Venice Bridges: 2. Ponte de Rialto

Along with the Bridge of Sighs (on which, more later), the Rialto Bridge is one of the most famous tourist sights in Venice. For over five centuries, it was the only bridge across the Grand Canal, until the first Ponte dell'Accademia was built in 1854.

The canal was first bridged at this site by a pontoon bridge in 1181, replaced by a wooden bridge in 1255, incorporating a drawbridge opening section (as depicted by the painter Vittore Carpaccio). This, and several successors, were to collapse and be rebuilt until in 1503 proposals were made to build a more permanent stone structure.

The timber bridges already incorporated the shops that were to become one of the Rialto Bridge's most distinguishing features. They provided a rental income which helped to cover the bridge's maintenance costs.

In the late 16th century, a bridge design competition took place, with submissions from Michelangelo, Giacomo Barozzi da Vignola, Guglielmo Marastoni, Gugliemo di Grande, Jacopo Sansovino, Andrea Palladio, and others (several of these can be seen online). Many of the designs, including Palladio's, were for three span structures, but the Canal is relatively narrow at this point and the authorities preferred a bolder single span solution to minimise the impact on navigation (and possibly because the cost of foundations was lower overall).

One of the three-span designs, by Vincenzo Scamozzi, was adopted in 1588 as a template for the single-span bridge which would be built over the next three years. The builder, Antonio da Ponte, adopted the proposed layout of shops and stairways, as well as the balustrades from Scamozzi's design. Scamozzi himself severely criticised the attempt to build a single span, which he thought likely to collapse.

Da Ponte's bridge spans 29m, the shortest of the Grand Canal Bridges, with the arch rising 6.4m. The arch thrusts are substantial, especially for the weak Venetian soils, and as with many such bridges the horizontal component is reduced by adopting a slender arch section (1.25m thick) at the crown, with more weight above the abutments. Part of the arch thrust is stabilised by the loads from adjacent buildings.

The bridge as built was painted many times by Canaletto, who also painted a vision of what Palladio's proposal would have looked like had it been adopted.

Today, it remains a fine bridge, although its attraction is as much as a vantage point from which to survey the Canal as for its appearance. Many inhabited bridges have a deep, stocky appearance, but the dark shopfronts allow the upper arcade to appear lighter than it really is. Because the arcades are set back, the arch can still be seen as a relatively slender element in its own right.

Closer at hand, however, the bridge is remarkably run-down, with considerable grime and graffiti adorning it. Generally, a certain level of dilapidation provides much of the charm in Venice's buildings, and if they were all cleaned up and restored to their original appearance, a certain Disneyland feel would probably be the result.

Nonetheless, I find it surprising that Venice allows a major historic landmark such as the Rialto Bridge to be left in this condition.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Bridges in Venice - Architectural and Structural Engineering aspects [PDF] (de Miranda, Pogacnik & Skansi, IABSE Symposium, Venice, 2010)
• Living Bridges (Murray & Stevens (eds.), 1996)
• Bridges (Judith Dupré, 1998)

Venice Bridges: 1. Ponte dell'Accademia

While in Venice, I visited all four of the bridges over the Grand Canal. I started at the north end, with the most modern bridge, Santiago Calatrava's Ponte della Costituzione, and worked my way south, but here I'm going to cover the four spans in reverse order, which is closer to the history of when they were built.

The Ponte dell'Accademia ("Bridge of the Academy") spans the Grand Canal fairly close to its southern end, running broadly between the Gallerie dell'Accademia and a large public square, Campo Santo Stefano.

The first bridge on this site was completed in 1854. Designed by Alfred Neville, it spanned 50m, comprising a flat iron truss girder. A paper presented at the Venice IABSE conference in September describes it as a "reticular girder" or a "Neville beam", but these aren't terms I've seen elsewhere.

