London Bridges: 27. Bridge of Aspiration

This post brings to a close my recent series of posts featuring bridges in London. Sadly, unlike the Plashet School Footbridge, I couldn't get permission to visit the interior of this one.

The Bridge of Aspiration links the Royal Ballet School with the Royal Opera House, spanning over Floral Street in Covent Garden in London.

It was designed by Wilkinson Eyre with Flint and Neill, and is admirably simple in its conception. A square cross-section rotates through 90 degrees from one end to the other, creating a fusilli-like twisted form. The two end portals are not directly opposite each other, and as can be seen from below, the bridge's geometry provides a rational way of dealing with the requirements of a skewed crossing.

Structurally, the primary element is an aluminium box girder, which helps reduce the weight and hence the loads on the supporting buildings. The girder twists in such a way that it is shallowest at its ends (rectangular in cross-section), and deepest at midspan (triangular), as befits the bending moment diagram.

The girder supports a series of square frames, with glazing between them, each of which is rotated approximately 4 degrees relative to its neighbour.

It's a very special bridge, both in how it responds to the geometric needs of the site, and in how the structural form combines the rational with the idiosyncratic. I expect that it's essentially unrepeatable, difficult to imagine it being re-created anywhere else. Judging from the various interior photos I've seen, my exterior photos fail to capture most of what makes it so charming, but even from this perspective, it's a very fine bridge.

Further information:

• Google maps / Bing maps
• Structurae
• Speirs and Major
• New Materials for Modern Footbridges (Firth, Footbridge 2002)
• The Bridge of Aspiration, Covent Garden, UK (Firth, Structural Engineering International, 2006)
• Footbridges (Schlaich and Baus, 2008)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)

Two bridge design competitions

Two bridge design competitions are in the news in recent days.

First, the serious one, a contest to design a replacement for the seismically vulnerable 6th Street Bridge in Los Angeles (pictured, courtesy of orngejuglr on flickr). This will be a major scheme, with a budget of US$400m available, and it's likely to attract some heavyweight designers. The LA city mayor is seeking an iconic structure to replace the existing landmark bridge.

Previous studies for the project have narrowed options down to a cable-stayed or extradosed bridge, and it appears that may be the only solution permitted in the contest, which should make it an interesting challenge.

Firms which are selected for the contest will each receive a US$50k payment to develop their designs, leading to selection of a winner later this year. A formal invitation to prequalify is expected any day now from the LA Bureau of Engineering.

The other contest just announced is for the design of a "New Contemporary Bridge" over the River Seine in Paris. This one is open to all contestants, well, nearly all - you must be an architect or have an architect involved somewhere in your team, and be aged between 18 and 45. Woe betide elderly bridge engineers, although I guess you could just don a black polo-neck and play pretend - there will certainly be a host of trainee architects pretending they know something about building bridges.

There is a registration fee to enter, and prizes on offer of US$1k for each of twelve winners at Stage One, with a first place prize of US$10k at Stage Two.

This isn't a real bridge, there's no client involved and no plans to actually build anything. However, I'd guess the prize money will be highly attractive to students and anyone else with a bit of spare time on their hands. Contest entries must be in by August 24th.

London Bridges: 26. Thameslink Borough Market Viaduct

Okay, that was my trip to East London out of the way, now I'll cover a couple of bridges in central London that I visited on the same day. After that, I've got some bridges over in Kent, and then, who knows, maybe one day I will get back to posting more often. Currently, I don't have time to talk about the results of the Amsterdam Iconic Pedestrian Bridge competition, the recent much-delayed opening of Calatrava's Peace Bridge in Calgary, the only-a-little-delayed opening of Poole's Twin Sails Bridge, or even Knight Architects' spiffy new proposal for a bridge at Paddington Basin (see video) to replace the knackered Helix Bridge. You will just have to make do with those links ...

Anyway, the penultimate bridge from my recent London tour was the new railway viaduct over Borough High Street at London Bridge. This forms part of the massive Thameslink construction programme, which expands capacity for trains on the Thameslink route through central London. At London Bridge, it involves the construction of a new twin-track railway viaduct alongside the existing viaduct, wrecking a conservation area and a number of historically important buildings in the process. The existing viaduct is pictured on the right (looking south).

Most of the viaduct is a relatively conventional half-through girder structure, pictured on the left. It looks rather peculiar mainly because of the large curved panels attached to the main girders, which provide acoustic screening.

The most interesting part of the viaduct spans 72m over Borough High Street. This part of the bridge has been designed as a "gateway" feature, with a 6m deep lenticular truss supporting its southern elevation.

