Manchester Bridges: 24. Brabyns Park Bridge, Marple

Now here's a tale of woe.

Nathaniel Wright exemplified the manner in which the industrial revolution wrought huge changes on British Society at the end of the 18th and beginning of the 19th century. He made his fortune from coal mining in Derbyshire, and associated closely with other entrepreneurs such as Samuel Oldknow, who was the main driver behind construction of the Peak Forest Canal, connecting the Derbyshire coal mines through to Manchester.

In 1800, while the Canal was just finishing work on Benjamin Outram's Marple Aqueduct, Wright purchased the Brabyns Hall estate nearby. The estate lies immediately to the east of the Peak Forest Canal, occupying the land between the canal and the River Goyt. As one improvement to the estate, Wright commissioned Salford Iron Works to construct a bridge across the river, providing access to and from nearby Compstall village. The Salford firm's engineers James Bateman and Thomas Sherratt had previously in 1795 supplied a pumping engine for one of Wright's collieries.

The new bridge was in the form of a cast iron arch, possibly the first such bridge in this part of England. The design was broadly patterned after Thomas Telford's 1797 Cound Arbour Bridge, with three cast iron arch ribs, each with a series of iron circles filling the spandrels. Variations on this theme were common, another example being the 1810 Tickford Bridge, but Telford had moved on, using the more rational diamond-pattern spandrels in his 1814 Craigellachie Bridge, for example.

The bridge is decorated with the date and name of its maker, and the lightweight iron parapets feature an ornamental "W" in the middle, for the owner Wright, who died five years after the bridge's completion, in 1818.

So far as is known, the Brabyns Park span is the only bridge to have been built by the Salford Iron Works. It spans approximately 14m (46ft) and is 2.7m (9ft) wide. The central rib was cast in two halves, joined in the middle. The external ribs were cast in six pieces, with each half subdivided into three parts: an upper rib, a lower rib, and the circular spandrel elements. The arch ribs were stabilised by tie bars at their lower level, and by cross-bracing at the upper level. The original decking comprised timber planks.

 In 1991, a structural assessment of the bridge labelled it dangerous, and a "temporary" bailey bridge was installed directly above it, remaining there until 2007, when the bridge was finally refurbished, following a lengthy campaign. Photographs of the bridge taken in 2007 immediately prior to refurbishment show the bridge to be horribly neglected, although how much of that was due to the difficulty of maintaining it with the bailey bridge in the way is hard to tell.

The bridge is Grade II Listed, but it was a lengthy battle to secure Heritage Lottery funding for its restoration, and then to agree exactly what works would be carried out. A design by engineers Scott Wilson Kirkpatrick (now submerged into AECOM) was eventually accepted after much debate.

In 2003, the campaigners had commissioned a report by local engineering lecturer and historic bridge specialist Tom Swailes. According to the report, the main issue with the bridge when it was closed in 1991 was the condition of the timber decking. Swailes took the view that:

"Future loads on the bridge will be no greater than the loads that it safely carried for 178 years. It is interesting to apply modern computer-based structural analysis techniques to old structures, but the results of such theoretical analyses must almost always be disbelieved unless verified by tests on the structure itself."

The subsequent involvement of "proper" consulting engineers seems to have been a curse. The designer's drawings proposed significant alterations to the bridge, over and above what may have been required for simple restoration. As Swailes noted in his report, despite its neglect the bridge had stood the test of time.

That was clearly not enough. The main structural alterations involved the bonding of steel plates to the central arch rib, the renewal of corroded bracing elements and the addition of new transverse bracing between the arches. These have relatively minimal impact on the appearance of the bridge, and you may struggle to see them on my photographs. In any event the underside of the bridge is not seen by most people, and is difficult to view even from the river bank.

The larger issues for the engineers (or for the technical approval authority at Stockport Council, perhaps), were the presence of a gas main, a small water main, and electric cables, which seem to have been previously laid on top of the deck planks; and the height and strength of the existing parapet railings, which were now judged to be inadequate.

