Mabey Bridge Ltd
Balfour Beatty-Carillion Joint Venture
Transport for London
The new East London Line is an overground line which runs north to south through the East End, Dockland and South areas of London. In December 2007 the line was closed for an extensive expansion project to take place which would include the overground network. It was forecast that 35.4 million passengers per year, an increase from 10.4 million, would use the line following completion of phase one of the extension plan.
The contract included new stations and large truss bridges and was scheduled to be opened in June 2010. Part of the contract was to fabricate and erect a £5.9 million package which included the bridge and approach span at New Cross Gate.
The flyover, north of New Cross Gate with a total of 690 tonnes of steel, allows trains to run through to West Croydon crossing over the Network Rail London to Brighton Line. Carrying a single rail line, 75m long and 8m deep, the Warren Truss structure comprises eight longitudinal main girder sections, each 20m long and weighing between 20 and 25 tonnes and 37 crossbeams 10m long and each weighing three tonnes. The side trusses were formed by the main girder sections: 36 diagonal beams six metres long and each weighing between two and four tonnes and 12 top members all varying in length and weight.
A close relationship was formed between the steelwork contractor and the main contractor to achieve the most cost effective solution, putting forward value engineering alternatives which would improve the scheme and ensure the best outcome. For the approach span, it was recommended that 8mm leg length fillets be used instead of 12mm fillets, offering a substantial saving to the fabrication price.
As an alternative to the erection method for the main span it was suggested that a very large crane could be used to lift the pre-assembled deck into place. It was concluded however, that the original tender option would be used due to the lift weights and available space for a crane platform.
All the steel was approved to BS EN 10025:2004 standards as per specification. The bottom chords and associated cross girders were provided from weathering steel and the remainder of the structure was painted with Network Rail N4 Paint System.
The bridge steelwork was trial erected at the steelwork contractor’s fabrication facility and the deck crossbeams were machined to length ensuring the correct fit within the end plate connections. The steelwork was delivered to the site and, with a planned closure of the busy line at New Cross Gate on 10 May 2008, a pre-possession assembly took place adjacent to the tracks. The steelwork forming the deck and the top chord of the truss was assembled and the whole structure fully bolted. The deck was cast and once it had achieved sufficient strength, the structure was jacked up approximately six metres high to enable a self propelled modular transporter (SPMT) to be positioned underneath.
In order to transport the structure slowly across the four lines it was necessary to cover the lines with a track protection. Once in place the structure was rolled across the tracks and lowered into position over the bridge abutments.
The extension is expected to play a major role in transporting people to and from the 2012 Olympic Village.
All the challenges of constructing a large bridge over a key rail route, with numerous site constraints and to a very tight timetable, were successfully addressed here. The team and client developed a Warren truss design, with the structure and deck being driven and jacked into place.
The judges were impressed by this solution to a very difficult logistical problem.