London Bridges: 12. Bellmouth Passage Road Bridges

Bellmouth Passage connects two docks together near the east end of Canary Wharf. Within the space of about 100m, there are no less than five opening bridges.

The first two were designed by Wilkinson Eyre with NRM Bobrowski, and each carry highway traffic above the waterway. The Passage isn't used much by waterborne vessels (especially right now, while West India Dock is occupied by the gargantuan construction site for a CrossRail station), but opening bridges are necessary for when waterway traffic returns.

The bridge pictured is the northern of the two identical spans, and is of an unusual, if not unique, design.

It operates a little like a Scherzer rolling lift bridge, which is a bascule bridge which rolls back onto a curved girder, with a motion like a rocking chair. In the Scherzer design, the centre of rotation moves horizontally, whereas at Bellmouth Passage, it stays in one place. The rolling girder sits on support wheels, and a rack-and-pinion motor drives the bridge.

A more conventional bascule bridge wasn't adopted because there was no space for the conventional counterweight, with the client having specified that pedestrian access had to be maintained at dockside level. The brief also requested that there would be no visible structure above highway level. The chosen design solves the problem by positioning the bridge's pivot point midway between highway and lower footway, allowing the bridge to rotate about a "virtual fulcrum". The counterweight is incorporated within the elevation of the ring girder, as can be seen in the photograph to the left.

The only similar design I've ever seen was one of the losing proposals for the Foryd Harbour bridge design competition.

The bridge has two steel box girders supporting the deck, which are continuous with the ring girders. The highway deck is an orthotropically stiffened steel plate with a thin asphalt layer.

One feature of minor technical interest is that trief kerbs are deemed sufficient to keep vehicles on the road, allowing the use of lightweight mesh pedestrian parapets and timber footways. Unsurprisingly, this has led to seepage staining below, likely to be followed by paint loss and corrosion if road salts are being used on the highway.

This is not a beautiful bridge, but it is an ingenious one, and the ring girder has an obvious appeal. It may have visually more successful if it could have been a full ring, rather than having the top truncated, but the experience of being able to walk through it is unique, and must be doubly so when the bridge actually operates.

Further information:

• Google maps / Bing maps
• Structurae
• Conceptual Design of Moving Footbridges, Footbridge 2008 (Cutlack, 2008)

London Bridges: 11. Limekiln Dock Footbridge

Okay, my summer holidays are now out of the way, so hopefully I can return to something like normal service.

First up, I've got a batch of bridges in London's Docklands that I visited earlier this year. These are not all marvels of the bridge design world, by any measure, and that's doubly true of this first structure.

The Limekiln Dock bridge spans what was once a busy inlet from the River Thames, connecting riverside footpaths. When the Docklands was re-developed, this area was re-populated with housing, and although there were once mooring rights along the Dock, I'm unclear whether these still exist. They certainly must have done in 1996, when the footbridge was completed, as it is a swing bridge.

The bridge, designed by YRM and Anthony Hunt Associates, spans 34m. It was built by Littlehampton Welding, and is an asymmetric cable-stay bridge in design, with the main span balanced by a large counterweight slung off the back of the mast. The entire assemblage pivots to open, requiring a large space on the quayside to be reserved for the counterweight and its support arms, so they can swing sideways without any risk of an accident.

In the picture, you can see the arms supporting the counterweight, as well as two arms which support the deck, which is S-shaped in plan (the Bing maps link below gives the best general appreciation of the bridge layout - rotate the birdseye view to get an idea of the tidal range). There's the idea of an elegant bridge there, but the support arms and stiffness make it more like a hunchback on crutches.

Indeed, I've seldom seen such a promising geometry ruined by such dismal detailing. There's almost nothing about the bridge which works well. The counterweight is clunky, and the loss of dockside space that results is criminal; the mast resembles a leaden drinking straw; an unfortunate fender arrangement below part of the deck detracts from what little style its flat-faced deck girder might ever have possessed; and it's all in a shade of grey which would give a pessimistic air even to an industrial estate, let alone blemishing the riverside next to some of the most expensive real estate in London.

