06 June 2017

London Bridges: 47. Ruckholt Road Footbridge

Here's another bridge built for London's 2012 Olympics, just a little further north-east along the same road. Footbridge L01 crosses above Ruckholt Road, while this bridge sits alongside it.

Ruckholt Road Footbridge is a sibling of Olympic Park Bridge 1, and it's interesting to note the similarities and differences. Both were designed by Knight Architects, although working with different engineers in each case.

Both siblings solve a common problem: to provide additional pedestrian/cyclist capacity alongside an existing highway bridge which is severely constrained in width, while spanning over a railway line. Not just any railway line, but part of the Channel Tunnel Rail Link, necessitating super-high barriers so that drunken parapet hurdlers cannot access the railway (although pole vaulters still could).

In both cases, the solution is a half-through plate girder bridge which takes as a starting point the conventional Network Rail footbridge design. In the NR design, the bridge is essentially a rib-stiffened trough, with the side panels serving as both girder webs and parapet screens, while the bottom of the trough acts as both a floor plate and as the bottom flange for the girders. Vertical stiffeners act as u-frames encompassing all three sides of the trough, to prevent the top flange from buckling under bending compression. A happy side-effect of this design is that there is no projecting bottom flange, discouraging vandals and trespassers from accessing the external elevations of the bridge.

Both Bridge 1 and Ruckholt Road designs are in weathering steel (along with several other Olympic Park bridges), reducing the need for future maintenance over what are clearly major railway lines. However, they are painted on the inside faces, where leaving the weathering steel exposed would leave it prone to damage from graffiti.

To achieve the height of screening specified by the high-speed railway authority, both sibling bridges have elevated grille screens above the level of the main girders. This creates an unpleasant tunnel effect for users, so the screens are inclined outwards along with the girders, to open up the view and make using the bridge slightly less uncomfortable.

Olympic Park Bridge 1 is a single-span bridge, and a very good design overall. However, I think that Ruckholt Road Footbridge is less successful.

The structure has two spans, a longer span over the railway with a shorter back-span over a minor road. The bridge girders have been shaped to be deeper over the main span, which I find peculiar, as a structural engineer would expect the girders to be deepest over the central support pier (where bending forces are greatest). The girder shape seems awkward when seen from the outside, although less so from the footway.

The external lines of the steelwork are clean, with the stiffening ribs looking sharp and giving the bridge an interesting visual character.

The upper parapet screens are more "tacked-on" than on Bridge 1, where their geometry was varied in an interesting manner. At Ruckholt Road, they are mostly above head height, and it's hard not to feel a little confined, even with the way the arrangement opens up towards the sky.

There are little oddities, such as the fact that the inner edge of the top flange is left unpainted, resulting in the inevitable rust staining, although this is a pretty minor impairment compared to the graffiti that adorns the girder webs.

However, the oddest thing about this bridge, for me, is to wonder why the tall screens are really needed. Here's what the existing road bridge looks like, which you can see does have footways on both sides. No effort has been made to raise the height of the parapets on this bridge, although it passes over the same railway. It's difficult therefore, to feel that the railway authority's requirements for tall parapets were genuinely necessary.

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1 comment:

Marcus Schodorf said...

Agreed that the height of the parapets is ridiculous. I did the AIP for Temple Mills Ln bridge, which crosses the same railway, and I believe the extra height (over and above standard NR standards) was due to the line being 'high speed'. This was also ridiculous because it is just a spur line to the Eurostar depot and the trains wouldn't be travelling very fast. The adjacent bridge wasn't modified, so no need to update the out of date parapets.