Measuring from the engineer's drawings, the original parapets are approximately 1.15m tall (although it will have been less when fill material had been piled onto the bridge decking to cover the gas main). This could not possibly do, the parapets had to comply with modern standards and be 1.4m tall (the modern requirement for cyclists). The solution to this, and also to accommodating the utilities, was to install new parapets connected to steel hollow sections sitting entirely on top of the existing bridge, with the hollow sections also used to contain the utilities.

The campaigners were evidently not very happy. On their website they indicated their regret:

"This means that in the future, perhaps with technological improvement or an enlightened approach to safety standards, or a combination of both, the modern intervention could all be removed and the old bridge will be revealed intact underneath."

No such technological improvement was ever needed: simply an "enlightened approach" that recognised that an existing historic structure could be considered to be perfectly acceptable whether or not it complied with modern standards. Nobody is going around rebuilding ancient castles (or installing 1.4m tall balustrades on the battlements) simply because the original builders couldn't foresee the stupidity of 21st century compliance-drive engineering culture.

Quite what the local planning authority, or English Heritage (now Historic England), thought is difficult to say. They certainly should have demanded that the engineers think again. Reading through the excellent restoration campaign website, one thing that jumps out at me is the absence of any mention of a specialist conservation engineer or conservation architect. The result, in my opinion, was deeply unfortunate.

The new parapets are painted a different colour to the original bridge, with a view to disguising them when the bridge is viewed from a distance. I don't think it succeeds: the elevation of the bridge is intact but the parapets are hard to miss. Seen from the perspective of crossing the bridge, the visual impact is much worse, essentially ruining what was a fine and historic structure. Reaching over the new parapets to push against the old ones, it seemed to me that the original parapets were quite strong enough.

The difficulty with hoping that this can be put right in the future is that not only was the opportunity lost at the time when funding was available, but that so much of the funding was seemingly unnecessary. It is perhaps unfair to criticise those involved when not in possession of all the information that they had, but at best the treatment of the Brabyns Park bridge is a crying shame, and at worst it's the inevitable but disastrous outcome of foolishness and incompetence.

Further information:

• Google maps
• Grace's Guide
• British Listed Buildings
• A Brief History of the Park, the Iron Bridge and the People Involved in its Construction (Whittaker and Clarke, 2006)
• Bridge restoration campaign website (well worth a visit - hugely informative!)
• Bridge restoration photographs

Manchester Bridges: 23. Marple Aqueduct

Yes, yes, I know, it's not really in Manchester. Want to make something of it? No? Good.

It's sometimes difficult to imagine how the pre-industrialised landscape would have looked near the end of the 18th century. Some canals had been constructed earlier (e.g. the Bridgewater Canal opened in 1761), but canal mania only really took off in the century's final decade. The Peak Forest Canal, authorised by an Act of Parliament in 1794, was part of that explosion of canal-building.

So far as was possible, canals needed to be level to be economically viable. The Peak Forest Canal connected mines at Dove Holes to Manchester, with two level stretches of water (the Upper and Lower Peak Forest Canals) separated by a 64m tall flight of locks at Marple. Immediately south of the locks, the Lower canal crosses the valley of the River Goyt, and between 1794 and 1800 a huge aqueduct was built here.

It must have been a tremendous endeavour, considering the scale of the stone blocks that make up the finished structure. The enterprise was largely promoted by industrialist Samuel Oldknow, and the engineers were Benjamin Outram and Thomas Brown, and William Broadhead, Bethel Furness and William Anderson were appointed as the contractors. It is claimed to be the tallest masonry aqueduct in the United Kingdom, and over the six years of construction, seven construction workers died.

