26 October 2011

Merseyside Bridges: 7. Silver Jubilee Bridge

Righty-ho, from Liverpool, this tour heads inland, following the course of the River Mersey, although with a sharp detour coming up soon.

In Liverpool, there are no bridges across the river, only tunnels below it. Heading east, the next opportunity to cross comes between Runcorn and Widnes. The River was first spanned here in 1868 by William Baker's Runcorn Railway Bridge, with three 93m spans, bridged by wrought iron lattice girders. It's visible in several of the photographs.

The second bridge here was the Widnes-Runcorn Transporter Bridge, opened in 1905. This was the first (and longest) of the only four transporter bridges to be built in Britain. In July 1961, the bridge now known as the Silver Jubilee Bridge was completed (originally the Runcorn-Widnes Bridge), with the transporter bridge being demolished immediately thereafter.

This was designed by Mott Hay and Anderson, at almost exactly the same time as they designed Tamar Bridge. As I noted a month ago, the two bridges make for an interesting comparison. The Runcorn-Widnes Bridge spans 330m, while the Tamar Bridge spans 335m. Both structures are in close proximity to a historic railway viaduct, but very different solutions were chosen.

Three-span and five-span truss designs were considered for the Runcorn-Widnes crossing, but rejected in favour of a suspension bridge design. However, wind tunnel tests showed that eddies in the wind caused by the proximity of the Runcorn Railway Bridge could destabilise the suspension bridge, and it had to be redesigned with an unusually stiff deck to prevent this. Nonetheless, the client eventually preferred a steel arch bridge option. At Tamar, the opposite conclusion was drawn when it was shown that the height of the road bridge deck relative to the railway bridge sufficiently reduced the effect of wind eddies.

The arch design was not without its problems. A two-pinned arch similar to the Sydney Harbour Bridge was shown to have a natural frequency of vibration not dissimilar to the rejected suspension bridge option. Making the bridge continuous with its side-spans altered the frequency sufficiently to resolve this concern, and offered the added benefit that the arch could easily be built by cantilevering from either side. Two very similar bridges built in the 1960s were the Bridge of the Americas (344m span, 1962) and Laviolette Bridge (335m span, 1967).

One aspect of the Silver Jubilee Bridge's construction which was perhaps surprising for the period was that it was built from riveted steel, a method essentially then obsolete. It was certainly one of the last, if not the last, large bridge in the UK to be riveted.

Any attempt to judge the aesthetics of a bridge like this might be considered irrelevant. The form and appearance are determined almost entirely by considerations of load-carrying function and of the demands of construction. Nonetheless, the not dissimilar Sydney Harbour Bridge and Tyne Bridge (the latter also designed by Mott Hay and Anderson) have both become city icons in spite of their industrial appearance. The Silver Jubilee Bridge is less iconic but much of that is down to context: power stations and chemical plants define much of the landscape of Runcorn and Widnes, and the Silver Jubilee Bridge seems an inevitable consequence of this rather than something which stands alone.

Its closeness to the railway bridge renders it difficult to see in isolation, as will be apparent from the photos of the main elevation. I find the choice of colour unconvincing as well, it's a little sickly and not really in keeping with a bridge where its sheer scale means it dominates the largely flat landscape. A difficult bridge to love.

Further information:

23 October 2011

Merseyside Bridges: 6. Paradise Street Bridge, Liverpool

Wilkinson Eyre built their reputation as specialist bridge design architects with a series of spans where the art of structural engineering was allowed to be the star. The architect's skill lay in pushing the engineering outside its normal comfort zone, in establishing a sense of sublime poise, and in ensuring quality of detailing. Paradise Street Bridge, in Liverpool, represents a major departure from that approach, although not the only such example in their portfolio.

Designed in collaboration with Arup, this £2.4m covered footbridge links a multi-storey car park to a John Lewis department store. The span, approximately 60m, is unusually large for such a structure, although certainly not the longest (the Ney & Partners bridge at Esch-sur-Alzette spans further).

Images of the bridge that I had seen before visiting it were oddly disturbing. The bridge's lack of lateral symmetry appeared forced, with little in the way of obvious logic. From above, a better sense can be obtained of how it works. The bridge kinks to either side of a central axis, but is rotationally symmetrical, with glazing offering views in and out in the areas furthest from the main axis.

