27 June 2017

Footbridge Awards 2017

The shortlist has been announced for the triennial Footbridge Awards. Winners will be announced on 6th September at the Footbridge 2017 conference in Berlin.

I've posted links below to sites where you can find more information about the entries. I've only visited two, Merchant Square Footbridge and Greenwich Reach Swing Bridge. Most of the others are new to me. In general, there are some interesting bridges here, please share opinions via the comments if you want to!

Short span
Medium span
Long span

21 June 2017

Unbuilt bridges of London

I've been thinking for some time about writing a few articles about unbuilt bridges, but it turns out there's so much good material elsewhere on the internet that I'm better off directing you appropriately.

The best online resource for this is the "Ian Visits" blog, which has uncovered a cornucopia of unbuilt London architecture, including bridges.

A feature on Victorian railway stations that tried to span the Thames discusses unbuilt railway bridges (and stations) at Pimlico (pictured) and Waterloo. These early proposals came to nothing, and it's only with the recent remodelling of Blackfriars Railway Bridge that a railway station has finally been extended to span both banks of the River Thames.

The Victorians were also responsible for an 1852 proposal to run a railway viaduct not across the Thames, but along it. This structure was proposed to run between Westminster and London Bridge, and can be seen as a forerunner of what is now the District Line, built in a  tunnel below the north bank of the Thames.

At the end of the 18th century, a number of proposals were invited for the replacement of Old London Bridge. The scheme eventually selected would not be completed until 1831, but some of the alternative designs are interesting. Thomas Telford proposed a 600-foot span cast iron arch (pictured), which would have been a tremendous technological achievement if built. Creating suitable foundations to withstand the arch thrust in London Clay would have been a significant challenge! "Ian Visits" also discusses George Dance's proposal for two parallel bridges.

Thomas Dunn was the engineer behind a proposal for a remarkably modern looking iron footbridge at Ludgate Hill in 1863. Before traffic signals brought order to the city streets, Dunn's design was for a walkway-in-the-air, allowing pedestrians to cross a busy road junction without the inconvenience of having to cross overcrowded and dangerous highways.

Over the years, there have been various proposals for a new bridge across the Thames near St Paul's Cathedral (see my previous posts for some examples). "Ian Visits" documents a 1909 proposal for a road bridge, and the amusing 1997 design from FAT for a kitsch garden bridge in tribute to the late Princess of Diana (pictured). This one was ahead of its time, I think, the market for ironic monuments to celebrity has surely expanded further over the last two decades.

There have been plenty of proposals for inhabited bridges across the Thames over the years (again, some of which I've covered in the past). It's a perennial theme from incurable nostalgicists blithely unconcerned with the visual impact of what they design. One such proposal was the Crystal Span Bridge, a 1963 proposal to replace the Vauxhall Bridge with a seven storey monstrosity complete with gallery, roof garden and ice rink.

Further curiosities include a proposed elevated viaduct along Oxford Street, to allow the area below to be pedestrianised, and a high-speed travelator for London Bridge.

What unites most of these unbuilt bridges is simple: we can be glad they were never built, given the number of potential eyesores avoided!

See also Unbuilt London: Bridges To Nowhere And Mad Masterplans at the Londonist.

12 June 2017

London Bridges: 48. Vauxhall Bridge

This is the third and last in this latest series of posts on the bridges of London.

Vauxhall Bridge sits on the site of a pre-Roman structure within the River Thames, only discovered in 1993 when shifts in the riverbed exposed its wooden piles. From that structure's demise until 1816, the only crossing of the river at this point was via ferry.

In 1806, engineer Ralph Dodd campaigned to build a new bridge at the Vauxhall site, leading three years later to the passing of an Act of Parliament and the creation of the Vauxhall Bridge Company. Dodd's 13-arch bridge design was, however, rapidly dropped in favour of a 7-span stone arch design by John Rennie. This then proved too expensive, and approval was obtained via a further Act of Parliament to construct an iron bridge instead. Rennie proposed an 11-span iron bridge, but a 9-span cast iron proposal from Samuel Bentham was preferred.

In its turn, Bentham's design also fell out of favour, and the bridge eventually completed in 1816 comprised nine cast iron arches supported on stone piers, designed by James Walker. This bridge proved highly successful, and was eventually taken into public ownership in 1879. However, by this time, the bridge foundations were in increasingly poor condition, and in 1895, permission was granted to replace the structure.