The girder solution is a sensible approach in Venice from one perspective, although only about 1 in 8 Venetian bridges are beams rather than arches. Girder bridges have the advantage of only transmitting vertical reactions, which is helpful, as Venetian soils are of poor quality and prone to long-term settlement, not a great characteristic when subjected to arch thrusts.

However, the beam bridge requires much higher approach ramps, especially if an appropriate navigation clearance is to be maintained. With a headroom of only 4.2m, the first Accademia bridge was a constant obstacle to navigation, and when it became increasingly dilapidated in the early 1930s, it was overdue for replacement.

The new structure was a 48m timber arch span, designed by Eugenio Miozzi, and opened in 1933. This required both concrete piles and screw-tipped wooden piles to withstand the arch loads. Miozzi's bridge lasted even less well than Neville's design, with the introduction of substantial steel arch components in 1948, and further stiffening added between 1963 and 1965.

The present bridge is yet a further evolution, as the bridge was rebuilt in 1986 as a steel arch structure, clad extensively in wood to preserve the general appearance of Miozzi's design. You can see in my photographs that there is a lattice steel arch lurking with embarrassment behind the wooden facade, and this mismatch becomes even more apparent from below, where the steel flanges are clearly visible (don't forget you can click on any photo for a larger version).

As with most of the Grand Canal bridges, this is a steep bridge to cross if carrying heavy baggage, and must be particularly awkward for anyone who is mobility impaired or has a child in a pushchair. The Venetian City Council announced in 2009 plans to modify the bridge to provide disabled access, but they are unwilling to stump up any part of the estimated €5m cost. Instead, they're hoping potential designers and contractors will fund the work via some form of sponsorship.

According to a report in The Art Newspaper, similar use of sponsorship elsewhere in Venice has attracted considerable yet unsurprising criticism (I'll come back to that in a later post). I suspect they have had few takers for this astoundingly cheapskate proposal, which is particularly ridiculous given the amount of money available in Venice for mega-projects such as the anti-flooding MOSE scheme.

I like the Accademia bridge - from a distance. The shape is attractive and well-proportioned, and the relatively open timberwork has a light appearance. From close at hand, however, it looks somewhat shoddy, and the conjunction of the poorly conserved timber shell with a duplicitous steelwork core is very uncomfortable.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Brief considerations on the Accademia Bridge in Venice [PDF] (Barbisan, Pogacnik & Skansi, IABSE Symposium, Venice, 2010)
• Bridges in Venice - Architectural and Structural Engineering aspects [PDF] (de Miranda, Pogacnik & Skansi, IABSE Symposium, Venice, 2010)

Back from Venice

I'm just back from a few days in Venice attending an IABSE Symposium. I've plenty of things to catch up on, but hopefully will get a chance soon to cover some of the bridges I visited, as well as thoughts on one or two of the papers and presentations which were particularly relevant to this blog.

Amongst many pleasant Venetian things, I also had time to visit an exhibition on the great Italian Engineer Pier Luigi Nervi, which runs at the Palazzo Giustinian Lolin until 14th November. The accompanying catalogue, Pier Luigi Nervi: Architecture as Challenge, is well worth seeking out. The exhibition moves on to MAXXI in Rome in December, and there are plans to bring it to London, Montreal, New York and elsewhere. I find the thought of seeing Nervi at MAXXI somewhat ironic, given his astounding ability to create beauty from the logic of gravity, whereas the MAXXI building's designer, Zaha Hadid, is an architect with a callow disdain for the same logic.

I also visited the Casso di Risparmio di Venezia, a bank building completed by Nervi in 1972, and an excellent chance to see one of his magnificent concrete waffle slab floors. I can't find a photo of the interior online, but there's one of the floor during construction which should give the general idea.

Once I get the Venetian reports done, I hope to get time to feature some of Nervi's bridge designs here. He was hardly noted as a bridge designer, but one of his most ambitious projects was for a crossing of the Straits of Messina, and his other bridge schemes should also be of interest.