The aqueduct's most distinctive visual feature is the series of circular openings in the spandrel walls of all three spans. These were presumably intended to reduce weight, and were not a new idea: William Edwards had used them for his fourth and final bridge in Pontypridd in 1756; John Smeaton included them on Coldstream Bridge in 1767, albeit filled with loose stone. No doubt there are other examples. They add visual interest to the bridge but I don't think they lighten its appearance: compare Rennie's Lune Aqueduct, completed in 1796 while the Marple structure was being built, which scarcely looks much heavier.

Tie bars were inserted in the central span in 1860 by engineer Charles Sacre. Use of the canal had declined during the 19th century, with Marple Railway Viaduct being built alongside the aqueduct in 1863.

Decline eventually lead to ruin, with one spandrel wall of the aqueduct partially collapsing in January 1962. British Waterways proposed demolishing the structure and replacing it with a pipeline, but additional funding was found from the local authority to rescue the aqueduct, and in 1966 the restored bridge was declared a Listed Building, Grade I. The canal itself was rescued from dereliction and reopened in 1974.

The aqueduct was altered again in 2018. Until that point in its history, the aqueduct had only one parapet, on the towpath side. The other side had only a very low wall. According to the Canal and River Trust, people had been recorded nearly falling from that edge, and some people were even playing a dangerous game of leaping across the canal to land on the unsecured edge.

It was agreed to add a new parapet, designed by Knight Architects and Arcadis, and manufactured by Bisca. This is a bespoke design in stainless steel, with the top rail left unpainted, and uprights painted matt black. In a nod to the Peak Forest Canal's benefactor Samuel Oldknow, the arrangement of the metalwork evokes the warp and weft of cotton-weaving.

It feels a little odd, having such high-quality metalwork in a place where the ordinary public are remote from it, but it must have been some challenge to navigate the demands of parapet standards and the difficult constraints of modifying such a historic structure.

Viewed along the aqueduct, the new parapet gives an impression of solidity, while viewed looking across, it is much more transparent. Engineers will note that the location where the woven bars intersect is the weak point. I did wonder if the design is not a little over-done when something more minimal may have sufficed, but I didn't find it too objectionable.

Further information:

• Google maps
• Wikipedia
• Structurae
• British Listed Buildings
• Engineering Timelines
• Grace's Guide
• CanalPlan
• Transport Trust
• Restoration of the Marple Locks and Aqueduct
• The National Trust Book of Bridges (Richards, 1984)
• Civil Engineering Heritage: Northern England (Rennison, 1996)
• An Encyclopaedia of British Bridges (McFetrich, 2019)

"Ordsall Chord - Manchester's Missing Link"

This is the third and last of a set of "souvenir" books I'm featuring which document recent major UK bridge projects.

The Ordsall Chord is quite a different beast to either the Mersey Gateway or the Queensferry Crossing, with the largest bridge span a relatively modest 89m. The Chord is a new railway connection linking Manchester's Piccadilly and Victoria stations, and although there were evidently plenty of bridge works involved, the nature of this book makes clear that it was more of a multi-disciplinary project in nature. The civil engineering construction accounts for only 38 pages, with substantial chapters given over to other topics such as railway signalling, track and overhead electrification.

The Ordsall Chord - Manchester's Missing Link (Mercury Group Limited, 2018, 168pp) was written by members of the project delivery team, and this gives it quite a different slant to either of the other two books I've featured, with a strong focus not just on the project objectives or the construction efforts, but more material on how the project was procured and organised. I think this book is therefore potentially of more interest to professionals than the general public, although I'm sure there are many railway enthusiasts who'd enjoy it.

The foreword to the Mersey Gateway book is by a politician, while the Queensferry Crossing book has multiple forewords from different perspectives. The Ordsall Chord book is introduced by the lead director from the delivery organisation, who introduces a theme that runs throughout the book, "great people working collaboratively". I think it's a interesting that this needs to be highlighted - it suggests that the construction industry is often populated by not-so-great people, not working collaboratively, so that anything else is an anomaly.

