Special report on rail: Robots keep Birse on track

Steel girders welded using robotic technology are playing a key role in the construction of a bridge on the last remaining two-track section of the West Coast Main Line. Martin Cooper explains.

Enabling trains on the West Coast Main Line (WCML) to achieve really high speeds is one of the most important aspects of the route's on-going £8bn modernisation programme.

To allow trains to reach speeds of 200km/h, a number of bottlenecks will be ironed out with the last remaining two-track sections upgraded to four tracks.

One of the most important elements of the work is the £300m Trent Valley four-tracking project. This 19km-long stretch of the WCML between Armitage and Tamworth is currently being upgraded to include four tracks, which provides one track in each direction for local and high-speed express trains respectively.

The scope of works includes a number of steel bridge replacements, with one of the largest being the River Tame crossing, located 5km north of Tamworth.

Here, main contractor Birse Rail, in conjunction with steelwork subcontractor Fairfield-Mabey, is using robotic technology to help it replace the bridge inside a tight timescale.

Birse has already built one new twin-track steel bridge adjacent to the existing 100-year old structure and is currently replacing this old structure with a second twin-track steel bridge.

The demolished 100-year old crossing was a three-span steel structure with a concrete slab, supported on brick arches. Roger Colton, Network Rail's senior civil engineer, says many of the old bridges on this section don't meet current standards and consequently have to be replaced.

"Trains will be travelling at 200km/h once the whole WCML upgrade has been completed in August 2008, and the Tame bridge wasn't designed for such high speeds," he explains.

The first of the new bridges to be constructed actually sits on the site of a much older bridge, which was the original Tame Crossing. "The recently demolished bridge replaced an older structure from the late 1890s, and we've now made use of these old piers," Colton says.

The new bridge consists of two main girders measuring 3.6m deep x 975mm wide, with a total of 69 cross girders.

The existing two tracks have now been diverted onto the new bridge, allowing for the old bridge to be demolished and replaced by a second identical new structure.

It is this second identical bridge, currently being fabricated for erection this autumn, which is benefiting from the robotic technology.

Fairfield Mabey has installed robot welders at its Chepstow fabrication yard, and says it is now producing bridge girders more cost effectively and at a higher throughput.

The firm specialises in producing bridge steelwork, and the robots are being used to weld stiffeners on girders.

"This is a real first for the steel construction industry, as these robots are directly linked to our CAD 3D modelling system," says Dr Peter Lloyd, Fairfield-Mabey managing director.

This link-up means there are no drawings to be seen on the workshop floor. All processes including profiling, drilling, marking, as well as the robots, are fully integrated into Fairfield-Mabey's computer system.

Within the CAD 3D programme, a virtual model of a steel bridge structure is created, including every bolt and plate. This is then peeled apart into components and the information sent directly to the shop-floor machines.

"Timing is critical on this project," Lloyd adds. "But by using robotic welding, we are able to fabricate the girders faster and have confidence that the finished product will be right first time."

Colton adds: "The steelwork was delivered within our timescale for the first bridge, but the new technology now means we have the added peace of mind knowing the second structure will meet our tight schedule."

The bridge should be completed by January 2007, with Birse finishing other associated structures by the following June.

Project factfile

• Scheme: Four-tracking of WCML near Tamworth
• Client: Network Rail
• Value: £9.6m (whole scheme)
• Main contractor: Birse Rail
• Steelwork contractor: Fairfield Mabey
• Start date: March 2006
• Scheduled completion date (whole scheme): June 2007

[Contract Journal, 6 September 2006, p 32]