Castleford Bridge, at night

I had the chance to get a set of photos of Castleford Bridge at night. There wasn't really room for them in the last post, so here's a second look at the same bridge.

Castleford Bridge

At last, I've found an opportunity to visit Castleford Bridge. This new footbridge over the River Aire in Castleford, Yorkshire, was the subject of a Channel 4 TV documentary that I discussed here a couple of years ago. It's one of the best British footbridge designs of the last few years, and I had been keen for some time to see how the reality compared to pretty photographs.

The bridge is the jewel in the crown of a wider town regeneration scheme, intended to bring investment into a town which once relied on the mining industry, as well as to help promote a more confident spirit amongst local residents. On a practical level, the footways on the only older bridge over the river were (and are) very narrow, and the new structure offered improved connectivity to the houses on the north bank of the river.

An invited competition was held in 2003, won by McDowell + Benedetti. As my previous post recounts, the bridge design eventually taken forward bears little resemblance to the one that won the competition, a floating bridge which could never survive the river's heavy floods.


The final design is 131m long, with a 4m wide deck. There are four 26m long main spans supported on V-shaped steel legs, with 9m across the top of each V.

The deck and lean rails are made from Cumaru hardwood, with the deck planks supported on fins from two steel box girders, one 500mm by 400mm, and the other rising to a maximum of 1000mm deep at midspan in a "finback" arrangement. This box is hidden below timber so that it also provides seating. Skateboarding along the seating is discouraged with a series of stainless steel "armrests". I've borrowed a sectional visualisation from World Architecture News which makes the form of construction a bit clearer (as with all my images, click on it for a larger version).

The preliminary structural design was undertaken by Alan Baxter & Associates. The bridge opened in July 2008, built by Costain, with detailed design by Tony Gee and Partners.

Channel 4 presenter Kevin McCloud heavily criticised the decision to adopt the design-and-build procurement route, saying in 2006: "I find it extraordinary that having done all this work, having prepared all this stuff, [design] is all squeezed out. The contractors and sub-contractors are provided with the minimum amount of information and effectively asked to redesign it from scratch."

Having seen some of the architect's early visualisations for the bridge, I find it hard to tell what has changed in the final design, although the support legs may be a little different and I believe the lighting units below the parapet handrails are of a cheaper design. McCloud's complaint seems exaggerated.

So, what makes the design so good, or at least, so interesting? Much of the attractiveness of the bridge comes from it sinuous layout, an S-shaped curve that meanders across the wide part of the river called "Castleford Bay". It makes the most of the opportunity for the bridge to be a space to visit rather than just a means to cross an obstacle. It's a promenade on which to stop and admire the view (the frothy weir to one side, and the older arched highway bridge on the other), or to sit and converse.

The extensive use of timber gives the bridge an informal feel, as does the shaping of the pier legs, with use of steel which is curved and varying in section. It's high-tech, but with a more organic sense than much high-tech steel bridge design. The lean rails are nice and wide, enhancing the idea that it's a place to stop and stare.

The curved deck seats, hiding the structural beams, are a very succcessful element. It's a good way to disguise most of the structural depth on what is structurally a fairly straightforward beam bridge.

I do wonder about the strength of the pier legs though, if they were to be impacted by large debris in flood conditions. They are also curiously blackened at their bases, presumably a result of abrasion from material in the river, and it's hard to see how this will be anything but a huge maintenance problem.

I'm also unclear quite how the bridge articulates - the top of the pier legs is hinged perpendicular to the bridge alignment - is there a hinge at the base of the legs as well? The bridge has guide bearings at its abutments, again aligned parallel to the bridge alignment, and I would have thought this combination of restraints would lead to twisting movements under thermal expansion.

However, perhaps the strangest thing about the bridge is its amazing attraction to spiders. Pretty much every part of the parapet was covered in spider-web (with accompanying spiders) when I visited - what makes it such an attractive home to them? (And why can't the bridge owner keep a supposed tourist attraction free of spider webs?)