Luckily, some of the other bridges I visited were a little better.

Further information:

• Google maps / Bing maps
• Structurae
• movablebridges.org.uk

Outcry over plan to rebuild the Ponte dell'Accademia, Venice

A repeat of the battle between city authorities and traditionalists over Calatrava's bridge in Venice may be in the offing. Proposals have been announced to replace the Ponte dell'Accademia, which spans the southern end of Venice's Grand Canal.

I featured the bridge here last year. It's a steel arch bridge built in 1986, clad in timber to resemble the previous structure, Eugenio Miozzi's 1933 bridge. As I reported then, plans have been brewing for some time to improve disabled access over the structure which, like many Venetian spans, has steep steps difficult for many users to negotiate.

The latest proposal (pictured), from Bolognese architects Schiavina, is for a complete reconstruction. The new bridge would cost about £5m, and depending on which website you read, would be built either in glass, steel and stone, or in glass, steel and timber.

In support of the plans, the city council claim that the cost of maintaining the existing bridge is exorbitant, and therefore reconstruction combines the ambitions of reducing ongoing cost and providing access for all. A recent refurbishment cost about £220k, and the authorities consider the future costs to be unsustainable.

Preservationists are up in arms, with the president of a Venice heritage group crying "The bridge has a certain dignity. Why don't we just restore it?" At present, there's no final commitment to build the new design, which has been considered by the local authorities and sent to the heritage ministry in Rome for approval. Protesters are however probably mindful of the precedent set by the construction of Calatrava's Ponte della Costituzione, where concerns over its contemporary design were ignored (in my view, probably rightly, as it hugely improves accessibility in an area where many visitors first encounter the city).

It's unclear how the work will be funded, with Venice notoriously unwilling to dig into its own coffers to maintain the historically valuable fabric of the city (as seen here). One report has multi-millionaire Renzo Rossi covering the costs.

I don't know whether replacement is the right solution. As an engineer, the question must mainly come down to the sustainability of bridge maintenance: is it really in such a bad condition that it is too expensive to look after? It did look a little on the shoddy side when I visited, but so do many bridges, and I suspect the cost of maintenance is being exaggerated.

Venice argues that its 1986 reconstruction means it is no longer a historic artefact and therefore not deserving of heritage protection. I don't buy that at all. A considerable effort was made to preserve the general appearance of Miozzi's bridge when it was rebuilt - it maintains a continuity of appearance which should not lightly be set aside.

The bridge's context is also highly significant. While I'm no fan of attempting to preserve the environment in aspic, there are few places where the cityscape is of such historic (and economic) importance as in Venice. The bridge is a key gateway to the Grand Canal, and in a place like this there should be a very good reason indeed for changing the bridge's appearance so much.

It is also worth considering the technical difficulties that imperil every bridge in Venice, particularly the inability of the ground to support large foundation loads. The first bridge at this site was a simple metal truss, imparting only vertical load. Miozzi's wooden bridge was relatively lightweight, but at 48m span even that required a large number of both concrete and screwed timber piles. The current bridge is heavier, but a new steel and stone bridge will be heavier still. If the Schiavina proposal proceeds, expect a lengthy political battle followed by difficulties with cost escalation (it is far from apparent whether an engineer is yet involved).

Finally, I have to ask why only a single architectural firm has presented a design for this site. Schiavina are not noted as bridge designers, but if the case for replacement is ever proven, then surely a site such as this merits the consideration of design proposals from competing designers?

We interrupt this usual service

Ok, I've been away, and it will take some to recover, catch up with work, and get back to posting.