The main part of the book opens with a short chapter explaining the history of the site (the project runs right across part of the historic 1830 Liverpool and Manchester Railway), and the need for the new railway line.

The heritage theme continues into a chapter exploring the project's constraints and how the design was developed. This is a better attempt at explaining design issues than in either of the other two books, crediting and naming specific individuals rather than submerging them in corporate anonymity, exploring the challenges of working amongst numerous protected heritage structures, explaining how visual sense was made from a disparate variety of structural forms, and of how the architecture and engineering work in conjunction. The focus is very much on the architecture, but that's a relief after seeing it largely ignored in the other books.

A thorough chapter discusses the Northern Hub Alliance, a contractual partnership which brought client Network Rail together with their contractors to deliver the scheme. My experience is that Alliance arrangements are unusual in UK transport infrastructure, and while I found this chapter very interesting as a professional, I can imagine some readers' eyes glazing over.

The main message I take away from the chapter on civil engineering is the difficulty of building a project of this sort in a constrained urban environment. This is reflected in the way a large number of smaller structural elements were constructed in a "piecemeal" manner, with plenty of off-site fabrication and precasting.

The text never delves into the level of detail that would satisfy a bridge engineer, and I was left with a large number of questions, while recognising that I'm not really the main audience for such a book. The word "success" is used relentlessly, but fortunately leavened with a few short acknowledgements of real problems encountered during the project, such as issues with the stressing of the network arch bridge hangers.

The remaining chapters cover the other railway disciplines and (briefly) the project's outcome and legacy. I'm no trainspotter, but I think I did learn a few new things from reading these.

It's very well-illustrated throughout, with plenty of photographs.

The book is available for £25 plus £5 postage from a dedicated website, or from Amazon.

Further information:

• Official project website

Salford Meadows Bridge delayed

The Architect's Journal has revealed that a "funding shortfall" has prevented progress on Salford City Council's Salford Meadows footbridge.

The bridge proposal is a design by Tonkin Liu and Arup, which was the winner of a RIBA design competition, back in January 2014.

It's a sinuous steel bridge, curved in plan and supported on legs of silver Emmental (well, actually steel box girders with cylindrical perforations).

When I first mentioned this contest in June 2013, I noted that it had grown out of a feasibility study identifying a possible budget of £1m to £1.8m. I said: "The first question is whether there's sufficient political will, and funding available, to ever build a bridge at this site ... Salford say they are looking for an 'iconic' structure, but it's far from clear whether they have the appetite for an 'iconic' cost."

After the winner was declared, I commented: "There's little in the contest submission to provide confidence that what is depicted can be delivered at a reasonable level of quality, and I think there's absolutely no chance of it being done for the £2m figure that the designer states ... There must be a strong possibility it will also end up as unbuilt".

I understand that after winning the competition, Tonkin Liu were commissioned to develop the design further, but the AJ makes clear that it has not yet been submitted for planning consent, as Salford have yet to secure sufficient funding. Salford's mayor, Paul Dennett, is looking to fund the bridge from Section 106 planning contributions (essentially a tax on nearby developments), along with external infrastructure funding from central government.

The bridge proposal was always somewhat speculative - it would provide very useful access from one of Salford's main roads to the Salford Meadows area, but can only be a luxury in current economic circumstances. It was always clear that the money didn't exist to build it, and that its future was entirely dependent on economic whim.

That's something of a shame not only for Arup and Tonkin Liu, but also for the other 171 competitors who devoted significant time and energy to offering ideas for this bridge scheme.

Further reading:

• Salford Meadows Bridge Design Competition
• Shortlist announced for Salford Meadows Bridge Competition
• Shortlist published for Salford Meadows bridge competition
• Bridge competition debris part 26: Salford Meadows Bridge
• Salford Meadows bridge design contest winner revealed

Salford Meadows bridge design contest winner revealed

Arup and Tonkin Liu have been announced as the winners of the RIBA Salford Meadows bridge design competition. I last mentioned this scheme back in December when the shortlisted designs were released for public consultation.