The new bridge also went through several incarnations before being built. Alexander Binnie's initial steel design was not popular, and he then proposed a 5-span granite-clad concrete structure. Work began on this, but problems during construction of the foundations led to a change in plan: it was decided that the concrete bridge was too heavy. The granite-clad concrete foundations and piers were completed, but Binnie and Maurice Fitzmaurice designed a steel arch superstructure instead, which was finished in 1906. This is the bridge that can be seen today.

The new bridge attracted further controversy even before it was complete. Complaints about the engineer-led design led to a decision to install statues on the bridge piers, four on each side of the bridge. The downstream statues by Alfred Drury represent the themes of Science, Fine Arts, Local Government and Education. On the upstream side, statues by Frederick Pomeroy illustrate Agriculture, Architecture, Engineering and Pottery.

Many users of the bridge will not even realise the statues are there: they face outwards to the river. They are visible from the river bank, but for a close-up view you either need to get into a boat or lean over the side of the bridge.

The bridge originally had wrought iron railings on each side, but these were replaced in 1973 with the squat parapets seen today. These are at least shorter than the original railings, which were some 8 feet tall! This was before the bridge achieved Grade II* Listed status, which only came in 2008, as part of a group of London bridges.

Apart from the parapets, Vauxhall Bridge is an attractive structure when viewed from the river. I think a large reason for this lies in the paintwork, which has been arranged to highlight a hierarchy of elements, with the riveted steel arch ribs in yellow, spandrel stanchions in white, and red, white, blue and yellow all used in different parts of the stringcourse and parapets.

There are thirteen arch ribs in each span, and the span dimensions vary from 45.6m in the centre span, via 44.0m in the intermediate spans to 39.8m in the end spans.

What struck me most about the bridge was its sheer width. It accommodates two footways, a two-lane cycleway, a bus lane, and a four-lane highway.

This is a major traffic artery, and there are further major highways running along the river at both ends of the bridge.

The result is a bridge that makes it easy cross the river, but which also plays its part in creating division, as it's almost easier to cross the river from north to south than it is to cross the bridge from east to west. Seen from the river, it's like a giant staple connecting the two river banks. Seen from above, it's like a giant pair of scissors, helping to subdivide London into smaller and more navigable city blocks.

Further reading

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.

Further information:

04 June 2017

London Bridges: 46. Footbridge L01

I've covered most of the interesting bridges built for London’s 2012 Olympics on previous occasions, but there are two structures that I’ve been waiting for a chance to visit for some time. Now, that chance has come, and the first of the pair is the excitingly named "Footbridge L01", designed by Atkins and Allies and Morrison.

This bridge carries a foot and cycle path above the busy Ruckholt Road, a major highway in the east of London. To the south, the path leads to the Olympic Velodrome, and to the north, the playing fields of Hackney Marshes.

I don't know how well the path was used during the Olympics, but it was not a popular route when I visited, mid-morning on a sunny weekday. For a while, I wondered if I would be the only person to cross it!

The bridge spans 42m clear across the highway, and is in the form of two tied arches. The arch ribs are single steel plates, some 90mm thick, while the deck consists of ribbed steel plate (topped with concrete) spanning transversely between small box section tie girders. The tie girders are suspended from the arches by 25mm thick steel slats.

When I go to visit bridges, I often have a number of questions in mind. These are sometimes the 5 Ws of basic journalism: Who? Where? When? What? Why? When applying these to bridges they can be informative e.g. What is it actually for? The additional sixth question of "“How?" is often more useful when considering engineering and design: How does it address its purpose? How does it stand up? How does it articulate?

For me, this is a "Why?" bridge, although this question is deeply entwined with the "What?" and "How?" As originally conceived by the designers, the bridge was a tied half-through arch, with the arch ribs passing below the deck level at the bridge's ends. Why it was conceived this way, I certainly don't know, it would seem to have made more sense for the arch and tie-girder to meet at the span support points, as is normal in half-through tied arches.

It was clearly not a straightforward bridge to design, as set out in various interesting technical papers by Atkins. The close spacing of the hanger slats (at 135mm centres) means that their aggregate in-plane bending stiffness is significant, enough to act as a Vierendeel girder restraining the arch rib against in-plane buckling. This inevitably leads to complexity in the structural analysis.

