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A record breaking and iconic bascule bridge is being constructed in Lowestoft to ease congestion on local roads and strategic roads.
The Suffolk coastal town of Lowestoft is split in two by Lake Lothing, a salt-water lake which forms the town’s inner harbour. Two bridges connect the two sides of the lake but a third is now under construction to ease congestion.
Motorists travelling across the town currently have a choice of taking the Bascule Bridge on the eastern side of the town or Mutford Bridge on the western side. Both are double leaf bascule bridges – bridges whose central span is split into two sections that can be raised to allow shipping to pass through.
Their low height above water level means that they have to open frequently, leading to congestion on the approach roads.
The new bridge is a single leaf rolling bascule bridge under construction between the existing ones, as part of a £146M project. It is expected to ease congestion on local and strategic roads.
The new eight span Gull Wing bridge will act as a tactical diversion route for the A12/A47 when the Bascule Bridge is closed. It will cross the East Suffolk railway line and several existing roads and will be longer than the town’s other two bridges at 342m.
The bridge has a 148m long north approach viaduct which has three spans, the northernmost of which crosses the railway. The lifting bascule section is 39.5m long and the four-span south approach viaduct is 154m long. There will be two piers in the water and five on land. The bridge will have a maximum width of 22m and will clear the water below by 12m.
“Many vessels will be able to pass under the Gull Wing bridge without the bridge needing to open, making it a more reliable vehicle crossing point,” says Suffolk County Council leader Matthew Hicks.
For a rolling bascule bridge, this is certainly getting to the limits of what is achievable with hydraulic cylinders
Traffic modelling showed that a new crossing could reduce traffic on existing ones by 50%. Hicks says that this will “enable further regeneration and improvements to Lowestoft town and the surrounding road network”.
Arup worked on the bridge design with Bam Nuttall, which was awarded the early contractor involvement contract by the Suffolk County Council in 2018. They used input provided to the council by Design Council Cabe, a design review provider for nationally significant infrastructure projects.
The Gull Wing bridge will have two vehicle lanes plus a shared use footway and a cycleway on each side. The bridge scheme includes building the southern access road bridge and two new roundabouts, plus 130 utility diversions.
The Department for Transport contributed £73.4M to the project, while Suffolk County Council provided £72.6M. The council and Bam Nuttall failed to agree on the terms of the main construction contract, which includes the design of the bridge’s mechanical elements. As a result Farrans was appointed main contractor in December 2020. Construction started in April 2021.
Complex design
“The council wanted a structure that was inspiring and one where the local community could see how it works,” says Arup project manager Conor Lavery.
This was achieved by opting for a rolling bascule bridge rather than a “traditional” one. Lavery explains that the counterweight of a traditional bascule bridge is under the deck, while a rolling bascule’s counterweight is above deck level, allowing people to see how it operates.
Lavery claims that Gull Wing will be “the largest rolling bascule bridge in the world lifted using hydraulic cylinders”.
Its 386t counterweight is made of concrete and is contained in twin steel structures – known as J-beams – beside the roadway. These structures rotate to lie parallel with the deck when the bridge is open and stick upwards when it is closed.
The shape of these structures was the inspiration for the bridge’s name, as the primary school pupils who won the naming competition said they resemble the wings of seagulls.
Lavery says that unlike other rolling bascule bridges that roll back on large reinforced concrete abutment structures, this bridge rolls back onto the approach span.
There will be two piers in the water and five on land.
Approach span structure
“The approach span structure consists of deep steel box girders which are designed to support the weight of the rolling bridge during operation and span 52.5m between the bridge piers,” he adds.
The bridge’s hydraulic rams are contained within the pier supporting the bascule section, while the hydraulic pumps will be housed in a plant room on the south quay. The rams raise the deck by pushing upwards against it.
The opening and closing of the bridge is helped by curved tracks known as racks attached to the
curved undersides of each of the longitudinal J-beams. These mesh with flat racks cast into the deck to help to keep the bascule in line when it is raised and lowered.
Farrans project director Neil Rogers explains: “When the hydraulic rams are extended the racks interlock and the bascule bridge rotates on the racks”.
When the bascule span is fully open, the tip of the bridge deck will be almost 62m above water. The longer you make the span, the bigger the technical challenge, says Lavery.
“For a rolling bascule bridge, this is certainly getting to the limits of what is achievable with hydraulic cylinders,” he highlights.
Adding to the complexity was the need to accommodate the client’s demand for a rapid opening and closing time. The bridge has been designed to open within 116 seconds and close in 106 seconds, 10 times a day, 365 days a year for its 120 year design life.
Fast progress
Construction has been progressing at a rapid pace too, with utility diversions and the new access road on the south end already completed.
Bridge construction involves on and off site work. Belgian steel fabricator Victor Buyck Steel Construction is supplying the steel structures for the spans. These are transported to site from Belgium by barge before Farrans attaches the reinforced concrete decks and they are lifted into place.
The first steel structure was delivered in February. This was for the 55m long section that will cross the railway. This structure was expected to be installed soon after this issue of NCE went to press, during a two day closure of the East Suffolk line.
While there is more than a year of work left to ready the bridge for opening, the team seems confident of the programme ahead.
Economic benefits
Rogers says that the better connectivity offered by the bridge to the area south of the lake could lead to more people and businesses moving there.
The local community and economy are already benefiting from the construction of the bridge. Hicks says that to date more than 1,400 local students and schoolchildren have been involved in science, technology, engineering and mathematics-related presentations and activities, while 71 local contractors and suppliers are involved in the construction programme.
Between them Farrans and Suffolk County Council have employed local graduates, apprentices and long-term unemployed people. Rogers says that half of the people working currently on site are locals.
“These benefits will continue to grow during the remainder of the bridge construction and will provide a lasting legacy to the local area,” says Hicks.
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