23 March 2012

London Bridges: 24. Stratford Town Centre Link

From the Green Bridge at Mile End Park, it is a very short hop north to Stratford, and to one of the many new bridges installed to improve transport links for the London 2012 Olympics. This bridge is largely independent of the Olympic developments, as it also serves to provide access between Stratford’s town centre and the gargantuan new Westfield shopping centre.

As such, it spans an unusually large number of railway tracks, and has capacity for crowds of up to 50,000 people per hour during the Olympics. This makes it a very unusual footbridge, and it must certainly be one of the heaviest pedestrian bridges I have ever seen.

The bridge was designed by KnightArchitects and Buro Happold, and built by Morgan Est with steelwork by Watson Steel.

The bridge is 135m long in total, 12m wide and curved in plan. To keep the pedestrian space completely clear of obstacles (other than occasional stone benches), it is supported by a 6.5m deep steel truss on each side.

These are Vierendeel in form i.e. non-triangulated, a structurally inefficient solution normally encountered on much smaller and lighter footbridges. Unlike an equivalent triangulated truss, the main members must be designed for considerable local bending moments, and high local shear forces are present within each of the connecting nodes. Additional steel is also required to provide sufficient overall stiffness. Nonetheless, it’s easy to see the attraction at this site, as it fits well with the full-height structural glass panels which provide containment on each side.

Bridges over railways in the UK often present something of an aesthetic challenge, as Network Rail and their predecessor organisations insist on parapets which are solid, and normally at least 1.8m high, especially at a site such as this where the railway below has overhead electrification. I know from my own experience as a designer that Network Rail can be fiendishly conservative, and persuading them to accept the use of glass is not always easy, although it is used on a number of other footbridges spanning rail tracks. At Stratford, the full height glazing is a great success, I think.

The second most obvious characteristic of the bridge is that it is constructed entirely in weathering steel (often known by the Cor-Ten tradename). This is a highly appropriate solution over a railway, as it should require much less maintenance than a painted alternative, particularly important given the number of tracks crossed. While weathering steel can be very vulnerable to graffiti, it is protected on its inner face by the glazing and on the outer face by its height above ground.

Weathering steel can also be highly vulnerable to trapped debris and moisture, and I would guess that the top and bottom chords of the truss have been given angled faces principally to reduce this risk. However, the upper face of the bottom chord is horizontal, and I can still see some potential for rainwater not to drain freely.

The final feature of the bridge which is of particular interest is the shaping of the truss on one edge. This truss runs along the inside of the bridge’s curve. Both truss are not truly curved, but faceted, as may be apparent in some of the photographs, but I don’t think this detracts from the appearance, indeed it is in keeping with the “industrial” character of the bridge. The inner truss is not vertical, except at its end supports. Over the central support, it is inclined outwards from the deck, with the effect that the truss top chord appears to be taking something of a shortcut.

Tilting the truss in this manner is a pretty bold choice but it has two clear advantages. One is visual – it reduces the tunnel-like nature of the high-walled trough which the bridge creates, by slightly opening up views around the bend. The second benefit is structural – the axial force in the truss top chord is tensile at the middle of the bridge (where it is in hogging over the central support), and therefore tends to pull the chord sideways, away from the bridge. Tilting the chord outwards must significantly reduce this effect, and hence reduce the forces applied to the vertical truss members which resist it. I suspect that the steel sections are heavy enough for other reasons that the saving in structural weight and cost is negligible, but it’s always nice to have a design feature which can be justified in more than one way.

Although I've focussed on the aesthetics, much of the overall concept was driven by the engineering. It was a highly constrained site, with limited access above the railway tracks, and a bridge suitable for launching from one end was a highly appropriate solution. The design has taken the demands of practicality (deep trusses) and still managed to create a bridge which is attractive and visually appropriate.

Further information:
  • Google maps / Bing maps
  • Structurae
  • Exploring Issues of Aesthetics in a Railway Environment: The Infrastructure of Stratford City, London (Knight & Wilson, IABSE Symposium Venice, 2010)
  • Combining Engineering and Aesthetics: The Town Centre Link, London (Knight & Fryer, Footbridge 2011)

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