Coming up when I get time:

• Reports on five or six more modern bridges from London's Docklands area
• Three or four bridges in Devon and Cornwall
• A review of Peter Lewis's book "Disaster on the Dee: Robert Stephenson's Nemesis of 1847"
• The bridges of two Spanish design firms, which were shown at Footbridge 2011
• A final batch of Where are they now? updates

Where are they now? Part 3

Here's another batch of bridges which I featured on this blog during 2009, and have barely mentioned since. What has happened to them? How many have survived the great black hole of indifference?

North Coast Harbor Bridge

In September 2009, I posted details of six alternative options for an opening footbridge in Cleveland, USA, designed by Miguel Rosales. Judging from a recent news story, the US$5.5m twin-bascule drawbridge option got the nod, and is due to be built starting in 2012. Aviation restrictions seem to have ruled out the city's preferred single-bascule option.

Sheffield Parkway

I revealed proposals from Scott Wilson for a new footbridge over the A630 Sheffield Parkway in October 2009 (pictured). This was a much cut-down design which had replaced Tim Nørlund and Ramboll's previous RIBA-competition winning entry, ditched due to lack of budget. I've tried to find out whether the bridge is still going ahead, but without any luck.

Copenhagen Harbour Bridge

October 2009 also saw the announcement of a winning design for a new opening footbridge in Copenhagen. The winner, from Studio Bednarski and Flint and Neill, is an unusual retractable bridge. The project looked to be well organised and well funded, but I haven't found any further information on progress. Does anyone know whether design is complete, or a contractor appointed?

Johnson Street Bridge, Victoria

I reported several times on the battle between historic bridge preservationists in Victoria, Canada, and their local council, who appeared determine to replace a rare heel-trunnion bascule bridge with a new structure designed by Wilkinson Eyre (pictured). The story rumbled on into 2010, leading up to a town referendum in November 2010 on whether to borrow the money required to build a new bridge. I didn't keep up with the news, and didn't report the referendum result: the council won by a significant margin. In January 2011, geotechnical investigations for the replacement bridge commenced. In March, defects found in a structural inspection led to the closure of the existing rail span. It is due to be demolished next year.

Preliminary design of the new bridge is currently underway, and due to be complete in Autumn this year. The overall project completion date is targeted as March 2016. The new bridge will be a highly impressive structure, and definitely one to watch.

The heritage campaigners have shifted their focus to arguing for retention of a rail route on the new bridge.

Galp Energia Bridge

I reported on the winner of a contest to design a new foot and cycle bridge in Lisbon (pictured). This was essentially a vanity competition, run to support a design biennale, with an attractive prize fund but no firm plans to actually build anthing. Unsurprisingly, I've not found any sign that the design went any further.

Okay, that will do for now. I will do a final batch of catch-ups from 2009 in a couple of weeks time.

Wrocław's Bridges: 16. Pipeline Bridge

This bridge is labelled "Gazociąg" by Wratislaveae Amici, which simply means "gas main" or "pipeline".

It was something of a surprise discovery - it isn't mentioned in any of the papers, guidebooks or websites that I looked at to work out what was worth seeing in Wrocław. I just glimpsed it in the distance and thought a closer look was merited, even though by this point myself and a companion were getting hot, tired and thirsty.

I guess its status as a utility bridge is responsible for its omission from guides to Wrocław's bridges, but that's a shame, as it's both a spectacular bridge, and also one of the most aesthetically pleasing that I saw.

I didn't have time to find a vantage point from which the whole bridge can be seen - it was simply too big. So you'll simply have to note that these photos show at most half of the bridge, and that the remainder is symmetrical (the Google maps link below makes it clear).

Spanning approximately 250m, this is a beautiful filigree suspension bridge. The cables are splayed, to provide the pipeline with a degree of lateral stability, and hence the pylons are also splayed, gigantic Y-shapes.

It would clearly be an absolute pleasure to walk across.

Further information:

• Google maps
• Wratislaveae Amici

Wrocław's Bridges: 15. Zwierzyniecka Footbridge

Heading eastwards and back out of town along the River Oder, you come to Kładka Zwierzyniecka.