Jury member Renato Benedetti describes the design as "poetic, innovative and elegantly engineered". Designer Mike Tonkin highlights its "biomimetic design". Local mayor Ian Stewart noted: "We will now be working with our partners to find the funding to create this stunning new bridge in the heart of Salford that will add to the city’s global reputation."

Sometimes you really just have to sit back in your chair and wonder quite what was going through the minds of all those involved.

The "poetic" nature of the bridge presumably relates to the designer's concept that it was some kind of silvery tendril, sprouting across the river from a seed, an idea which informs not only the bridge's S-shaped plan but also the layout of landscaping at one end. This landscaping is vital to providing a DDA-compliant ramp gradient at a site which, from the contest submissions, appears to have a very large level difference. The Arup / Tonkin Liu design is made to work by eliminating large areas of car parking in front of a pub and adjacent university building.

The structural engineering is interesting. The arch is a triangular steel box section which curves in plan and also twists. Arup's analysis indicates 10mm steel plate to be sufficient, and deformation and local buckling is prevented by the insertion of steel tubes through the box, creating the bridge's distinctive Swiss-cheese appearance. They state that these tubes eliminate the need for the normal box stiffeners and diaphragm plates, and also that the geometry is arranged in such a way that each face of the box can be bent from a flat plate. I find the latter point very hard to believe, given the twisting geometry shown on the visuals, and would reserve comment on the ability of the tubes to eliminate other stiffening. The FEA diagram included in the contest submission is certainly not very convincing.

The bridge deck is of similar geometric complexity, consisting of two paired box girders, again in triangular form, which curve in plan, vary in depth, and intersect and diverge at different places along their length. Although lacking in cheeseholes, the geometry of their surfaces is probably even more difficult to fabricate, and if 10mm plate is used throughout, likely to be subject to all sorts of out-of-tolerance imperfection. It will be a shame if it doesn't look good close up, because the perforated arches provide a pretty handy climbing frame from which it can be examined.

The parapets are a much more conventional system, arranged vertically with steel posts, a timber top rail, and a steel mesh infill. The designer seems to have made the entirely reasonable decision for them not to compete visually with the hyper-real brushstrokes of the main structural elements, which is not unreasonable in the circumstances.

 

This is an extremely bold design, and one which will be challenging for any designer (let alone the poor builder) to progress. There's little in the contest submission to provide confidence that what is depicted can be delivered at a reasonable level of quality, and I think there's absolutely no chance of it being done for the £2m figure that the designer states.

 

As with the ill-fated River Wear Crossing, which also emerged from a RIBA competition jury, this is a bridge which is technically demanding to the extent that the risks involved in choosing it are very high indeed. There must be a strong possibility it will also end up as unbuilt, not that this would an unusual outcome for RIBA, with only 2 out of 7 of their bridge competitions that I looked at in 2009 resulting in construction.

 

Of course, none of these issues are reasons to reject the design. If the customer wants flash and whizz, and can tolerate the likely budget, then this does seem the flashiest and whizziest of the four shortlisted entries. The other three contenders appeared more pragmatic and certainly in some cases less expensive, but I admire the desire to reject conventionality and pursue a flight of fancy. Perhaps that is what was in the jury's heads.

 

 

For me, the technical issues are secondary to the aesthetic in this instance, and to me the winning design looks both alien and ugly. Alien, in its twisted melange of retro-futuristic detritus, like the disposable plastic parts from an Airfix model of some end-of-70s science-fiction TV show. And ugly in its carefree trashing of such old-time conventions as innate elegance or harmonious relationship to context.

Shortlist published for Salford Meadows bridge competition

The four shortlisted entries to the RIBA Salford Meadows footbridge design competition have been made available online, and also as part of a public exhibition. Comments are invited from the public, with an online deadline of 6th December.

I previously showed some of the shortlisted entries here, as well as a selection of the many unsuccessful entries. I gave some more detailed comments on the contest back in June.

