporary jigs, test-assembled into their final bolted assembles and then dismantled for transport to the Perth site. They are skinned before erection, but re-skinning, which will not be required for at least 25 years, can be done at height, in situ.
The river piers are joined to the bank by temporary causeways to aid assembly, most of which will be done using cranes on the bank. The central span will require a 400 tonne lift over the river, which will be closed to water traffic for around six days.
Questions from the audience referred to detailed design of the structure, and the dynamic behaviour analysis. The durability of the membrane was also of interest, as was maintenance and in-service inspection. There is an internal walkway within the truss structure. However, sad to say, there is no prospect of, public access, Sydney Harbour Bridge fashion.
All present agreed that Ilario’s presentation had another successful instance of our tradition of selecting topics of general materials engineering interest for this annual event.