Spanning 86m, this suspension footbridge's main function is to carry two large diameter water pipes across the river, although it was also well used by pedestrians when I visited.

It was built in the mid-1970s, and designed by M. Wróblewicz. The deck and towers are both in steel, and the bridge is self-anchored in form i.e. the main cable force is carried by the deck rather than anchored into the ground. The attachment between the deck and the cables can be seen on the right: triple cables are carried through a single pin into a stiffened cantilever bracket.

Zwierzyniecka Footbridge has a simple, highly functional design, to the point of near-brutalism. However, the lack of bright colour and odd protuberances makes me more sympathetic towards it than the other suspension footbridge I visited in Wrocław, the Frog Bridge.

It's not by any stretch of the imagination beautiful, but I quite like its sense of austerity.

The next post will be the final one in this series, and I've saved one of Wrocław's least known, but most impressive, bridges for last.

Further information:

• Google maps
• Wikipedia (Polish)
• Wratislaveae Amici
• Footbridges in Wrocław, Poland, Biliszczuk, Rabiega & Tadla, Footbridge 2008

Wrocław's Bridges: 14. Zwierzyniecki Bridge

The most striking feature of Most Zwierzyniecki is the overhead bracing which connects its twin truss arches.

The bridge was completed in 1897 (replacing a two span timber bridge) but according to Wiesław Jurewicz, it was modified in 1910 "due to the hazardous vibration of the arch girders". That may account for the presence of just so much overhead bracing, which functions both as X-bracing in plan, and portal bracing in cross-section.

The bridge was designed by Karl Klimm, A Fruhwirth, and Richard Plüddemann. It spans approximately 60m and is 12.5m wide between the truss centrelines, 21.8m wide in total. It carries a highway and tramway across a side channel of the River Oder, in the vicinity of Wrocław's Zoo and Centennial Hall. An earlier span was reportedly called the Paßbrücke because it formed a checkpoint at the city limites.

The main arch trusses have more than a hint of the later Hell Gate Bridge or Sydney Harbour Bridge about them. The upper chord is essentially a stiffening rather than primary load-bearing element, terminating in mid-air vertically above the lower-chord springing. This is disguised by the presence of decorative sandstone obelisks at each corner, just as the same effect is disguised by monumental abutments on the two larger and better know spans.

The yellow-gold colouring on the bridge's metalwork, and the intricacies of the truss and overhead bracing, put me in mind of a gilded cage, where the structure is as much as a shelter as a means of support for the floor.

You couldn't build a bridge quite like this today. Quite apart from the cost of the heavily laced metalwork, the hanger bars are unprotected against impact from traffic. While it's acceptable to design a bridge for the possibility of loss of a single hanger, this bridge clearly suffers from the potential for multiple hangers to be lost in a single incident.

Technicalities aside, I found Zwierzyniecki Bridge to be a lovely bridge, with a great deal of antique charm.

Further information:

• Google maps
• Structurae
• Wikipedia (Polish)
• Wikipedia (English)
• Wratislaveae Amici (includes pictures of the previous bridge at this site, and the current bridge under construction)
• The best known bridges in Wrocław
• Wrocław Mosty - Bridges - Brücke, Wiesław Jurewicz (undated book, ISBN 978-83-926759-5-2)

Wrocław's Bridges: 13. Frog Footbridge

The Kładka Bielarska (or Kładka Żabia) connects Bielarska Island to the main river bank. There has been a suspension footbridge here since 1975, although it was largely reconstructed in 2002, including extensive reinforcement of the foundations with micro-piles. Before that, a truss bridge occupied the site. The bridge was designed by Kazimierz Gałajda, with the reconstruction attributed to Joseph Rabiega.

The bridge spans 56.5m, with pylons 10.5m high. The main suspension cables are 46mm spiral strand ropes. Both the pylons and the main deck beams are made from 457mm diameter steel tubes.