Below deck level, the arch rib was originally proposed to fly clear i.e. unconnected to the deck. However, the designer’s analysis showed this would not be feasible, as the arch rib would be too slender and would buckle in compression. The obvious solution is to use the vertical slats in these end zones, but as compression struts rather than as hanger plates. Again, however, the vertical slats would buckle, as they are too slender to carry any useful compression loads.

This answers one of the bridge's several "Why"s, the question of why the end-zone elements below the bridge deck are in solid plate – this internally stiffened plate can carry vertical compression, and it's there to prevent the below-deck elements of the arch rib from buckling.

A second "Why" asks why there is a similar solid plate above deck level, but only towards the ends of the arch. There is a similar explanation – under uneven pedestrian loading, there are situations in which the slats in this zone can also experience compression load. Again, a solid plate was inserted to prevent the slats in these areas from buckling.

Before visiting the bridge, I anticipated the effect of both these engineering compromises would be awkward. This is, I think, true only to an engineering eye expecting a single clear structural rationale. I think in practice that the arch can be seen as springing not from its end supports but from the nodes where arch and tie-girder connect i.e. the main piece is held aloft in mid-air by giant orange corbels.

Another way to see these compromises positively is to see them as framing the central part of the bridge, while allowing the bulk of the structure to be visually transparent. In short, it’s an oddity, but not a complete failure.

Transparency on this bridge is only ever relative. Seen from the highway below, the hanger arrangement is reasonably transparent, with only the horizontal bar of a handrail indicating that this is as much a balustrade as a structure or a screen. While crossing the span, the bridge user exists within a local "bubble" of transparency, able to see clearly to left and right, but with their view obscured when looking at an angle. This "bubble" stays with the observer as they pass across the bridge. This louvre effect is something that can be exploited usefully where there is a need to balance issues of privacy screening and transparency, neither of which apply on Footbridge L01.

Setting aside the overall structural form for a moment, the bridge is well detailed. In particular, the approach parapets use the same slat type and spacing, albeit painted in black rather than in blazing orange. This is highly effective, and the simple continuity of the handrail element plays a positive role as well.

The substructure adopts the robust finish used elsewhere in the Olympic park: prison-style gabion baskets, here hiding a concrete support structure which is integral with the steel superstructure. Mountainous earthworks behind the grey retaining walls were hopefully used to avoid off-site disposal of spoil from excavations, as otherwise their scale is quite unnecessary.

I've left the most significant "Why?" for last, which is to ask: Why this, rather than something else?

Here I find it harder to quell a persistent nagging feeling that there was simply no especially good reason to adopt this peculiar hybrid of tied-arch and Vierendeel slats. An arch bridge is certainly an appropriate solution for the span, where a gateway structure will initially have acted as an entrance signpost to the wider Olympic site. A slender arch rib is an admirable aim, but could as readily have been achieved with, for example, a network arch design.

I suspect here that there is a degree of both affectation and of muddle: a specific notion taken to its logical conclusion, however difficult the resulting structural engineering, and a notion that is something of a muddle between architectural and engineering aspiration. There's a phrase in one of the papers on the bridge which I think may be revealing and which certainly suggests a certain kind of design relationship: "The key challenge of the design was resolving the structural integrity of the bridge without compromising the architectural vision".

Nonetheless, I enjoyed visiting Footbridge L01. In particular, the bold juxtaposition of orange and black seems quite apposite for what will for most tastes be a marmalade and marmite experience. I like the use of colour, I like the bridge detailing and I admire the willingness to depart from what might otherwise have been quite a humdrum, conventional solution.

Further information:

14 May 2017

Bath Bridges: 3. Destructor Bridge

This is the third and last bridge that I visited on my recent trip to Bath.

The original Destructor Bridge was built as a railway bridge in 1870 by the Midland Railway Company, at an entirely different site. In 1905, the railway bridge was relocated to the current site, and nicknamed the "Destructor Bridge" after the nearby "Destructor", an incinerator. The present-day Midland Bridge, a road bridge, now occupies the original site.

This part of Bath is experiencing significant redevelopment, so far as I could see mainly to build new apartment blocks, the easy-money choice prevalent throughout the country. The replacement of Destructor Bridge was ordered as a planning condition for the Bath Riverside development. The new structure was designed by COWI and Knight Architects, and built by Britannia Construction, with steelwork by Cordioli.

