London Bridges: 38. Chobham Academy Footbridge

It's time to finish off my round-up of bridges from London's Olympic Park. There are a couple of interesting bridges I've not visited yet, so hopefully I'll follow these posts up on a later occasion.

Chobham Academy is a brand new school built as part of the Olympic "legacy" developments, to take advantage of the infrastructure left behind from the Olympic games. The school is separated from its playing fields by a road, Temple Mills Lane, and a footbridge has been built to save the students from dodging cars on their way to and from the playing fields. It's difficult to escape the notion that a pedestrian crossing would have cost a lot less, but hey, we get a bridge to look at, so never mind.

The bridge has been designed by architect Allford Hall Monagham Morris, with engineers AKT II, and as with many Olympic Park bridges, is built in weathering steel. Like the nearby Bridge 1, it uses the common footbridge form of a half-through arrangement, where the two edge girders form the parapets and the floor plate forms the bottom flange. This minimises the height of approach gradients.

As with the standard Network Rail rural footbridge design, to which this bears a clear family resemblance, stiffening U-frames wrap around the floor and webs, restraining the web and top flange against buckling.

The bridge trough is supported on two main supports, each a tetradactyl arrangement of weathering steel fingers or branches, on which the trough is perched. The "finger piers" are in turn supported on chiselled concrete plinths.

The trough varies in height, being tallest at the middle of the centre span. This is a decidedly odd arrangement, as it makes the bridge strongest at what must be one of the most lightly stressed places: as a continuous bridge, it should be most heavily stressed above the pier supports.

The most significant design feature on the bridge is that the u-frame stiffeners have had their spacing varied, concertina fashion, ostensibly so that the ribs are bunched most closely together in areas of highest shear stress. The effect appears grotesquely exaggerated from most viewpoints, and has certainly been played for effect, as the spacing of the stiffeners becomes much closer together than can provide any real structural benefit. I guess the u-frames are also doing very little to stabilise the top flange, which will be in tension over all or most of the span.

Whatever the rationale, I find the effect highly visually uncomfortable. I guess it has been inspired by the other stiffened-girder Olympic Park bridges, but it has been done far more crudely.

Although the deck is clumsy, I do really like the finger piers. The shaping of the weathering steel branches is excellent, and very well integrated with the support plinths. It makes me think of Constructivist art, or perhaps a skeletal Richard Serra.

It's interesting to see the ageing process on the bridge's weathering steel. The weathering to the two sides is different, presumably because of the wind or sun, or a combination, and the weathering to the underside is very different to the side faces, presumably due to the shelter and lack of moisture running along the underside. Various parts seen close up show "streaking" due to where rainwater has run, although I am sure this will vanish over time.

As with the other weathering steel bridges in the Olympic park area, it will be interesting to visit again in years to come and see how their appearance has changed.

Further information:

• Google maps / Bing maps
• AKT II
• AHMM
• Timelapse construction video