I'm a bit pushed for time right now, but if I get a moment, I'll post some more images and thoughts here.

Bridge competition debris part 26: Salford Meadows Bridge

Wow, now these people are quick.

These unsuccessful contest entries were all up on the internet even before the finalists had been announced for this footbridge design competiton, which I first discussed in some detail back in June.

So, here are the submissions which a fairly quick trawl of the internet has found. Follow the links to find more images and details of each design - it really is worth following most of them to see some of the weird and wonderful things that were submitted.

I will make no comment on the individual entries, other than to offer my sympathies to the competition judges.

Update: 21 October 2013: I've added a few more entries found on the internet, and understand RIBA Competitions are preparing their own gallery of all the entries.

ADAPT architects

Atelier Architecture 64 / Laufs Engineering Design

Avery Associates

BAI Design International

Stathis Eleftheriadis

Fala Atelier

InHolD (1)

InHolD (2)

JBMC

Magna

OOBE

Penda

Luca Poian Forms

Pruthiphon Buakaew

Standard

Studio 06

 

WCEC Group

Shortlist announced for Salford Meadows Bridge Competition

I discussed this contest when it was first announced in June. It was run as an open competition to design a new pedestrian bridge across the River Irwell in Salford, and I predicted the lack of other UK bridge design competitions would make this one very popular.

I didn't guess quite how popular: it attracted 172 entries, from 31 countries, which must be some kind of a record!

Salford Council have just announced the shortlist. These winning submissions will each pick up a £4,000 prize, and go forward to interviews with the judging panel in November to allow an overall winner to be selected.

There are also quite a few of the unsuccessful entries on the internet already, so I'll put together a separate post with those when I get a moment.

Atelier Zündel Cristea, Paris

Mott MacDonald with Moxon Architects, Altrincham and London

Toby Savage Design Limited with Wolfgang Buttress Studio and LDA Design, Stockport

Tonkin Liu Limited with Arup, London

Salford Meadows Bridge design competition

What amazing timing, for a new bridge design competition to follow so quickly after IABSE's design contest guidelines event!

Salford Council have announced an international bridge design competition for a footbridge over the River Irwell at Salford Meadows. It's being run on their behalf by RIBA's Competitions Office, currently the subject of a RIBA committee of inquiry, so you may imagine that how it is conducted will be watched closely in the architectural press.

The contest is in two stages, with the first open to anyone who can come up with the £50 registration fee. I'm going to bet they get over 100 entries, based on similar past exercises from RIBA, and the scarcity of similar competitions in the UK recently. The designs will be judged anonymously, with the top three each receiving £4,000. A second stage will see the shortlisted trio of designs being developed further, with anonymity removed, before a winner is decided.

Submissions are to be made digitally via the competition website, by 5th September, with the final winner to be announced in late November.

The bridge is intended to increase pedestrian access to the Meadows parkland area, and form a link in a network of walking routes connecting Salford to Manchester city centre. A total bridge length of up to 75m is anticipated, with a significant level difference between the two ends of the bridge posing the main challenge. A feasibility report made available online indicates a cost of between £1m and £1.8m, and reviews various options, such as the one illustrated here, none of which resemble a likely competition design.

It's interesting to read the competition arrangements in the light of the IABSE guidelines, and against what was discussed at last week's workshop. So much is wrong with this contest that it's difficult to know where to begin.

The first question is whether there's sufficient political will, and funding available, to ever build a bridge at this site. The competition brief makes precisely no commitment on this point, indeed there's not even a commitment to offer a design contract to the winner. However, Salford's track record of building "landmark" footbridges is good - I've visited their Trinity Footbridge, Greengate Footbridge, and Spinningfields Footbridge in the past, all of which span the same river. It's also apparent they've been planning further bridges for some time.