A similar sense can be obtained from below. It's more ordered than the crumpled-paper architecture of Frank Gehry, it has more in common with the crystalline geometry of Daniel Libeskind, but it's still more than a little uncomfortable in its disdain for rationalism. While you begin to get a sense of how it works structurally (it's a varying section steel box girder supporting a set of clad and glazed frames), it refuses straightfoward comprehension.

It becomes clear that it's a skewed, distorted cousin of Wilkinson Eyre's earlier Bridge of Aspiration, which introduced the basic idea of a disguised box girder supporting a series of continuously varying frames. It's only when you go inside the bridge that it really starts to make any sense, as it's immediately clear that the outlandish geometry is there simply to obfuscate the possibility of a simple linear crossing, where you can see from one end to the other where you are going, and consequently focus on your destination more than on the journey.

At Paradise Street, there is no clear view from end to end, and instead your attention is drawn to the geometry of the enclosed space, and its relationship to the world beyond. The varying angles of the bridge relative to the sunlight allow the already peculiar geometry to project new patterns against itself, varying with time, season, location and orientation.

As a piece of structural engineering, I hate it - if the relationship between engineer and architect was truly collaborative, that's nowhere made visible. As a way to walk between a multi-storey car park and a department store, it's an interesting and unusual experience.

Further information:

20 October 2011

Merseyside Bridges: 5. Stanley Dock Bascule Bridge, Liverpool

My Merseyside journey continued south from Bootle into the centre of Liverpool. Travelling along Regent Road, I crossed the entrance to Stanley Dock via this bridge. It's of the Scherzer Rolling Lift type, a patented design where the bridge deck rocks back onto a roller girder in order to raise it to allow vessels to pass through. It's a little like a rocking-chair, pulled back by mechanical arms.

I've previously reported on a twin-leaf example of the genre, Queensferry Bridge, although that is no longer able to open. The bridge at Stanley Dock has only recently been refurbished, at the cost of £600,000. It was originally built in 1932, and allowed to run into disrepair by the owners, Peel Ports, leading eventually to its closure in May 2008. In 2010, repairs were carried out, the machine house re-roofed, and everything given a lick of new paint. The work won a Historic Bridge and Infrastructure Award earlier this year.

The rolling girder can be seen in the photo, complete with copious riveted stiffening - the entire bridge sits directly on this when it opens. One advantage of the arrangement is that the centre of gravity changes position with respect to the point of support - the counterbalance can be adjusted to make sure that when closed, the bridge tends to stay open, and when fully open, it tends to stay that way. The photo also illustrates one of the things I like most about bridges of this vintage - you can't escape the visual evidence of its making and assembly. Modern bridge technology tends to erase rather than emphasise this aspect, and such bridges often appear as if wished-into-place without having experienced any birth pains.

The bridge's machine room spans the roadway, and is something of a blot on the landscape, although not without character. I've included a couple of links at the end to websites with photos of the interior.

The bridge isn't unique, even locally, as there's a very similar span, Egerton Bridge, on the other side of the Mersey at Birkenhead, although I didn't get to visit it on this trip. Examples elsewhere in the UK include those at Barrow-in-Furness, Inchinnan, Keadby and Poole.

The bridge sits amidst some spectacular disused dockside structures, most notably the absolutely enormous Stanley Dock Tobacco Warehouse building visible in the final picture. There is still a lot of industry nearby, but much of the area is derelict. It's clearly ripe for some kind of Docklands-style redevelopment, but presumably the money simply isn't there.

Further information:

19 October 2011

Merseyside Bridges: 4. Pennington Road Footbridge, Bootle

Ok, heading south along the coast from Southport, we come to Bootle, at the northern end of Liverpool.

In 2006, RIBA organised a bridge design competition for a replacement of a rather grim pedestrian bridge spanning the Leeds to Liverpool Canal in Bootle. They received 88 entries (some of which I've shown here before), and in early 2007 announced the winning design, from Softroom and Eckersley O'Callaghan.

