12 April 2012

London Bridges: 26. Thameslink Borough Market Viaduct

Okay, that was my trip to East London out of the way, now I'll cover a couple of bridges in central London that I visited on the same day. After that, I've got some bridges over in Kent, and then, who knows, maybe one day I will get back to posting more often. Currently, I don't have time to talk about the results of the Amsterdam Iconic Pedestrian Bridge competition, the recent much-delayed opening of Calatrava's Peace Bridge in Calgary, the only-a-little-delayed opening of Poole's Twin Sails Bridge, or even Knight Architects' spiffy new proposal for a bridge at Paddington Basin (see video) to replace the knackered Helix Bridge. You will just have to make do with those links ...

Anyway, the penultimate bridge from my recent London tour was the new railway viaduct over Borough High Street at London Bridge. This forms part of the massive Thameslink construction programme, which expands capacity for trains on the Thameslink route through central London. At London Bridge, it involves the construction of a new twin-track railway viaduct alongside the existing viaduct, wrecking a conservation area and a number of historically important buildings in the process. The existing viaduct is pictured on the right (looking south).

Most of the viaduct is a relatively conventional half-through girder structure, pictured on the left. It looks rather peculiar mainly because of the large curved panels attached to the main girders, which provide acoustic screening.

The most interesting part of the viaduct spans 72m over Borough High Street. This part of the bridge has been designed as a "gateway" feature, with a 6m deep lenticular truss supporting its southern elevation.

Normally, this type of truss is vertical, but here it is three-dimensional in form, with curved upper and lower chords offset from a main boom which connects to the bridge deck cross-girders. All the truss members are in tubular steel, and the diagonals are tapered in form. I can't think of anything quite like it anywhere else, although there is perhaps a bit of a nod to Brunel's Royal Albert Bridge.

The structural rationale isn't entirely clear, and I do wonder about the advisability of the curved bottom boom, which brings it below the level of the more robust bridge deck slab and hence makes it easily the most vulnerable part of the structure to vehicular impact. It looks like vehicles coming from the north can pass below the existing viaduct before they would hit the new truss structure.

The north edge of the new viaduct is supported from a more conventional plate girder, which is essentially invisible to the public. You can get a better idea of the overall structural form in the picture on the left, showing the bridge from below.

The bridge steelwork weighs about 1200 tonnes, and was launched longitudinally across the roadway. As can be seen in the video below, the bridge was assembled piecemeal on top of the main viaduct, with the main viaduct girders being used as the launching rails for the High Street span.

I find the design of the truss a little odd, and it does seem completely out of scale with its surroundings. Nonetheless, it's a very impressive feat of engineering.

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