To their credit, Salford have done some legwork to confirm that the location for the bridge is appropriate, and that a bridge of some sort is economically feasible. However, the design parameters offered to competitors are skimpy, to say the least. The council seeks an "iconic structure" (don't they all), and notes that no supports should be placed in the river, but the published brief says nothing about required vertical clearance to the river, and acknowledges the river banks have yet to be surveyed. Is the ground suitable for suspension anchorages? Designers should just guess. The end result of all of this is the risk they may accept winning designs which are subsequently shown not to be suitable; or that they reject as unsuitable designs which are perfectly feasible.

In any design contest, the promoters need to consider how the rules and remuneration incentivise entrants, and whether the incentives align with the scheme objectives. As noted above, we can expect large numbers of entries, and to stand any chance of winning, designs need to stand out amongst competitors, to be eye-catching in a way which could impress a largely non-specialist jury (more on that in a moment). This may attract a large share of designs which are less appropriate in terms of maintenance, construction, structural economy etc.

The very low prizemoney and low odds of success means that many experienced bridge design firms simply won't bother entering, especially if they are busy with other, commercially rewarding work. Some of the better designs which could be prepared will therefore never even be submitted.

The low prize money is particularly disappointing because the organisers state that for the winner it will be considered an advance on future fee. This begs the question "what future fee?", as there is no commitment to award any design contract. Worse, it means the winner is less well off than the two runners-up: all three receive the paltry £4,000 as a prize, but the winner is then expected to do a further £4,000 of work for which they cannot charge. Prize money and design fee should always be separate, as otherwise the winner receives no reward for the hard work done in preparing their entry, or to recompense them for the significant risk they have take in allocating their time to the project.

The jury includes noted bridge architect Renato Benedetti, and only one engineer, Urban Vision's Shoaib Mohammad. Urban Vision is a joint venture between Salford Council, Galliford Try and consultant Capita Symonds. One of the recommendations of the IABSE competiton guidelines is to have an expert bridge engineer on the jury, but Mohammad is described variously as a Director for Engineering, Streetworks and Landscape Design (on the Urban Vision website) or as a Highways Director, specialising in highways maintenance (on LinkedIn). The clear concern must be that structurally challenging submissions will not be properly evaluated, leading to greater risks should the project proceed further. While an engineering review will be completed at Stage 2, that's simply too late, if good and bad proposals have been ruled in and out unfairly at Stage 1.

One of the biggest causes of competition failure is a mismatch between the promoter's budget, their aspirations, and their understanding of what the submitted designs may cost. There seems to be nothing in the rules about a proper evaluation of costs for any of the designs, and there is a real probability that large numbers of submissions will depict something well beyond the likely budget. Salford say they are looking for an "iconic" structure, but it's far from clear whether they have the appetite for an "iconic" cost.

An article in the Salford Star reveals the council have a budget of £25-30k for the competition, which gives some idea as to RIBA Competitions costs given that £12k of that is set aside for prizemoney. The Architects Journal notes that Salford have suspended their own standing orders to appoint RIBA as the competition organiser, stating that no other body could run such a contest in the UK. Salford apparently describe them as "an exclusive provider of such services" - the source for the AJ's article appears to be an internal council paper available online.

Quite what other UK architectural competition organisers like Malcolm Reading Consultants or Designed2Win make of that claim is anyone's guess. Only a minority of UK bridge design competitions in recent years have been run by RIBA, so it's a very odd claim indeed.

Despite all these issues, I expect the lack of bridge design contests will make Salford Meadows a very popular competition, and it will be very interesting to follow over the months ahead.

Manchester Bridges: 22. Spinningfields Footbridge

This is the second of two new footbridges over the River Irwell, connecting central Manchester to neighbouring Salford. I covered the other one in the previous post.

Neither bridge could be justified on a cost-benefit analysis of journey time saved. Greengate Footbridge is particularly absurd in that respect, being only 5m away from an existing foot-accessible bridge. The £1.5m Spinningfields Footbridge is a little better, shaving maybe 3 or 4 minutes off a journey walking across the nearest existing bridges (Bridge Street to the north New Quay Street to the south). However, both bridges act as gateways, as ways to improve the permeability of the urban fabric, enhancing the feeling of accessibility as much as the reality of it.