Out of seven RIBA bridge design competitions that I considered in July 2009, this is the only one which has resulted in a bridge actually being built. Such a rare specimen clearly merits close attention. Several of the RIBA bridges foundered over funding, either because there never was any, or because of unacceptable cost increases. The Pennington Road bridge suffered from the same issue, costing £750k to build against an original budget of £400k. That's a very large sum for what is basically a very short span pedestrian bridge, so someone must have thought it was worth it.

The bridge spans east-west across the canal. At its east end, it ramps straight down to ground level. On the west side, the deck spills you out onto the top of a large mound, which can be descended to the west and south by staircases. Between the staircases, there are a series of switch-back ramps, some of which you can see on the right of the photo (click on any image for a larger version). This is a fairly horrid solution to providing mobility-impaired access, and it's far from clear that there wasn't enough space for a more direct ramp arrangement.

When it was built, the ramps had slopes with planting on them, currently being rebuilt in block-paving, presumably a response to vandalism, although not one that improves the bridge's aesthetic.

The staircases are bordered by timber walls, which give the impression of flowing seamlessly into the timber beams which carry the main bridge deck. The trough-type deck is bounded by two glulam timber beams, but the timber continuing down the ramps and stairs is essentially cosmetic.

These timber parapets support a handrail and also a post-and-wire fence. I'm not sure whether this is to provide a required balustrade height, or just to stop people skateboarding along the top of the parapet beams. I do like the jagged relationship between the handrail and parapet on the staircases.

The floor of the bridge is in timber as well, and I have to say it's great to see timber being used on such an "architectural" bridge. I can imagine there were a few questions asked about durability, resistance to vandalism, and fire risk, but the wood has worn well so far, with very little visible damage (although a number of recessed light fittings have been broken, unsurprisingly).

From underneath, the structural form is perhaps clearer. It's also possible to get an idea of the original, unweathered colour of the wood, which is much warmer in tone. I think this is one aspect of the bridge which is a little unfortunate: wood is often commended for its organic approachability, but as it silvers with age it does lose a lot of what initially makes it attractive. A darker timber may have worked better, as from above, the bridge does now look cold and washed-out.

You can't really tell from the soffit photo, but if you squint at the first photo you can just about see that the main span is simply supported at either end on very small lugs, which project out from the concrete abutments (this is made clearer on some of the images at the Eckersley O'Callaghan website linked below). Initially, this offended my structural engineer's sensibility, that the structural form and the geometric form should be closely partnered - the deck supports break up the sense of visual flow. However, it's a reasonable compromise in achieving the architect's concept, so nothing to worry about.

I was left with a sense of appreciation for the bridge's modesty. It has been built in a slightly run-down area where functionality and robustness are laudable aims, and it rightly avoids a level of flash that would never have suited the site.

Further information:

18 October 2011

Bridges news roundup

I thought I'd briefly interrupt the posts on Merseyside bridges to catch up with some bridges news from the last week or two ...

Happy birthday to a transport of delight
Middlesbrough Transporter Bridge celebrates its centenary. Nice little article from The Guardian.

Barnard Castle footbridge would be 'longest in UK'
265m suspension footbridge proposed. Close examination of the image supplied suggests a commendable desire to keep to keep it cheap and cheerful rather than unnecessarily embellished. I think that's a good thing, and it would be a remarkable span if they can only work out how to fund it.

Crazy Swiss Swingers Suspend Hot Tub from Bridge
Yes, yes, but which bridge is it?

Rehberger's Slinky Footbridge
Designer of new footbridge in Oberhausen interviewed. It was designed in collaboration with Schlaich Bergermann, and the artist's spiral enclosure notwithstanding, it's an impressive stressed ribbon bridge (the deck for the main span is 66m long and a mere 12cm thick). It's almost a shame, given the inspiration, that it doesn't appear to wobble. More details here.

Thousands of jobs secured as Mersey Gateway gets green light
This has been extensively reported elsewhere, but I thought a couple of points were worth noting. Can we take seriously the project director's quote that "Only six organisations in the world are capable of delivering this complex project." Really? Really? The local councils' chief executive also said: "We will keep the iconic design of the bridge but we will be challenging engineers to come up with different ideas and designs." It will be interesting to see what that means when tenders are submitted: there would seem limited scope for contractor value engineering unless at least some significant variation on the current design is permitted.

