18 Oct 2020 Old Māngere Bridge, Auckland
A stunning new bridge in New Zealand is offering excitement and fresh challenges for the Contech team. They are supplying and installing BBR VT CONA CMI internal post-tensioning for the piers of a new and dramatic bridge structure which promises to be so much more than just a crossing it is set to become a destination in its own right.
Visit New Zealand’s largest city and you’ll soon discover it’s often called the City of Sails, owing to its numerous yacht marinas and the sailboats anchoring offshore. If you travel to-and-from the airport though, you might prefer to call it the City of Bridges as there are some glorious examples to be found along the route.
The newest bridge
The latest addition – currently under construction – is a shared walking, running, fishing and cycling bridge that will cross Manukau Harbour and connect the Auckland suburbs of Ōnehunga and Māngere Bridge. Strikingly elegant, it will replace the ‘Old Ma¯ngere Bridge’, a 250m-long, reinforced-concrete structure that opened as a vehicle bridge in 1915 and which itself was preceded by a single-lane timber bridge built in 1875. It has cultural significance too. Local iwi resided in the area as kaitiaki (guardians) of this important portage and where waka (canoe) travelled from one coast to the other. Before the old bridge was built, it’s said that crossings were possible on stepping stones at low tide.
A Waka Kotahi NZ Transport Agency project, the new bridge’s design – which is the result of years of work with Mana Whenua (the Māori people that have an ancestral connection to the land), the community and other stakeholders – follows that of a traditional waka, and recognizes and celebrates the cultural and historical significance of the old bridge and the surrounding area.
The concrete bridge will be 300m long and feature a 60m central deck span and arch in structural steel, together with nine approach spans in reinforced concrete. The central deck span and arch are being fabricated in Napier (New Zealand). Designed to be a community meeting place, it will include benches so that people can relax and enjoy the views and there will be two fishing bays dedicated to those catching kai moana (seafood). It will also provide up to 6.5m of clearance underneath to allow waka and small watercraft to travel into the Upper Māngere Inlet. Where possible, parts of the old bridge will be salvaged and reused as artworks in the surrounding area, and cultural artwork will feature on the bridge structure and at the entry and exit points.
To access the seabed, temporary ‘cofferdam’ structures, which create dams so that water can be pumped out to create completely dry working environments have been used. This setup enables McConnell Dowell and the Contech team to anchor a total of 32 prefabricated, 12-strand post-tensioning tendons in the concrete foundations at seabed level.
As each pier is constructed, the CONA CMI internal post-tensioning tendons are cast into the concrete pier elements. A steel plate is then fitted to the top of the concrete piers as part of the base of the steel arch structure. Then, using these steel plates as a bearing surface, the tendons are stressed – thus creating a seamless connection between the arch and foundations. The challenge then is to ensure that the tendons remain bone-dry for 10 weeks in this demanding marine environment, in preparation for post-tensioning. To ensure the integrity of the arch and deck, they’ll be coated with a zinc thermal metal spray with a sealer. An additional colour coat will be applied to the visually exposed arch, with a protective anti-graffiti finish.
The BBR Contech team is thoroughly enjoying being part of this exciting project, which has attracted significant public interest. It’s due for completion in 2022, and will no doubt attract visitors from far and wide – whether they simply want to cross the bridge or spend time appreciating and enjoying the environment.