Greengate Bridge's minimalist appearance shows the structural engineer in full retreat, a functionary whose role is quite literally to support the architectural vision with as much discretion as can be mustered. Sited between Greengate and Spinningfields, Santiago Calatrava's Trinity Footbridge is the exact opposite, the structural engineer as flamboyant showman, the centre of attention.

Spinningfields Footbridge lies somewhere between the two extremes. The structural engineering takes centre stage, but not at the visual expense of everything else in the immediate vicinity. It's a very sophisticated piece of engineering, but retains a sense of modesty and a great deal of elegance. I reckon it's the finest contemporary footbridge in Manchester, and one of the best in Britain.

Designed by Ramboll (based on appearance, their former Whitbybird team rather than their former Gifford team), this is a design that distils many years' bridge design experience, matching a simple and beautiful overall concept to a super-careful attention to detail. It was built by Eric Wright.

The River Irwell at this point has high banks and so far as I can tell is not regularly navigated, nor prone to flooding. This gives designers the opportunity to "hide" all the bridge structure below deck level, something that is not always an option. Ramboll have used that as a cue for a lenticular form, deepest in the middle where the highest bending forces are found.

I described a generally similar bridge in Manchester two posts back, and discussed how the form can be seen in several different ways, one being that it is an "under-spanned" self-anchored suspension bridge. This is like a normal suspension bridge but with the suspending cable below the bridge deck rather than above it. Instead of being hung below the cable, the deck sits above it on struts. Instead of the main cable's tension force being anchored into the ground, it is anchored to the bridge deck itself, which therefore must resist compression. It's a design which can sit much more lightly on the ground than the normal suspension bridge.

Bridges of this type can be prone to torsional flexibility, a tendency to twist. I've seen several designs where the deck and cable are connected by a triangulated structure to create a skeletal "torsion box" which can resist this twisting – indeed, the Leech Street Footbridge is of that type. The Spinningfields bridge is considerably more adventurous.

The key feature of the bridge is that it is curved in plan, as is the supporting cable, although it follows a different line. The two are connected by individual struts, with no triangulation. The struts themselves are of folded steel plates, giving them necessary stiffness, and this is cleverly integrated with folded-plate deck bracing. There are clear echoes here of the ribbing on Lune Millennium Bridge (by the same designers), which I featured here not long ago. I have to say that I find it startling that so trimmed-down a structure is stable, and would need to see the engineering drawings to really understand how it works.

The Y-shaped parapets and the knife-edge fascia are reminiscent of the Chelsea Bridge Wharf Link Bridge, yet another Whitbybird / Ramboll design, and in my view are very attractive. Lighting is incorporated into the handrail, and there's further lighting below the deck level to illuminate the bridge structure at night. The triangular fascia does a great job in making the edge of the deck look extremely slender. The decking is permeable aluminium sheeting, allowing rainfall to drain freely through to the river.

I found vibration to be quite perceptible, although the amplitude is low enough not to cause any distress. Feedback on the Skyscrapercity website indicates I'm not alone in noticeing the movement.

A seating bench is provided on the deck, which I guess may make it a popular sandwich or smoking spot for workers from nearby offices. The deck is up to 4.5m wide, which is pleasingly generous.

Overall, I think this a real gem of a bridge, structurally daring yet in no way ostentatious, and one which deserves wide recognition.

Further information

• Google maps / Bing maps (both sites currently show images pre-dating bridge construction)
• Structurae
• Skyscrapercity (includes pre-construction visualisations, and construction photos)
• Manchester Confidential: New city centre footbridge lands
• Salford Online (installation video)
• Suave Aerial Photographers (great aerial photos of bridge installation)
• Salford City Council