A thrust arch structure, the new Walton Bridge across the River Thames between Walton-on-Thames and Shepperton, in Surrey, United Kingdom, will replace two temporary bridges. Atkins
The sixth bridge to be constructed between Walton-on-Thames and Shepperton, in Surrey, United Kingdom, is designed to stand the test of time.
September 24, 2013—Since 1750, when the first bridge over the river Thames between Walton-on-Thames and Shepperton, in the county of Surrey, United Kingdom, was constructed, maintaining a crossing at the site has posed significant challenges. Five bridges have been erected and demolished there. But with the construction of the latest bridge, which is formally named the Walton Bridge, the two towns will finally have a crossing that they can rely on for decades to come.
The first bridge at the site was made of timber and by 1783 had deteriorated beyond repair. As a result, it was replaced with a masonry arch structure, but that crossing washed away in 1859 and was replaced with a Victorian-style bridge of wrought iron. The iron bridge stood strong until it was damaged by bombing during World War II, rendering it unfit for vehicles. It became a pedestrian crossing, and in 1953 a temporary bridge, of the Callender–Hamilton type, was erected alongside it for vehicles. But being made of steel, the vehicular bridge slowly rusted, resulting in significant weight restrictions. In 1999 the Victorian iron bridge was demolished, the Callender–Hamilton bridge was converted for pedestrian use, and a new temporary bridge was erected for vehicles. Now, more than 260 years after the first bridge was erected, a permanent bridge has opened approximately 5 m upstream of the two temporary bridges, both of which will soon be demolished.
The Surrey County Council issued a tender for the new bridge project in 2004. In addition to the bridge, the project includes a new viaduct that crosses the adjacent floodplain, a new roundabout, a modified junction, and new pedestrian and equestrian paths that serve the park along the riverbank in Walton-on-Thames referred to locally as Walton Beach. As a result of the tender process, the council appointed Costain, an engineering and construction firm based in Maidenhead, United Kingdom, as the primary contractor. Atkins, a global design, engineering, and project management firm headquartered in Epsom, Surrey, is the structural engineer for the project.
The council considered several bridge types, including cable-stayed and conventional girder bridges, but ultimately deemed that a thrust arch bridge would be the most appropriate for the site. “Driving all of the options was the need for a clear span over the river because the Environment Agency, which looks after the river, did not want any new piers to be placed in the river,” explains Rob Wheatley, the chief engineer for Atkins’s highways and transportation department. “That will improve the navigation and the views on that section of the river.”
While it didn’t take long to determine the bridge type, advancing the project took years. In 2006 a public inquiry process revealed strong opposition from residents who were concerned that the project would negatively affect Walton Beach. As a result, the project team was asked to reconfigure the scheme to better preserve the popular recreational area. The team presented an updated planning application the following year, and in 2009 a new public inquiry process approved the scheme. By that time new government leadership had conducted a comprehensive funding review, and the project was stalled until December 2010, when it finally received all of the appropriate approvals. Construction commenced in January 2011 with site work, including the relocation of an oil pipeline. “The challenge was for the team to keep together and keep the focus on getting the scheme sorted out to resolve all of these issues,” Wheatley says.
Despite those hurdles, the Walton Bridge opened in July. Designed in accordance with European standards known as Eurocodes, the 148 m long bridge has five spans: a 96 m long main span and two smaller spans on each side. Its deck is sufficiently wide to accommodate two traffic lanes at a total width of 7.3 m, as well as two 3.5 m wide sidewalks. It carries Walton Bridge Road, officially known as the A244, which continues toward Heathrow Airport on the Shepperton side. The bridge is one of five to cross the Thames in Surrey, but since two of them are subject to weight restrictions, this bridge will be especially vital for commerce. “There’s a lot of local traffic going to Heathrow Airport, particularly for employment,” Wheatley says. “So it’s quite important from that respect.”
The bridge is founded on large concrete pad foundations that are 20 m long and 25 m wide and extend 2 m deep to Thames gravel. “We had to resist a lot of the lateral thrust, so we needed a massive foundation,” Wheatley says. “We supplemented the concrete foundation by incorporating temporary sheet piles into the permanent structural design to give it a little more lateral resistance. We tied the sheet piles into the foundation to increase the shear resistance.”
The foundations are capped by springing points, from which the bridge’s two steel arch ribs rise to a height of 14.7 m. Considering that the span is 96 m, the span-to-rise ratio is 6.5, resulting in quite a slender arch, Wheatley says. The arches incline 5 degrees toward the center of the bridge and are hexagonal to impart visual interest. “The client was very clear in his mind that he wanted the arches made in a hexagonal section rather than a circular section or diamond section or square section,” Wheatley says. “It’s slightly more expensive, but it does give an interesting appearance.”
The arches arrived at the site in segments and were assembled using a temporary tower. “We built a temporary tower in the middle of the river, lifted the large sections, and adjusted and stressed the arch sections down to the foundations,” Wheatley explains. “Then we welded the section at the top, and the tower was removed.”
Ten hangers made of high-grade steel extend from each arch to a relatively thin (850 mm) superstructure that comprises 600 mm deep cross-girders and a 250 mm thick concrete deck. “There are two main longitudinal girders, one on each edge, and then a series of transverse girders with regular centers,” Wheatley says. “Every other transverse girder has short outriggers that connect to a hanger.” The main girders are J shaped to conceal the bottom of the flange and streamline the bridge’s appearance. “That is entirely for aesthetic purposes,” Wheatley says. “It’s less efficient, but it’s much better for appearance.”
In addition to making the bridge aesthetically pleasing to residents, the designers had to ensure that the structure would be sufficiently visible to the many swans that frequent the site and the swan sanctuary upstream. “Swans are not very maneuverable creatures, unfortunately, so we had to paint the bridge a light color to help them see the bridge and avoid colliding with it,” Wheatley explains. “That was a little bit of a different design aspect.”
The bridge project now complete, the residents of Walton-on-Thames and Shepperton finally have a safe, reliable permanent crossing. “They have a structure now that will last them for some time,” says Wheatley, who hopes that the bridge will create new economic opportunities in the two towns.