The pylons consist of portal frames, with the crossbar and legs each comprising twin tubes, "stitched" together with cross-stiffeners. These are ungainly enough, but made worse by the addition of frog-like protrusions at the top which carry lighting units. Presumably these give the bridge its "Frog Bridge" nickname.

Sadly, the paired tubes are not unique to this bridge - I've seen them on a photo of another Polish footbridge. Overall, the styleless reliance on tubular steel in combination with bold colouring is emblematic of an undesirable trend which seems to have informed a great deal of Polish footbridge design.

The bridge evidently suffered from considerable vibration prior to its reconstruction, and tuned mass dampers were added to the midspan and quarter span points. These are large dampers, and quite easily visible in the first photo of the bridge above.

Further information:

• Google maps
• Structurae
• Wikipedia (Polish)
• Wratislaveae Amici
• Footbridges in Wroclaw, Poland, Biliszczuk, Radiega & Tadla, Footbridge 2008
• Redukcja drgan kladek dla pieszych z wykorzystaniem tlumikow masowych, Dalmer, Fiebig & Rabiega, 2004 (Polish, explains the addition of the tuned mass dampers to the bridge)

Wrocław's Bridges: 12. Malt Footbridge

Not far from Kładka Piaskowa, another footbridge can be found which was built at the same time (2003), linking Słodowa island to the main riverbank. Kładka Słodowa is a steel bowstring arch bridge, with inclined hangers forming a triangulated system (this is stiffer than using vertical hangers, but is prone to load-reversal in the hangers if they are not sufficiently pre-tensioned). The designers were Research & Design Office Mosty-Wrocław with ISBA architects.

The arch spans 48m, and is 15.6m tall. It consists of two tubes, 508mm in diameter, and parabolic in profile, intersecting at a high level. Some form of connection is essential to brace the arches against buckling, but the choice to intersect in this way is essentially an arbitrary one.

The bridge deck consists of a cellular steel box girder with a curved soffit plate, similar to the nearby Sand Footbridge, and which picks up reflected light from the river in a similar manner. The deck is paved in blue-grey sandstone slabs.

The bridge elements were fabricated in a shipyard and brought to site by boat for final assembly. The arch was assembled lying on its side, with a temporary bowstring for support, before being lifted by crane into the vertical position. Three deck sections were then erected one by one using temporary vertical strands for support.

This is a reasonably straightforward and attractive design. The view of the cables gets a bit confused because of the variation in angles seen from anywhere other than side-on to the bridge. As with the Sand Footbridge, the olive-lemon green colour is not unattractive, especially amongst trees, and the deck looks slender.

I'm not sure why they went for a different parapet here - mesh infill panels rather than horizontal wires - and as at its sister bridge, the parapets look quite utilitarian.

It's interesting to see one of the alternative options that was considered for the site, an asymmetric cable-stay bridge without any back-stays supporting the pylon (shown below). This would clearly have been more expensive, both in terms of the superstructure and the foundations.

Further information:

• Google maps
• Structurae
• Wikipedia (Polish)
• Wratislaveae Amici
• Examples of newly built footbridges in Poland, Biliszczuk, Berger, Machelski, Wegrzyniak, Onysyk & Prabucki, Footbridge 2002
• Footbridges on the River Odra's Islands in Wroclaw, Styrylska, Boniecki, Biliszczuk, Barcik, Rudze & Stempin, Footbridge 2005
• Footbridges in Wroclaw, Poland, Biliszczuk, Radiega & Tadla, Footbridge 2008

Wrocław's Bridges: 11. Sand Footbridge

Most of the bridges in Wrocław that I've covered so far in this series have been historic in nature. The next two or three, however, are all relatively contemporary designs.

Kładka Piaskowa was built in 2003, and is a three-span pedestrian bridge (spans 4.5m, 45m, 4.5m), designed by Research & Design Office Mosty-Wrocław with ISBA architects. The deck is 3.19m wide and 1.06m deep, and comprises a steel box girder with a curved soffit. The deck is painted a lemon-olive colour, while the steel parapet post are painted light grey. The deck surfacing consists of Iroko hardwood planks.

