Protecting Marine Environments
THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS
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THE BROADMEADOW bridge in eastern Ireland crosses a shallow marine estuary.
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THE SIXTEEN PIERS that support the twin spans of the bridge are designed to last 120 years.
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THIS ECOLOGICALLY SENSITIVE tidal zone will not have to be disturbed to facilitate maintenance work on
the bridge.
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THE 313-METRE-LONG concrete spans contain about 169 tonnes of S31600 stainless steel.
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STAINLESS STEEL REBAR added less than 3% to the total cost of the bridge.
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EACH PIER HAS a circular skeleton fashioned from rebar in diameters of 25, 32 and 40 millimetres (mm) for
the upright strands, and 12 and 16 mm for the hoops, or shears.
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STAINLESS STEEL REBAR is now required to attach parapets to all new bridges in Ireland.
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ONCE CONNECTED, a strip of concrete is poured to encase the bars and fuse the components.
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OVER THE NEXT 10 YEARS highway building in Ireland will use a significant amount of S31600 stainless
steel.
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Español
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Ireland uses stainless steel reinforced bridges to span sensitive marine estuary.
By Dean Jobb
Nickel Magazine, March 2005 -- Think of the time and money to be saved if a bridge spanning a saltwater estuary were to require no maintenance for, say, 120 years. No need to break into the concrete piers to replace rusted rebar, no traffic tie-ups while road crews undertake repairs.
Dublin-based Arup Consulting Engineers not only envisioned such a trouble-free bridge; they designed and built it using stainless steel rebar. The twin spans of the Broadmeadow Bridge in eastern Ireland, part of a motorway that links Dublin and Belfast, opened to traffic in June 2003.
"We had an aggressive environment – salt water, wetting and drying – where future access for maintenance is very, very difficult," says Troy Burton, Arup’s associate director and the principal design engineer for the bridge. "We wanted to guarantee a 120-year design life ... and we needed to convince our client that we had a durable solution that would cost little money in the future to maintain."
The solution was to use stainless S31600 rebar to reinforce all 16 piers that carry the 313-metre bridges across the estuary.
Using stainless rebar was a first for Arup. "It pretty well ticked all the boxes in terms of a permanent, durable solution," Burton says.
In all, 169 tonnes of stainless were used. Most of it went into the piers, which have circular skeletons fashioned from rebar in diameters of 25, 32 and 40 millimetres (mm) for the upright strands, and 12 and 16 mm for the hoops, or shears. All but the 32-mm rods are stock items, but Arup built in plenty of lead-time, ensuring the larger gauge was available when needed and the bridge was completed ahead of schedule.
Burton says using stainless rebar added less than three per cent to the approximate 12-million-Euro cost of building the bridge – a negligible expense, given the savings in maintenance and repairs over its lifetime. It is difficult to reach the Broadmeadow Bridge’s piers without damaging the ecologically sensitive mudflats, making it essential that the structure not require maintenance.
Besides, Burton added, "bridges get forgotten about, and maintenance budgets are a fair way down the list of priorities. You can’t guarantee that maintenance is going to happen."
Carbon steel was used in the pilings driven into the riverbed and in other parts of the structure where corrosion was not seen as a problem. An exception is the 16-mm, S31600-grade rebar protruding from the precast parapet sections and the bridge deck. Once connected, a strip of concrete was poured to encase the bars and fuse the components.
Here, stainless is again essential. The edge of the parapet is vulnerable to corrosion from road salt and, once weakened, may fail if a car or truck strikes the side of the bridge.
The Broadmeadow Bridge inspired Ireland’s National Road Authority to mandate the use of stainless steel to attach parapets to all new bridges. When a 35-kilometre stretch of motorway across the Irish Midlands, from Kinnegad to Kilcock, opens at the end of the year, the metal will be incorporated into all 32 of its bridges and overpasses.
Arup designed those bridges, which Burton estimates contain in excess of 220 tonnes of stainless steel rebar. With Ireland embarking on a 10-year highway-building program, Burton foresees a strong demand for the product.
Dean Jobb is a Wolfville, Nova Scotia-based freelance writer.
PHOTOS: Arup Consulting Engineers
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