Pihl is building new bridge in Copenhagen
Danish contractor Pihl have been awarded the contract to build the Inderhavnen "Kissing Bridge". I've discussed the design, by Flint and Neill, Studio Bednarski, Hardesty and Hanover, and Speirs and Major, previously. It's an unusual opening bridge where the two halves of the main span retract along curved guideways.

Pennine Bridleway bridge design gains plaudits
It's an environmentally appropriate and structurally remarkable timber footbridge, built in the Yorkshire Dales, and already receiving awards. Its 24m main span is in stress-laminated timber, with short lengths of timber in laid in the form of an arch, and tensioned together with transverse tie rods. The pictures at outdoorsmagic.com make that a bit clearer. It has been recognised with a Wood Award and a BCIA Award.

City release new renderings of ped-bike bridge
Knoxville, Tennessee are planning a major river crossing, with the 200m span arch depicted being the preference from six options discussed in a detailed report. It's hard to believe it's the least expensive, and the arches as currently shown look unnecessarily bulky (possibly because they seem to want to do it in concrete rather than steel).

Council chief says ‘bold design’ of bridge will create ‘distinctive’ gateway to the city
Work to start on landmark Bradford footbridge. Bold, yes. Beautiful? Nah.

17 October 2011

Merseyside Bridges: 3. Venetian Bridge, Southport

There are three bridges which cross the artificial lake in Southport. I've already covered the first two, and the third is here for completeness rather than special interest. It has three sections, separated by two islands, all built in 1931, with concrete main structural members supporting a timber deck. They were refurbished in the early 1990s.

Further information:

16 October 2011

Merseyside Bridges: 2. Marine Way Bridge, Southport

Marine Way Bridge lies right next to Southport Pier, the subject of my last post. The contrast is remarkable. It's perhaps unusual to find a landmark bridge of this size located quite so centrally in a predominantly low-lying urban area.

Opened in 2004, Marine Way Bridge carries a local road across an artificial lake, which dates back to 1887. As well as improving highway links, it sought to provide a "catalyst for future investment and regeneration" in what is a somewhat run-down seaside resort. A previous road bridge had been demolished 15 years earlier, and the new bridge restored a key connection between the seafront and the main part of the town.

It's an asymmetric cable-stayed bridge, with the deck supported from an A-frame pylon sitting to one side of the lake channel. It has a main span of 80m, and is 150m long and 18.5m wide. The tower is 56m tall. Both the deck and the pylon are in reinforced concrete.

The bridge was designed by Babtie (now part of Jacobs) with Nicol Russell Studios, and built by Balfour Beatty. It shares a number of similarities to Nicol Russell's previous Leven Crossing bridge at Glenrothes. The design was reportedly the winner of a competition, although I've not found any details on that. The bridge won the ICE's North West Merit Award in 2005.

Seen in elevation, the tower gives the impression it is taller than it needs to be, with the forespan cables steeper than is normal in a cable-stay bridge. A 40m tall tower would be sufficient to get the average cable angle at 45°, which is more conventional. It's not clear whether the height was chosen to reinforce the structure's landmark status, or to reduce the horizontal forces in the deck, and hence on the foundations.

I never really like A-frame pylons, visually. I can't quite figure out what it is about them that I dislike. At Marine Way, an effort has been made to shape the head of the tower, but the legs themselves are still a bit too bland.

The underside of the deck is more successful, taking the form of a concrete ladder beam with two large edge beams and a series of crossbeam ribs. It's more attractive than the underside of many bridges, but from an engineering perspective, the crossbeams seem unnecessarily deep, and the whole thing just too heavy - a steel structure would have considerably reduced the cost of the foundations.

From above, the decision to anchor the cables directly into the edge beams, rather than through small cantilever stubs, seems a little unfortunate. It creates a level platform likely to attract people to hop over the main parapet, and hence needs protecting with awkward cables, as shown.

On the whole, it's certainly not an unattractive cable-stay bridge, but it does strike me as a little out of place, especially set against the other bridges which span this artificial lake (I covered one last time, and the other will be my next post). There seems no pressing reason to have to span the lake without intermediate piers, and an attractive bridge could have been designed which was far less expensive. Whether the bridge fulfilled its aim to act as a catalyst for regeneration, although looking at the surroundings, I doubt there is much that a more modest bridge wouldn't also have facilitated.

