16 October 2014

London Bridges: 36. Olympic Park Bridge 20

Right next to London's Olympic Park Bridge 14, you find Bridge 20. Don't ask me how they numbered these things!

Bridge 20 is an 85m span highway bridge, which carries the roadway across two levels of railway tracks, the Channel Tunnel Rail Link and the Docklands Light Railway. Like Bridge 14, it was designed by Arup and Knight Architects, and built by Morgan Sindall.

The steel arch is 18m tall, and sits in between two single-lane carriageways. The tied arch is a fabricated steel box, which varies in cross-section from base to crown in an unusual manner. A weathering steel spine box girder sits below the deck and is suspended from the arch by cables. The deck is supported on either side of the spine girder with weathering steel cantilevers.

The lower end of the hanger cables is protected from vehicular impact by a concrete upstand beam. The edges of the bridge are protected by concrete parapets. Both parapets and upstand beam contain low-level lighting units, eliminating the need for conventional streetlighting.

The strength of the bridge's design is in its simplicity, particularly the use of the single arch, which renders it an immediately legible landmark. The arch is nicely shaped in elevation, being wider at its base than at its crown, but its width is a little disconcerting - it is wider at the crown than at the base. From some angles this gives it a top-heavy appearance, and it doesn't seem to make structural sense to me, as resistance to lateral buckling of the arch is to a large extent determined by the transverse fixity at the ends.

The cantilevers supporting the deck from below are another detail which is not entirely happy. They make use of the full depth of the spine beam, meaning they are stronger and stiffer than the shallower cantilevers more normally seen. This allows large parts of their material to be cut away, but also has the effect of making these essentially secondary elements visually quite prominent.

Perhaps this doesn't matter, as the underside of the bridge cannot be seen from most public vantage points. Although my photos show a small road passing below, this is a private access road behind a secure fence. Perhaps future developments in the area may render this part of the bridge more visible.

The issue of perspective is significant. To photograph the underside of the bridge I had to walk down a dead-end road and peer through a security fence. To photograph the bridge in elevation, I had to peer over a bridge parapet which clearly was not meant as a vantage point - there was no footway in front of it!

Along with the other interesting bridges to emerge from the Olympic park, Bridge 20 can't disguise the essential issue with the whole area, which is that it is criss-crossed with waterways and railway lines, both of which render significant parcels of land unreachable. The bridges are essential to make connections across this landscape, but the focus on their external appearance belies the fact that they are principally viewed by the people actually crossing each bridge. With this in mind, both Bridge 20 and Bridge 14 present a somewhat sterile, industrial appearance to their users, almost brutalist in their treatment of the streetscape.

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1 comment:

Imre said...

As opposed to the bridge presented yesterday, I really like the weathering steel parts of this bridge, especially the deck cantilevers. The whole bottom side - being protected from runoff water by the deck itself - makes good contrast with the light colour of the deck concrete. Also, I find the shape of the cantilevers nice - it shows that the designers did pay attention to these elements, as opposed to cases where they are really treated as "secondary" structural members.