Essentially, it operates as a fixed-ended beam, which allows the relatively large span-to-depth ratio to succeed. The deck is of a constant depth, but is lightly arched. The photo on the left illustrates the support system. The transition to the much shallower approach ramps looks as good as can be expected for something which is always very awkward.

The bridge was erected by "pull-launching", with its front end supported on a barge while the whole deck was towed across the river from one bank.

What's especially pleasing about this footbridge is its undemonstrative nature - there is no attempt to be unnecessarily showy, unlike some Polish footbridges. The low profile was a deliberate attempt not to interfere with historic riverbank views. The parapets are somewhat utilitarian, but not so much as to ruin the design.

The curved underside of the deck picks up shimmering reflections from the water below, and enhances the impression of slenderness. The colour seems initially a little odd, but it is muted rather than garish.

Further information:

• Google maps
• Structurae
• Wikipedia (Polish)
• Wratislaveae Amici
• Footbridges on the River Odra's Islands in Wrocław, Styrylska, Boniecki, Biliszczuk, Barcik, Rudze & Stempin, Footbridge 2005
• Footbridges in Wrocław, Poland, Biliszczuk, Radiega & Tadla, Footbridge 2008

Wrocław's Bridges: 10. Saint Clare Bridge

What's that? You think I have included Most Świętej Klary in this series solely so I can feature a photo of a nun on a bridge? Ok, I confess.

This appears to be a two-span timber truss bridge, although there are steel members visible below as well.

Further information:

• Google maps
• Wikipedia (Polish)
• Wratislaveae Amici

Wrocław's Bridges: 9. Słodowa Bridge

I'm including Most Słodowy simply for completeness, and because it is the only road crossing onto the Słodowa island.

There are a series of islands at this point in Wrocław, within the boundaries of the River Oder, and for most of the town's history, access to them has been limited. The construction of two footbridges (both of which I'll come to soon) in recent years has greatly improved accessibility.

The present structure is a temporary Bailey bridge, with a separate pedestrian walkway to one side. It spans only about 8m. From what I can determine, this must be relatively recent, as most of the information on the internet and linked below refers to a steel beam structure which is clearly no longer there.

Further information:

• Google maps
• Wikipedia (Polish)
• Wratislaveae Amici

Wrocław's Bridges: 8. Mill Bridges

The two Mosty Młyńskie originally spanned over the streams for two watermills, named Maria and Feliks. The latter mill has been demolished, but the former is still present, although disused.

As with Sand Bridge, there were bridges here for several centuries. The current steel parabolic arch trusses date from 1885, and are clearly a more developed design than the Sand Bridge. The general form of the truss is such as to follow the typical bending moment diagram for a simply-supported span, making a more efficient use of material than in a more uniform truss. The web members are also more open than in the lattice truss form which was previously prevalent.

The two bridges span 28m and 36m respectively, and are of slightly different heights. The slightly flattened top to the trusses is characteristic of the so-called Schwedler Truss. I presume it was done this way to cater for the variability in live load positioning, responding to the envelope of load effects rather than simply uniform loading over the entire span.

The northern of the two spans is currently being refurbished, and the bridge is closed to its normal roadway and tramway traffic.

In their current form, I am not greatly enamoured of the bridges. They are very honest structures, but as Schwedler himself is reported to have believed, the flattened arch form is not the most attractive.

Further information:

• Google maps
• Structurae
• Wikipedia (Polish)
• The best known bridges in Wrocław
• Wratislaveae Amici (includes historic photos of the bridges and their timber predecessor, and images taken during the present and previous refurbishments)
• Wrocław Mosty - Bridges - Brücke, Wiesław Jurewicz (undated book, ISBN 978-83-926759-5-2)