For me, these points don't detract from what's easily one of my favourite footbridge designs of recent years.

Further information:

• Google maps / Bing maps (Bing image visible at the time of posting shows the bridge under construction)
• Structurae
• Blogs: designboom, Dezeen, World Architecture News, dexigner
• Channel 4: Kevin McCloud And The Big Town Plan (includes photo gallery with construction photos)
• McCloud blasts changes to Castleford bridge (Building Design, 2006)
• McDowell + Benedetti's Castleford Bridge, part of Kevin McCloud's Big Town Plan (Architect's Journal, 2008)
• Masterpieces: Bridge Architecture + Design (2010) (see review)

Bridges news roundup

Plans revealed for Cumbrian bridge wrecked in floods
Capita Symonds design revealed for new Navvies Bridge. It reminds me a bit of Parson Brinckerhoff's Redhayes Bridge.

Leamouth Bridge plan still in the pipeline
Leaside Regeneration are still trying to drum up funding for an unusual opening footbridge, planning permission has been extended while they wait (see my previous post for more details on this bridge).

Ordinary house-hunters have the chance to become lord or lady of a remarkable Banffshire estate
More importantly, if you have £2.2m to spare, you could become the proud owner of Scotland's version of the Pont du Gard, the little known but thoroughly splendid Craigmin Bridge, described by one author as "a wildly picturesque, two-storeyed, wavy-parapeted practical folly".

Budget for new Bay Bridge span nears $2 billion
So, what is the most expensive bridge ever built?

How Britain Bridges the World
Documentary for the History Channel will feature Michel Virlogeux, Keith Brownlie, Norman Foster and others.

Port Authority commits $1B to Bayonne Bridge
How do you raise the clearance on a mammoth bridge like this? Options include everything from raising the road deck, to adding a lift span in the middle.

Bridge architect Miguel Rosales lands commissions for three pedestrian bridge projects in Cleveland
There are some engineers involved too, apparently. One of the bridge proposals is pictured below.

Providence River Pedestrian Bridge Competition

Don't jump out of bed too quickly for this one.

For one thing, having only just read about it in some three-day old news, it's already pretty much too late to put in a prequalification submission: the deadline is 4pm EST tomorrow (September 17th).

For another, it's yet another contest where the client is expecting lots of effort, for little reward. Entrants who are shortlisted (there may be as many as ten) are given a month to prepare their entry, and rewarded with a paltry US$500 for their time. There seems to be a real commitment to build the bridge, but while the winner will receive some form of design appointment, its scope is unclear. There's no prize money.

The bridge is part of a scheme to replace the I-195 highway bridge, which is to be partially demolished. The new pedestrian bridge is to be supported on the old bridge's piers, left in place.

The project website states that the bridge is to 450 feet (137m) long and at least 20 feet (6.1m) wide. At least US$2m is available for construction, although the promoter is seeking funds to double that amount.

Eight things you didn't know about this blog

Do you read this blog through an RSS newsfeed reader, or by an email subscription?

If so, I thought I'd remind you of what you miss if you never visit the blog's home page at http://happypontist.blogspot.com/.

On the right of the home page, you'll find my "profile" which includes my email address if you want to tell me bridge-related news.

You can see all my previous posts sorted by date ("Blog archive"), or subject ("Labels").

You'll also see "Related links", websites hopefully of interest to readers. Check out the Bridge of the month competition, or help out fellow bridge engineers at the Eng-Tips forum.

"My blog list" shows other bridge-related blogs which may be of interest, and you can see at a glance which ones have anything new to read.

If you don't use an RSS newsreader or are fed up opening the blog page regularly to check what's new, you can now subscribe by email to get a message every time there's a new post. Again, the link is on the right-hand side!

