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Paddington bears it

Spotlight

Contractor Hochtief, with specialist lifting and jacking firm Dorman Long and steelwork subcontractor Cleveland Bridge, is gearing up for the next phase of a modest-sized but technically challenging bridge launch at London's Paddington station.

Westminster City Council is replacing a hodge podge of 19th century bridges with a new composite six lane structure to remove a traffic bottle neck.

The project is jointly funded by the council, British Airports Authority and Network Rail.

Largest of the original bridges was a steel Parker truss. Straddling a number of main line rail tracks, it was too risky to attempt dismantling this in situ. The challenge of replacing the Parker truss was solved by jacking it into the air using 400t strand jacks on temporary towers. Lifted out of the way, the new bridge could be slid in underneath it. Once completed, the truss could then be lowered onto the new structure and hauled away to the north abutment for demolition.

The new bridge consists of two sections. From the north abutment to the first pier, spanning the Grand Union Canal, it has been constructed insitu using steel beams and concrete. The two spans across the railway are being bridged with a launched structure - the first 60m to a new pier on Paddington's platform 10/11 was launched in May (NCE 12 May). A pause has been needed while the tail of the bridge is added to the nose. Launch phase two, completing the new bridge, will take place next month.

The launched section has been built on two sets of specially designed, heightadjustable trapezoid support frames, mounted on the new canal span. The support frames are equipped with standard Hillman rollers to enable the bridge to be slid forward.

Adjustment of the frames' height is needed to accommodate the changing geometry of the bridge's underside - its longitudinal beams have a variable depth and the structure flares from five to six lanes wide. An additional complication is that the bridge is skewed across the railway at 30°. Frames are controlled by hydraulic rams.

Progress of the launch is controlled by two restraining and two pulling strand jacks of 400t capacity each. The first 60m launch was carried out over six days. The longer launch still to come will take nearly a month. During this second launch, the bridge's geometry demands that the strand jacks must be repositioned twice.

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