Further information:

13 October 2011

Merseyside Bridges: 1. Southport Pier

Okay, before embarking on a set of reports from a tour of Merseyside, I ought to point out straight away that I'm using the term in its broadest sense, a sort of Greater Merseyside which stretches from Southport in the north, along to Warrington at its easternmost extremity. I know locals get touchy about these designations, but really, it's just an excuse to go visit some interesting bridges and lump them all together here, okay?

I think Southport Pier is the first pier that I've featured here. Is a pier a bridge? A pier is clearly bridge-like in its structural form if not in satisfying the usual definition of "spanning an obstacle". Southport Pier, however, makes the decision easy, as not only does it jut out into the sea, but part of it also spans an artificial lake. So it's definitely a bridge.

It was designed by Sir James Brunlees, and opened in 1860. At 1108m long, it's the second longest pier in Britain, although a large part of its length is over land. It was even longer in the past, having been extended first in 1864, and then again in 1868. From the point of view of engineering history, its use of jetted piles was pioneering, but I'm afraid I just can't get that bothered about piles.

It was proposed to be demolished in 1990, but saved and Listed (Grade II) following a vigorous campaign. I find it hard to fathom why a seaside resort like Southport would want to demolish one of its main attractions, and I think they were perhaps swayed by unrealistic estimates of the cost of maintenance (a figure of £250,000 for repainting every 5 years was quoted).

One of the methods used to help save the pier was by a campaign to sponsor the planks in the deck, the legacy of which is clearly visible via individually named plates embedded in each plank.

The pier is described as having been refurbished, with the work completed in 2002, but it's clear that large parts of it were completely rebuilt. Looking close up at the main lattice beams, it's apparent they are welded, which clearly isn't part of any original construction. The current aerial image shown at Bing (linked below) also makes clear how extensive the rebuilding was. I guess the cast iron columns and wrought iron tie bar bracing may be original, and for me these are the main attraction of the pier - their delicacy is what makes it an attractive structure, as the modern decking and parapets, and the awful arched lighting supports, are certainly not very appealing.

The pier also carries a unique battery-powered tramway, added in 2005. The links below provide far more information on the pier's history than it would be sensible to recount here.

Further information:

12 October 2011

Where are they now? Part 4

Okay, I did three previous posts where I looked back at some of the bridge schemes I had covered in previous years, asking what had happened to them after I had mentioned to them. Here is the final such roundup, for now at least.

Kraków Footbridge
Now, here was a fine one. The winner of a 2006 design competition was much panned at the time, largely because its architect had designed something that couldn't possibly stand up. In November 2009, I noted the project had undergone an interesting metamorphosis, with the ultra-slender concept being substantially fattened up until a structure emerged which could actually cope with iniquitous gravity.

At the time, I commented that funding seemed uncertain, but there has clearly been enough money in the pot to treat the design very seriously indeed. There were a couple of papers at the recent Footbridge 2011 conference on the Kazimierz-Ludwinów bridge (to give it the proper title), both from the architect and the engineers. These showed clearly that considerable effort has gone into the structure's development, and I've used an image from one paper as the illustration (click to enlarge).

The engineer's paper reported that the bridge had a PLN 37m budget, but that the lowest tender received in 2009 was PLN 3m above that, leading to the scheme's cancellation. I'm not sure, however, whether that is the end of the story, or whether there is more life in this project yet.

St Patrick's Bridge, Calgary
The main bridge story in Calgary has been Santiago Calatrava's Peace Bridge, which has been a fiasco from beginning to end, except for the fact that the sorry tale seems so far to have no end in sight. I last discussed it extensively in April, reporting on delays caused by shoddy welding.

The footbridge scheme at St Patrick's Island was always seen as its better-organised neighbour, procured through an open design competition, and won in March 2010 by a bridge which obeyed the laws of structural common sense (pictured, design by Halsall and RFR).

That's pretty much all I can report. The bridge concept was approved by a city committee earlier this year, but I haven't seen any other news. The promoter's website suggests construction will start before the end of 2011, but I haven't seen any sign that construction tenders have been invited yet.