From the home page you can also search all my past posts (all 292 of them).

If you do read the blog at its home page, don't forget you can click on any image to see it at its full size. You can also comment on posts (please do!) by clicking the link which appears below each post.

York Millennium Bridge

I first visited the York Millennium Bridge several years ago, but had been waiting to get some better photographs before featuring it here. It's a remarkable and fascinating structure, and I'm glad to finally be able to discuss it.

It was built in 2001 for £4.2m, by C Spencer Ltd. The bridge was designed by Whitby Bird, now part of Ramboll. Six years previously, they had designed an inclined arch bridge in Manchester, but the York structure is far more technically daring, if also a little less logical in its conception.

In Manchester, their Merchants' Bridge design had the arch inclining away from a curved deck, such that both elements counterbalanced and reinforced each other. In York, the deck is straight, and the decision to incline the arch is therefore more arbitrary. I don't know who was first to design a bridge of this type, but Calatrava's unbuilt Gentil Bridge of 1988, and his La Devesa Footbridge, completed in 1991, are amongst the earlier precursors.

The York Millennium Bridge is 150m long in total, with a main span of 80m. The structural steel deck has a clear width of 4m, and is exceptionally slender at its free edge, largely because its main structural element is a torsion box running along the suspended edge and hidden beneath seating. As well as stiffening the arch and distributing vertical loads on the deck, the box resists the twisting that's caused by out-of-balance live loads on the deck.

However, the bridge's most interesting feature is clearly the arch, inclined at a very flat 50° angle to the horizontal. Both the arch and the hanger cables are stainless steel, and the arch has a very slender cross-section, at only 200mm deep (it's a welded box, 600mm wide). The cables are staggered so that some cables attach to the front of the box but rear of the deck, while others attach to the rear of the box, but further forward on the deck. This helps prevent the arch from buckling out of plane.

The arch is protected against wind-induced vibration by the presence of two tuned mass dampers.

The arch forms a segment of a circle, which is projected onto the ground as an ellipse and used to define the geometry of approach paths at either end (a nice conceit, but perhaps irrelevant and invisible to the bridge users).

I was amused to read online that a study by the bridge owner suggests de-icing salts should not be used to clear snow from the bridge in winter, because they will accelerate corrosion of the bridge's steelwork. A council officer is quoted:

"In order to make the best use of the limited maintenance resources and not compromise the effectiveness of the coating, the council’s bridge engineer would need to be completely convinced any treatment would not be detrimental and would oppose any proposed treatment involving the application of a corrosive de-icing material."

Well, duh. How did they think they would maintain the bridge when it was built? What is the paintwork on the bridge for if not to protect it against corrosion? Attempting to defer the inevitable repainting of a 9-year old structure may be understandable on pure cost grounds, but failing to keep the structure clear of ice seems a crazy consequence to accept. In any event, there are surely deicing products available which are less corrosive than the traditional road salts.

Overall, I think the bridge is attractive and well-detailed. However, the arch becomes somewhat awkward at its junction with the deck. Structurally, it's a tied arch, with the thrust from the arch withstood by the deck rather than by the bridge supports. However, as can be seen in the photo on the right, part of the arch member continues downwards past the deck for a considerable distance. This "tail" is present at both ends, and from some angles is quite disconcerting - it looks like the arch is supported in thin air. I imagine that the detailing of the arch/deck connection was geometrically very awkward, and this is a way of visually resolving it, but it's still unfortunate.

The support pier also seems oddly shaped, to say the least. I suspect the aim is to reduce the visual mass of the pier by incorporating a small corbel at the top (see especially the photo at the top of this post), while doing everything possible to continue the line of the arch thrust to ground. Accordingly, the pier is inclined both towards the arch and towards the river, even though in fact it mainly resists a combination of vertical load and overturning load. Structurally, it's an inefficient arrangement, but I'll bet the more efficient vertical pier (without corbel) would have looked completely wrong.

Further information:

• Google maps / Bing maps
• Structurae
• Wikipedia
• LUSAS Case Study
• York Millennium Bridge - A Footbridge with an Inclined Arch, UK (Mairs, SEI, 2001)
• Bridge Builders (Pearce & Jobson, 2002)
• Pedestrian Bridges in Stainless Steel [PDF] (Helzel, euro inox, 2004)
• Critical Analysis of York Millennium Bridge [PDF] (Isaacs, Bath Student Bridge Conference, 2007)

Worcestershire Bridges: 5. Holt Fleet Bridge

Continuing north from the Sabrina Bridge, the last structure I visited on this particular trip was Thomas Telford's Holt Fleet Bridge, which is the next span across the River Severn (there's an 1844 iron arch bridge at Bevere Island, but it only spans part of the river).

Telford's cast iron arch at Holt Fleet was completed in 1828, and spans 46m across the Severn, carrying a roadway. It was built by William Hazledine, and is now Listed Grade II. It replaced a ferry, and is now the only highway crossing in the 12 mile stretch of the Severn between Worcester and Stourport.

As originally built, the bridge had five cast iron ribs supporting the deck via X-shaped lattices. The ribs were cast in 6.8m long segments. The bridge was strengthened and widened in 1928, by encasing both the upper and lower members of each 1m deep rib in a slab of reinforced concrete. The more vertical spandrel struts were also encased in concrete, and the original 32mm thick iron deck plates were replaced with a concrete beam-and-slab deck. I imagine that access for inspection and maintenance between the two arch slabs must be difficult if not impossible.

The concrete works have deteriorated considerably since then, with a 7.5 tonne vehicular weight limit recently placed on the bridge following inspection and assessment work. At the time of writing, considerable work is planned throughout 2011 and early 2012 to refurbish the bridge, with design by Halcrow. This will involve essentially reconstructing the reinforced concrete struts from scratch. The photos here show that the condition is poor, with extensive spalling, and rust staining in places.

It will be good to see the bridge improved from its current somewhat dilapidate state. Nonetheless, it's a fine structure, and a good example of Telford's work. There's a similar Telford bridge at Eaton Hall in Cheshire, which preserves the cast iron spandrel tracery and is much better looking, plus of course his classic span at Craigellachie. Galton Bridge, at Smethwick, is also cited as similar, but seems to me to have a different spandrel bracing pattern.

Further information:

• Google maps / Bing maps
• Structurae
• Engineering Timelines
• Wikipedia
• Civil Engineering Heritage: Wales & West Central England (Cragg, 1997)
• A Century of Bridges (Witts, 1998)

ARC Wildlife Crossing Competition Shortlist announced

The ARC International Wildlife Crossing Competition has announced five shortlisted design teams, chosen from 36 entrants, to go forward to the contest's second stage. I've covered the competition here previously: entrants are to design an innovative wildlife crossing over Interstate Highway 70 in Colorado, USA.

The shortlisted teams are:

• Balmori Associates (New York), with StudioMDA, Knippers Helbig Inc., David Skelly, CITA, Bluegreen, John A. Martin & Associates, and David Langdon
• Janet Rosenberg & Associates (Toronto), with Blackwell Bowick Partnership, Dougan & Associates, and Ecokare International
• Michael Van Valkenburgh & Associates (New York), with HNTB and Applied Ecological Services
• The Olin Studio (Philadelphia), with Explorations Architecture, Buro Happold and Applied Ecological Services
• Zwarts & Jantsma (Amsterdam), with OKRA Landscape Architects, IV-infra and Planecologie

Details of the second stage competition process are available on the ARC website. They seem to have a well-informed jury (including Marc Mimram), and reasonable prize money, although much of that will be eaten up by the need for entrants to deliver a physical scale model of their concept.

The organisers are very keen to promote innovative use of materials, so it will be interesting to see what emerges in January when the winning design is to be announced.

