Construction World December 2018

SPECIAL MENTION

T he cable stayed bridge and its temporary works were designed by a team of engineers in SMEC South Africa’s office in Cape Town between 2013 and 2017. The team first conceived the winning tender design, for contactors Salini Impregilo, in a design and build tender. It then went on to develop the detailed design and undertook the erection engineering work for its construction. It was a mammoth task that absorbed a team of up to 15 staff for over four years. Curved cable stayed bridges are rare and curved cable stayed rail bridges even rarer. The conceptualisation, detailed design and construction of this bridge was a huge technical challenge that went beyond conventional cable stayed practice and technology. To take on this challenge and to succeed with such a landmark project in the international market is thought to merit the entry of the project. The project is especially noteworthy in that the deck superstructure was first built as a seven span continuous girder on temporary supports using an overhead self-launching gantry. Thereafter the bridge was converted into a three span cable stayed viaduct that is part of the new Sydney Metro. The metro is the largest infrastructure project currently underway in Australia and its first phase, the Northwest line, includes eight new stations, approximately 15,5 km of tunnels and the aforementioned ‘skytrain’ viaduct. SYDNEY METRO CABLE-STAYED BRIDGE Located in the north west of Sydney is a unique, curved in plan, three-span cable stayed rail bridge constructed using precast segmental concrete. The bridge forms the tail end of a 4,5 km elevated

PROJECT INFORMATION

• Company entering: SMEC South Africa • Client: Salini Impregilo • Start date: 2013 • End date: 2018 • Main Contractor: Salini Impregilo • Consulting Engineer: SMEC South Africa

bridge and the temporary supports were removed. The use of the precast segmental concrete for the deck superstructure was a required continuation of the main viaduct. The challenge for the SMEC team became the integration of the techniques and temporary works used to build precast segmental concrete bridges with the demands of cable stayed bridge construction.  The finished form is beguilingly simple as intended by Sydney’s urban planners. The bridge’s presented its complexity to the designers and construction team at every turn but this fact is now an unseen internal feature of the structure. The project itself will make a significant impact on improving the lot of commuters in Sydney with self-driving trains expected to run at five minute intervals. The cable-stayed bridge is the landmark of the project and the restraint and care taken in its design are positive reflections of the city’s investment in public infrastructure. On a technical level this project has undoubtedly proven that longer suspended precast segmental bridges can be erected using conventional erection methods. This method of construction had major safety benefits in that it eliminated the need for any day time closures of traffic lanes across Windsor Road. On a commercial level the project can also be considered a success because it was a critical path item that in the end it in no way delayed the perway contract that followed on. Design and build projects such as this, are a tremendous opportunity for engineers to develop both their technical and construction technology skills. It’s vital for the growth of our local industry that South African engineers have such opportunities. 

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