Four Mile Run
I was accused of being a little unkind to the US bridge design competition at Four Mile Run, when in November 2009, I used it as an example of "How not to run a bridge design competition". Amongst my complaints were the lack of any engineering information or justification for the structure; an opaque judging process; the lack of guidance as to what the promoters actually wanted; and the fact that no funding appeared to be in place to actually appoint a designer, let alone build the bridge.

In April 2010, the three contest winners were announced, with a proposal by Arup and Grimshaw coming top. I was too busy at the time to say much about the top three designs, all of which can be seen on the competition website. Since that announcement, the contest website has not been updated and I could find no further news, none of which is any surprise given how the competition was set up.

Since I was so remiss at the time, here are the three shortlisted designs:

Arup / Grimshaw / Scape

Olin / Buro Happold / Explorations Architecture / L'Observatoire International

Rosales + Partners / Schlaich Bergermann / Simpson Gumpertz and Heger

New Royal Victoria Dock contest
Very near the end of 2009, I discussed a "competition" being held for a proposed new crossing of Royal Victoria Dock in London, to improve pedestrian access to an exhibition centre during the 2012 London Olympics. Entrants were being sought via an odd sort of competitive interview where teams were set a design challenge and observed in how they responded to it. I never covered the subject again.

It turns out that the contest was won by Ian Ritchie Architects and Atelier One. They came up with several ideas, such as pontoon structures and opening bridges, as well as the one pictured here, described as a "water boatman" bridge, essentially a ferry or cable-car structure re-imagined in rotational rather than translational space. Apparently, all involved with the scheme agreed this was the option which would be taken forward, but pressure on Olympic project costs then led to the entire scheme being cancelled.

11 October 2011

Blogs roundup

It's time for a quick roundup of what you can read about bridges on various other blogs, as something of a stopgap before I embark on a set of reports covering nearly a dozen bridges in the Merseyside area, as well as catching up on several posts that I've had gestating for some time now.

Tallbridgeguy and Bridge Photo of the Day form something of a triumvirate of the more prolific bridge blogs (together with this one, of course).

TBG is the most diverse, with everything from off-the-wall design concepts through to the sort of practical day-to-day issues that actually take up an engineer's time. It even strays away from bridges quite a bit - I have a self-imposed rule not to do so (simply because I spend more than enough time on bridges without getting distracted!) TBG also writes the Google Sketchup for Engineers blog.

BPHOD is the Ronseal of the two blogs, often featuring bridges chosen from fact-finding trips around the world to areas which have been hit by earthquakes, which in recent months means India and New Zealand.

Tabikappa is a similar blog from Japan, although with a focus on photographs of bridges, hardly ever much text. They've been quiet recently, but returned with a quite marvellous double lenticular truss bridge.

ArchDaily chucks out posts like bullets from a machine gun, corpuscles of architectural gloss splattered across an internet canvas. They featured a bridge recently, Caltrava's Bac de Roda highway bridge, one of the Spanish designer's earlier and hence marginally more orthodox designs.

It's not a blog, but a monthly email, but Bill Harvey's Bridge of the Month is often interesting. Bill is an expert in masonry arch bridges, and generally finds an interesting technical angle on the bridges he covers. The most recent issue features Pyne Bridge, a very unusual set of brick arch spans supported on granite columns.

I can also recommend Bridgink's Bridge of the Month, a monthly "name that bridge" quiz with a book given away to one of the successful entrants every month.

Finally, tired of all these bridges? Perhaps you need to refresh yourself with the blog from the British Water Tower Appreciation Society. Or lift your head up high with the Pylon of the Month blog.

09 October 2011

Manchester Bridges: 18. Media City Footbridge

I had been waiting to find out when the new Gifford and Wilkinson Eyre-designed Media City Footbridge in Manchester was due to open to the public, only to discover on a chance visit at the end of September that it was in fact already open.

It's an asymmetric bobtail swing bridge, spanning across the Manchester Ship Canal. It's 83m long overall, with a 63m main span. To the north, it provides access to the new Media City development in Salford, the new home to several departments of the BBC. On the south bank, it currently connects to the Imperial War Museum North, but will eventually link in to a new ITV studio development.