Worcestershire Bridges: 4. Sabrina Bridge

From the brand-new Diglis Bridge, head north through Worcester town centre. First you'll come to John Gwynn's 1781 road bridge, the only road bridge in Worcester. Next, it's the Worcester to Hereford Railway Bridge, built in 1860 (with its river spans replaced in 1905).

The next bridge is, like Diglis, a cable-stayed footbridge, the Sabrina Bridge. This was built in 1992 for £617,000 by Morrison Shand Construction Ltd, designed by YRM Anthony Hunt Associates. As at Diglis, it has a single A-frame tower on the west river bank, with an asymmetric span arrangement. The main span is 62m, with a 3m wide deck.

The bridge is generally in steel, with timber deck planks with added anti-slip surfacing. When first built, the bridge attracted criticism for its slippery surface (presumably untreated timber), which led to a number of insurance claims and had to be replaced.

The really interesting feature of this bridge is the form of its deck, which consists of a series of steel tubular trusses spanning between the points of cable support. The deck is hinged at the connection points. This converts the bridge from the static indeterminacy of a normal cable-stayed bridge into a determinate structure.

This simplifies design, and also the construction of a bridge, as the cable stiffness has no effect on the stresses within the deck, allowing the cables to be adjusted freely during construction. The bridge then largely comprises a series of rigid triangles pinned to each other.

However, this is not a structural approach I've seen anywhere else. The hinges may be difficult to maintain, and the advantageous load distribution offered by a continuous stiffening girder is lost. You might expect the bridge to be more vulnerable to dynamic excitation, but I didn't find that to be the case.

An asymmetric cable-stayed bridge results in significant out-of-balance horizontal reactions at the end of the deck, due to the force in the deck balancing the main span cables. On Diglis Bridge, I guess this force is taken into the pylon base and then perhaps balanced against the back-stay anchors via a buried strut. At Sabrina, the force is tied back by diagonal bracing members visible in the photo above right, running from top right to bottom left.

The pylon is of the simplest form possible, and is noticeably more slender than on the Diglis structure. It's adorned with the arms of Worcester's twin town, Kleve (in Germany), but this doesn't detract too much from the pylon's appearance. Personally, although Sabrina's pylon is slimmer, I prefer the more sculpted form of the one at Diglis.

Sabrina Bridge is an unusual structure, and I'd be interested to know of any other cable-stayed bridges which share its hinged deck form.

Further information:

• Google maps / Bing maps
• Structurae
• A Century of Bridges (Witts, 1998)

Worcestershire Bridges: 3. Diglis Cycle Bridge

Diglis Bridge is one of a series of structures being built around the country with funding from Sustrans, who secured £50m of government money in late 2007 for their nationwide Connect2 scheme. I've recently featured another example, in Bradford. The Diglis bridge links cycleways and footpaths across the River Severn in south Worcester. It struck me that there are surprisingly few bridges in Worcester over the Severn, with this being the second of two footbridges to be built, and with there being only one highway bridge within the urban area.

It's the most modern of the various bridges I saw in Worcestershire, indeed, they had only just opened it in time for my visit (it opened on 20th July). The 66m span bridge cost £1.8m, and was built by Alun Griffiths to a design by Yee Associates and Mott MacDonald. The steelwork was fabricated by Rowecord.

The bridge's 28m tall steel A-frame pylon inclines toward the river at 22.5°. I'm not sure whether that's for engineering, aesthetic, or commercial reasons.

In terms of the structural engineering, it complicates erection of the bridge, but makes the cables slightly more effective in supporting the bridge deck, as they are closer to the vertical. However, the rear cables are less efficient, as their angle to the tower is reduced, and they must also support part of the weight of the tower. Writing in 1977, Michael Troitsky knew of only one cable-stayed bridge with a tower inclined towards the span (the Batman Bridge, in Tasmania), and I think it's probably still very rare today.

Aesthetically, I think it looks somewhat awkward. I instinctively feel that the tower should look as though it is helping to hold up the bridge. If it is going to incline, it should tilt away from the span, like a man leaning back to pull the reins on a horse.

I suspect, however, that the real reason is commercial - it may allow the cable back-stay anchorage to be brought closer to the foot of the tower, which can reduce the need to purchase additional land.

The pylon legs and the pylon crossbeam are both diamond-shaped in cross-section, as are the main deck edge members. It's an attractive feature, in particular bringing the edge of the deck to a nice crisp edge. However, looking closely along the edge of the deck, it's apparent they didn't quite manage to achieve a perfect smooth curve when the various deck sections were bolted together (it looks like it's fully welded, but the project's construction newsletters say that bolted joints are hidden under cover plates).

The pier at the end of the deck continues the diamond section theme, but unfortunately the approach ramps do not, leading to an untidy junction as shown on the photograph on the left.

The rest of the bridge is unexceptional although reasonably well detailed. The bridge decking consists of aluminium planks, while the parapets are of a post-and-wire type that's now very common, with the main feature of interest being the "Y"-shaped arrangement for the posts (surely not a nod to the architect's surname?) Lighting is recessed into the parapet handrails, as now seems de rigueur.

Overall, it's an unspectacular but attractive structure. It wasn't especially busy when I visited, but to a certain extent it's just one stage in a wider plan to expand footway and cycle links south from Worcester along the Severn. It will be interesting to contrast it with Worcester's other cable-stayed footbridge, which I'll cover in the next post.

Further information:

• Google maps / Bing maps (aerial imagery visible at the time of posting dates from before bridge construction)
• Worcestershire Council bridge website (includes construction newsletters and photographs)

Bridges news roundup

New bridge plans attract criticism
Will Perth cycle bridge ruin golf course?

How we construct our reputations
Does the reputation of engineers stand or fall with their structures?

Elements of new Frick lab join to create 'best infrastructure' for chemistry
One of the new works now opening at Princeton University is Christian Menn's excellent Streicker Bridge, a deck-stiffened arch design with ultra-slender steel pipe supports. There's a lot more on this splendid design here, here and here.

Newport cuts it fine for Ryder Cup
New station footbridge by Atkins and Grimshaw looks like two giant hairdriers mating.

Bradford Manchester Road Bridge goes for planning

Proposals for a new £3m cycle bridge in Bradford have been submitted for planning consent. The structure replaces an existing footbridge which has steep ramps and is too narrow for cyclists, and is funded by Sustrans as one of their Connect2 schemes. It forms a link in a proposed "living street" (which appears to be code for a tree-lined pathway). If planning consent is granted, work on site should start in 2011, with the bridge completed in 2012.

The planning application (currently available online - search for "Manchester Road Bridge") reveals that the bridge has been designed by Bradford Metropolitan District Council's in-house engineers and landscape designers. It takes its inspiration from a series of local bus shelters designed in a purportedly Deconstructivist style. The design and access statement tells us, for example, that the bridge columns "have a simple human profile aimed at providing a positive link to the ground", which is something I always find nice about columns.

The bridge's only feature of interest is a distinctive angular gateway which extends one of the columns above deck level (17m tall in total), but serves no structural function. The structural logic seems somewhat odd, since this column is positioned in the middle of the roadway, and it would clearly have been possible to omit this support position entirely and suspend the bridge deck from a pylon to one side, which would also fulfil the gateway feature.

The choice of red paintwork (matching the bus shelters) is a bold decision, especially for a structure which appears so keen to adopt an "industrial" look. To me, the whole effort is uninspired and unattractive, a step backwards visually, and certainly a lost opportunity to exploit the opportunities of structural engineering. The use of the non-structural gateway reminds me of some footbridges in Stoke-on-Trent, a bit of a throwback to the 1970s.