The bridge is an absolute delight.

Its layout is asymmetrical, although this is not apparent from a distance. The 6.7m wide roadway sits between two footways, with the western footway/cycleway being wider (4m) than on the east (2.5m). Beneath each edge of the road there is a steel box girder, with the road deck supported on cross-beams laid out ladder-style. The western girder is the larger of the two, and its load capacity is augmented by a shallow steel arch. The bridge deck is constructed in orthotropic steel plate, a series of welded troughs. Anti-pigeon fillets have been added to the cross-beams.

The arch is one of the elements of the bridge that I found most pleasing. The arch rib is a super-slender steel box, more of a ribbon than a conventional box, and the arch hangers are simple flat steel plates of the same width as the arch. It's a stark and beautiful piece of architecture, and will have been very difficult for the engineers to achieve. It exposes the sheer laziness of so many similar bridges.

I suspect that it's made possible by putting more reliance on the deck girders than is immediately apparent. I think they are carrying a greater share of the load than is usually the case on an arch bridge, and most importantly the stiffness of these girders is perhaps relied upon to the restrain the arch against in-plane buckling.

Most designers put the stiffness in what they see as the main load carrying member in the arch, or share it fairly equally between the two. The precedent for Destructor Bridge's boldness might be traced all the way back to the deck-stiffened arches of Robert Maillart, and indeed the full-width hanger plates remind me of the leaf piers adopted on several of Maillart's designs.

The second box girder is therefore serving two main functions: it carries a share of the highway and footway loads, but mainly it reduces the torsional effects on the primary box girder.

Unfortunately, the arch is the source of what is at present the bridge's biggest problem. The shallowness of the rise, and the flatness of the cross-section, present a compelling invitation to anyone wishing to climb the arch, and some minimal anti-climb plates and signs fail to offer serious discouragement. Inevitably, the bridge has been plagued with visitors walking across the arches, even doing handstands, and I'm surprised not to read any mention of skateboarders and unicyclists giving it a try.

Temporary barriers have been erected around the two ends of the arch, but these were clearly ineffectual and a security guard is now present on the bridge at all times. The problem is inherent in the design, and I've seen other steel arch bridges deal with it in different ways. A bridge in Nottingham has a smoothly tapered "steeple" detail on the arch, while one in Sheffield has anti-climb "blisters". Neither solution is compatible with the Destructor Bridge design.

The same issue arose (just as predictably) on the Tradeston Bridge in Glasgow, where it seemed to be something of a novelty, a craze that just went away over time. I'm not clear whether any different anti-climb solution is under consideration on the Destructor Bridge, or whether the same process of gradual loss of interest will apply.

The low-speed nature of the road carried by the bridge has been used to good advantage to dispense with the vehicular parapets normally mandated by standards. Instead, low-profile barriers are used to keep vehicles from mounting the footways. This allows the main edge containment to be in the form of pedestrian balustrades, with the western one slightly higher than the east, as a cycle path is accommodated on this side (although not presently marked out on the paving).

The parapet design echoes the arch, with a series of flat "blades" supporting a timber handrail (with integral lighting). The taller parapet is mounted with an additional stainless steel top rail. The effect is visually attractive – transparent when viewed directly, yet solid when viewed obliquely, and I was interested to see that although visually the parapet looks quite massive at an angle it still "feels" transparent. The device of attaching the blades to the outside face of the deck edge beam is also interesting, and quite different to what is normally done to make deck edges appear thin.

I'll finish with a few of the bridge's interesting details.

The bridge abutments are massive concrete constructions, but sloped back to follow the riverbank. The corners are faceted and the front face inscribed with parallel recesses, both of which serve the purpose of breaking up what would otherwise be a large mass. The recesses also discourage graffiti.

The central part underside of the bridge is left largely untreated, but the two edge sections have been hidden behind "cheesegrater" cover plates. Service pipes run below the deck in these areas, and the covers serve both to hide them visually and to reduce the risk of vandalism.

I don't really like this detail: it makes the bridge deck seem far bulkier than it really is, and it will surely hinder future inspection and maintenance.

The final detail worth noting is the benches which have been inserted between the arch hanger plates. These are a very nice feature, minimal and entirely in keeping with the geometry of the rest of the bridge. Soft lighting below the benches is an effective treatment, although unfortunately some of the lighting seems already to be broken.


Further reading: