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)

14 March 2012

London Bridges: 23. West Ham Station Footbridge

Here's another bridge from London's East End, very different to the last one in both form and function.

It's a footbridge again, but part of a railway station interchange. West Ham station was rebuilt as part of the Jubilee Line Extension project in 1999, and connects together two London Underground lines, one overground rail line and the Docklands Light Railway.

The revamp of West Ham was designed by Van Heyningen and Hayward, with structural engineering by WSP. It's in a much more modest architectural style than most of the JLE stations, several of which are gargantuan in scale, gaining much of their architectural cachet from spectacle rather than subtlety. West Ham was reportedly the least expensive of all the JLE stations, but its dignified simplicity is still very attractive. It's a direct descendant of Charles Holden's classic London Underground station designs such as Oakwood.

The station is set out on a 6 metre square grid, and the platform interchange footbridges stick to this plan.

Most of the station canopies and building are cube-like in nature, but the structural needs of the bridge vary this slightly by incorporating truss diagonals. The spans could have been achieved without these, but it allows the main structural members to retain a certain degree of slenderness. The bridge trusses are in reinforced concrete, in keeping with the design vocabulary of the rest of the station, which mixes concrete, brick, and glass blocks.

The bridge is uncluttered, minimalist in style, and the use of glass blocks allows in plenty of light while avoiding the shopping-centre feel that conventional glazing might have have presented. It's unusual to see a reinforced concrete truss bridge in the modern era, but it makes sense in this context. The covered footbridge is always a difficult design challenge, and this is an excellent response to it. Later in this series of posts, I'll feature a very different taken on the covered footbridge problem.

Further information:

08 March 2012

London Bridges: 22. Green Bridge

I recently paid a visit to four interesting bridges in London’s east end, plus a couple more in central London, which I’ll cover over this and the next five posts.

The first is the Green Bridge in Mile End, which spans the A11 Mile End Road close to Mile End underground station. It was built in 2000 as part of a wider urban park scheme. It carries a footpath across the highway and thus provides a key link in a wider pathway running north to south through Mile End Park.

The conventional solution would have been the sort of bland pedestrian bridge which blights so many cityscapes, an identikit steel truss or concrete beam just wide enough to carry a walkway and cycle path. At Mile End, the designers, CZWG Architects and Mott MacDonald, came up with a concept which I think may be unique in the UK. Instead of simply carrying the pathway across the road, the landscape of the park is also continued.

The bridge spans 30 metres and is approximately 24 metres wide, with the path bordered on both sides by large areas of planting. These were being renewed at the time when I visited - it presents a fairly barren scene right now, but if you visit one of the various links at the foot of this post (or go to the designers' websites), you can see it in its more "fully clothed" glory.

From below, an effort has been made to recognise the structure’s status as a land bridge rather than a footbridge – it has a monolithic appearance, as if the soil that it carries had been solidified and merely wrapped to prevent it against weathering. In reality, it is more prosaic, as the glass-reinforced plastic panels which form the underside encapsulate an essentially conventional composite steel girder span.

The bridge is supported on reinforced concrete abutments and foundations, although these are again well disguised with a green-tiled façade which also serves to house a number of small shops. This façade is probably the most visually appealing element of the bridge. The GRP soffit panels have an attractive scale-like appearance arising from the curved layout of the bridge, but otherwise are looking somewhat careworn in places.

It’s an unspectacular structure, which is entirely appropriate to the site and function, and therefore admirable.

Further information:

03 March 2012

Bridges news roundup

In the next week or two, I'll be doing another series of posts on bridges in London, I've got some really good ones to feature. Until then, some news ...

Motril Footbridge, Granada
ArchDaily features a very interesting pedestrian bridge in Motril in Spain (pictured (C) Fernando Alda). From some views, it appears a rational structure, approximating a Warren truss in elevation. It's curved in plan, and the deck and support frames are also triangulated, with every individual truss member constructed from welded steel plate. It's a very heavy design for the relatively short road it spans, but I like its wilful idiosyncrasy.

The Ada Bridge, Serbia
ArchDaily also showcases a very different design, a 950m long cable-stayed highway bridge in Belgrade. At this scale, the scope for eccentricity is limited, and it's a relatively conventional design seeming to be largely determined by the geometry of the riverbanks. The main architectural attention seems to have been lavished on its split-legged conical mast, which is attractive but perhaps a little too stark for me - oddly reminiscent of the puritan monumentalism of the pre-democratic Eastern Europe.

£3m Strabane bridge plan shelved as tenders are too high
Bridge design competition ends in nothing, with one of the main funders, Sustrans, withdrawing from the scheme. The project already looked unlikely to go ahead last November, when only two tenders were received, both 50% over budget. Regular readers of this blog may experience a sense of deja vu, as this has been the outcome of a few other bridge design competitions in the UK. A local man is unhappy about money and time wasted.

The design which won the 2006/7 competition, pictured above left, was by Roughan O'Donovan, and is described in detail online. I can't work out from the picture whether there are any backstays on the upper half of the pylon - can anyone more familiar with the project comment?

Moxon completes 'looping cradle' bridge in St Helens
It's good to see that subtlety is not dead. New Steel Construction magazine has an article about the construction of the bridge, which is pictured on the right.

Victoria’s contentious $77M bridge aims for landmark status
As the ultra-rare Strauss bascule bridge in British Columbia was demolished (more photos here), locals begin to look ahead to its replacement.

New River Hull footbridge, which works like a pinball table flipper, set to open in May
After a great deal of faffing about, this highly unusual bridge is set to open. Then open again. And again, and again. It's an opening bridge, geddit? Sorry.