26 June 2011

London Bridges: 10. Chelsea Bridge Wharf Link Bridge

Nestling just below the south side of Chelsea Bridge, there's a modern walkway which carries the Thames Path past the bridge and along this bank of the river (although not very far along - the Path is currently blocked in front of Battersea Power Station by construction works).

This structure was installed in 2004. It was fabricated by Littlehampton Welding, and erected by being floated along the river. The designer was Whitbybird, now part of Ramboll. The structure is about 4m wide, 65m long, and cost £650,000.

The walkway structure cantilevers out from the bank either side of the Chelsea Bridge's abutment, assisted by two extremely slender supports founded on the river bed. The structure is designed to resist uplift in flood conditions, and so that it will not collapse even if one of the two supports is destroyed by boat impact.

The walkway itself consists of a shallow steel box girder, sufficiently shallow that vibration was readily perceptible on the structure even with only one or two people using it when I visited it.

There are holes in the edge of the deck, which puzzled me when I visited the bridge, but which I gather allow concealed lighting behind to "punctuate" the line of the bridge at night.

The parapets use alternately slanting posts in a sort of "V" configuration, which I have seen on other Whitbybird / Ramboll designs, both built and planned. Presumably this adds sufficient longitudinal stiffness to justify using more slender posts than would be possible if they were vertical.

The parapets have stainless steel hand and bumper rails, and the main parapet infill comprises tensioned wires drawn through the parapet posts. There is a clear rust spot at every intersection, which may be attributable to poor fabrication (lack of paint penetration into the hole), design detailing (how could paint ever fill the hole?), or even to bimetallic corrosion (lack of an insert to prevent contact between dissimilar metals). Whichever is the cause, it will be awkward to rectify.

There's clearly no engineering reason why the walkway has to cantilever so far from the Chelsea Bridge's abutment - this seems to me a conscious design choice to facilitate the more attractive curved alignment and create a visual distance which respects the gap in age of the structures. It also minimises the sense of it being a "pedestrian subway" on a structure in close proximity to the dark underside of the bridge above, by establishing open space around the walkway rather than clinging to a wall.

This is no major footbridge, but a positive example of simple, well-organised design appropriate to its location and pleasingly lightweight in conception.

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