Normally, this type of truss is vertical, but here it is three-dimensional in form, with curved upper and lower chords offset from a main boom which connects to the bridge deck cross-girders. All the truss members are in tubular steel, and the diagonals are tapered in form. I can't think of anything quite like it anywhere else, although there is perhaps a bit of a nod to Brunel's Royal Albert Bridge.

The structural rationale isn't entirely clear, and I do wonder about the advisability of the curved bottom boom, which brings it below the level of the more robust bridge deck slab and hence makes it easily the most vulnerable part of the structure to vehicular impact. It looks like vehicles coming from the north can pass below the existing viaduct before they would hit the new truss structure.

The north edge of the new viaduct is supported from a more conventional plate girder, which is essentially invisible to the public. You can get a better idea of the overall structural form in the picture on the left, showing the bridge from below.

The bridge steelwork weighs about 1200 tonnes, and was launched longitudinally across the roadway. As can be seen in the video below, the bridge was assembled piecemeal on top of the main viaduct, with the main viaduct girders being used as the launching rails for the High Street span.

I find the design of the truss a little odd, and it does seem completely out of scale with its surroundings. Nonetheless, it's a very impressive feat of engineering.

Further information:

• Google maps / Bing maps
• Londonist
• Wikipedia
• Watson Steel
• Railway Strategies

London Bridges: 25. Plashet School Footbridge

The fourth in this series of posts regarding East London bridges finally brings us to the bridge which was the reason for my visit. It’s a short distance and a few tube stops east of the Mile End, West Ham and Stratford bridges covered in the last three posts.

There must be few schools in Britain which can boast a footbridge of such architectural distinction as this one. It was built in 2000 and links the two halves of a girls’ school across Plashet Grove, providing traffic-free and weatherproof access between the buildings for the students. It cost £560,000.

The designers were Birds Portchmouth Russum Architects with Techniker, and to my mind this is easily the best bridge that Techniker have designed (I’ve covered some other examples at Royal Victoria Dock, CanaryWharf, and of course in Sunderland).

The footbridge meanders for 67m in an S-curve at first floor level, and the presence of curvature is a recurring motif in the design. Despite the fact that it is fundamentally a hard, metal bridge, comprising a half-through twin girder layout with steel decking, the bridge is overwhelmingly soft in nature, thanks to the use of a fabric membrane roof carried on curved support frames. It looks a little like a series of wagons out of a Western film stuck end to end, and I don’t mean that in a bad way.

The support frames for the roof alternate along the length of the bridge, such that from any perspective, the rhythmic geometry is always interesting.

The ribbed-tunnel effect is punctured at midspan by two angular steel-and-glass belvederes. These are the only windows on the bridge, which is an unconventional approach on any modern covered footbridge. The conventional wisdom for such bridges is that users prefer transparency, but this is a bridge over an urban road – the truth is that there is not much to see, and the belvederes are therefore more relevant as meeting points than as viewing platforms.

The roof membrane is not sealed at its lower edge, with a gap allowing free ventilation. This is covered by a mesh, presumably to keep out pigeons. Concern over pigeons is also apparent from the presence of an anti-perching wire along the outer flange of the main beams, which I suspect was an afterthought. Details like this are a perennial curse of the bridge designer’s life.

The roof covering lets in considerable light. It is somewhat covered in grime at present, but apparently gets cleaned every few years. In general the bridge appears well-maintained and the design to have been robust. I was told on my visit that the original drainage (which ran through a tube forming the upper flange of the main beams) failed, and was replaced by separate drain-pipes which deliver water from the roof directly to the roadway below. Drainage is another detail which can challenge even the most conscientious bridge designer.

The bridge deck was noticeably prone to vibration when I visited, but not seriously so.

The bridge superstructure is visually unusual and attractive, but certainly not the only interesting feature of the design. The steel piers which support the bridge are also possibly unique, being formed out of single steel plates only 32mm thick. They are prevented from buckling by their curvature, and an example of remarkable lateral thinking.

I think this is one of London’s best footbridges, highly innovative in its design but not self-consciously so. The unusual design details are all in the service of a clearly-expressed and very attractive aesthetic.

Further information:

• Google maps / Bing maps
• Structurae
• LUSAS
• Architen Landrell
• Plashet School Footbridge (MacLaurin, The Structural Engineer, 2001)
• Bridge Builders (Pearce and Jobson, 2002)
• 30 Bridges (Wells, 2002)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)