The bridge deck is supported by a harp-style cable-stay layout. The cables support one edge of the deck, and connect to eight steel masts in a fan arrangement which I previously likened to a giant skeletal hand. I think perhaps the design visualisations were a little unflattering, as now that I've seen the (nearly) finished structure, it is a much more attractive creature.

The odd assemblage of masts arises from the formalism of the geometry. The cables establish a harp arrangement in elevation (i.e. they are parallel when viewed from that angle), and are tangential in plan to the curved edge of the deck. This creates a geometry where the cables must be supported at eight separate positions in space, rather than from a single mast.

The supported edge of the deck is a deep triangular steel box girder, paired with a shallow box below half of the deck itself. These provide the necessary torsional and bending stiffness, and support the other half of the deck via a series of cantilevered ribs, visible in the photo (click on any image for a full size version). The alignment of the edge girder is not quite perfect, but I don't imagine the casual observer will notice. The image also shows the flared width of the backspan, which conceals a concrete counterweight used to balance loads on the pivot pier.

The sharp edge of the box girder is such that it's depth is almost completely disguised, with the upper surface appearing as a streamlined fascia, and the lower surface disappearing into shadow along with the rest of the bridge soffit. On the face of the girder adjacent to the deck, glazed panels act to obscure it, and these conceal light fittings which help illuminate the deck at night and distract further from the beam's structural mass.

The cables connect the masts to the rear of the back-span, and the anchorage points have been adapted to provide jaunty little seats. Between the anchorages, users can walk straight into the future ITV site, although the steps and handrails were still under construction when I visited. People walking towards the Imperial War Museum bypass the anchorages completely. The detailing of the anchorages and seats is very nicely done. In the background of the photo you can see the Museum on the right, and on the left is the Lowry Millennium Bridge, which I've visited previously.

As with many Wilkinson Eyre designs (see for example South Quay and Swansea), a conscious effort has been made to treat the two bridge parapets differently. I've already mentioned the east parapet, which does an impressive job of hiding the main deck girder. While I was on site, there was at least one panel of the lightbox where the glass had shattered, and several more temporarily dismantled - I'm not sure whether this was to work on the glass or on the light fittings. This is a reasonably well-used and well-policed location, so I would hope that vandalism isn't a recurring issue.

The west parapet is a post and rail system in stainless steel, with some lighting concealed in the uprights. You can also see in the photograph above right how the deck surfacing is divided, with a resin/grit finish to the underlying steel box girder on the right, and aluminium decking panels on the left.

One element of the bridge I particularly like is the detailing of the mast pedestal (the large silver cylinders are uplighters). Each mast sits on a stainless steel ball bearing. You can also see the bottom edge of the mast, which is lens-shaped in cross-section. The cigar-like masts have been cut, bent and re-welded from basic circular hollow sections, and I think the result is visually very attractive.

Not all the detailing is perfect. I particularly disliked the cover plate to the north expansion joint, which is in two hinged sections to be manually flipped out of the way before the bridge is opened. It looks like an afterthought rather than something properly considered.

The bridge's main visual feature is the way the mast and cable array changes as it is viewed from different perspectives. The simple harp-like arrangement of the cables is clear only from the main elevations, and from other angles it is clear that their geometry is much more complex. The photo above right shows that they appear warped when viewed from the north end of the bridge.

Not every part of the bridge adjusts to changing perspective so well. The deck's vertical curvature is a careful compromise between canal navigational clearance demands and the geometry of the canal banks, with the deck ramping down at 1:15 to meet the ground. While the curve looks gentle from most locations, the S-shaped curvature of the west edge of the deck is such that in a foreshortened view, the deck looks to have broken its back. The same thing can be seen on the Tradeston Bridge in Glasgow, which shares the S-curve in plan. I doubt that anyone other than over-fussy bridge designers will spot it, however.

Quibbles aside, I was very pleasantly suprised by the Media City Footbridge. I had completely underestimated the quality of the design beforehand. I would now say it is well up there amongst the best major pedestrian bridges built in the UK in modern times, a genuine rival to the same design team's Gateshead Millennium Bridge. It has been shortlisted for an Institution of Structural Engineers Award, and deservedly so.

Further information: