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.

Bridges news roundup

Here are a few news links that you have probably already seen. I'm travelling over the Christmas period, so it will probably be January when I finish off my reports on the IABSE study tour of north-east England.

New Thames footbridge gets planning
I really dislike this so-called Golden Jubilee bridge, which is proposed to cross the Thames between Battersea and Chelsea Harbour in London. The proposed span arrangement, with a short middle span flanked by two larger spans, with correspondingly larger support arches, just seems to have a really awkward rhythm to it. Although the bridge has now won planning consent, it still appears to have no funding. Incidentally, if you don't have a BDonline subscription, you can read their news stories by simply copying-and-pasting their headline into Google, then following the link from there: this bypasses the subscription protection.

£60m boost for Thomas Heatherwick's garden bridge
The Golden Jubilee Bridge is a ridiculous enough proposal, but the Garden Bridge is an absolute disgrace, an absolute eyesore where money is no object if it serves to boost the vanity of all involved. The total project budget is £150m, which is absurd for a bridge which is so utterly unnecessary.

Striking but useless… just like Boris
It seems I am not alone in this opinion. "It has just the right combination of whimsicality and instant-wow theme-park tackiness to make it a likely Boris project."

Shortlist announced for Hisingsbron in Gothenburg
So far as I can tell using Google Translate, five proposals out of 24 submissions have been shortlisted in this competition to design a new moveable bridge. The finalists are Snøhetta / WSP, Zaha Hadid, Tyréns / Beam / Schlaich Bergermann, Dissing + Weitling / Lenonhardt Andrä, and Wilkinson Eyre / Ramboll. That's a mighty impressive roll call of bridge architecture names, along with one or two less so. All five shortlisted entries are available online, where you can play guess-the-architect. All I can say is that three out of the five submissions are quite horrible, and you really have to wonder what the participants were thinking.

Beautiful bridge planned for China's Hunan province evokes knots, Möbius strips
This depressingly awful bridge proposal has been all over the architecture blogs, but few have managed such an inappropriate title. Beautiful? In the eye of the beholder, perhaps. If they are blind.

I told you so, St. Patrick’s Island Bridge
Following flood damage during construction earlier this year, parts of the RFR / Halsall designed footbridge had to be disassembled, setting back the date for the bridge's opening. Tallbridgeguy takes the opportunity to say "I told you so".

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.

Durham Bridges: 3. Framwellgate Bridge

Framwellgate Bridge pre-dates the nearby Prebends Bridge in Durham, but is a significantly more attractive and impressive structure.

A bridge was built at this site in the early 12th century, by Bishop Ralph Flambard. It was protected by a gateway and tower, until destroyed in a flood in 1400. The replacement bridge consisted of two segmental arched river spans, and one much shorter land span. The land span is no longer visible, although it may be hidden by later buildings. It can be seen on a painting by Thomas Girtin, and if it does still exist, it may be a survivor from the original bridge.

The river spans are ribbed arches, originally with five ribs, and later widened to seven. As with its predecessor, it was originally protected by a gatehouse, removed in 1760 because they hindered traffic.

The shallow spans are very attractively proportioned. They are separated by triangular breakwaters. I think the slight recessing of the arch facing stones is a large part of the attraction, and spandrel tie bars have been added in a way which complements rather than detracts from the bridge's appeal.

This was pretty much the last bridge we visited on the first day of our IABSE study tour. We did visit the Gateshead Millennium Bridge in te evening, but I'll cover that when I run through all the Tyneside bridges that we visted on our second day.

Further information

• Google maps / Bing maps
• Wikipedia
• Structurae
• Bridges on the Tyne
• British Listed Buildings
• Durham World Heritage Site
• The Ancient Bridges of the North of England (Jervoise, EP Publishing Ltd, 1931, reprinted 1973)
• The Bridges of Northumberland and Durham (Graham, Northern History Booklets, 1975)
• Civil Engineering Heritage: Northern England (Rennison, Thomas Telford Publishing, 1996)
• An Encyclopaedia of Britain's Bridges (McFetrich, Priory Ash, 2010)

Durham Bridges: 2. Prebends Bridge

The centre of Durham is dominated by a large loop in the River Wear, which passes around the peninsula on which sit both Durham Castle and Durham Cathedral. While this may have been a highly defensible site when the Castle was built in the 11th Century, it must have proven an increasing constraint on travel as the centuries wore on.

Prebends Bridge was one of three mediaeval bridges to be built which spanned the river. Originally, it was the site of a ferry. In 1574, this was replaced by a timber bridge built on stone piers, although that was swept away in the floods of 1771. The present bridge was completed in 1778, funded by the Cathedral and designed by George Nicholson.

The bridge has three semicircular stone arches. The parapet is generally solid, but opened with balustrades near the crown of each arch. There are triangular cutwaters, with a semi-hexagonal geometry above the base. The face of the arch ring is "dentilated", and you can see waterspouts on the elevations which assist with drainage.

These are an unusual feature. Poor drainage is a common feature in old masonry bridges, but the details of this bridge seem better thought out than most. Viewed from above, you can see a succession of scalloped humps in the road, assisting water in draining towards the waterspouts. I can't remember seeing this feature anywhere else.

Despite this, the bridge has suffered from masonry deterioration, and it was closed to traffic entirely in 2011.

It's not the prettiest of Durham's bridges. The regular procession of arches seems stilted, and I think it would have looked better if the open balustrading was continued over more of its length. I imagine it looks much more attractive on a calm and sunny day, when it reflects better in the water.

Further information

• Google maps / Bing maps
• Wikipedia
• Structurae
• Bridges on the Tyne
• British Listed Buildings
• Durham World Heritage Site
• The Ancient Bridges of the North of England (Jervoise, EP Publishing Ltd, 1931, reprinted 1973)
• The Bridges of Northumberland and Durham (Graham, Northern History Booklets, 1975)
• Civil Engineering Heritage: Northern England (Rennison, Thomas Telford Publishing, 1996)
• An Encyclopaedia of Britain's Bridges (McFetrich, Priory Ash, 2010)

Durham Bridges: 1. Kingsgate Bridge

The last few posts dealt with bridges on Teesside that I visited as part of an IABSE study tour. On the afternoon of the first day, we travelled a little further north, to Durham. Our first stop: Kingsgate Bridge.

Built in 1963, this footbridge was personally designed by Ove Arup, and is reported to have been one of his favourite designs. At the time, Arup was probably more involved in running his Partnership than in the specifics of individual projects, but for the Kingsgate Bridge he immersed himself in every detail.

Famously, the University in Durham had only £35,000 to spend on a new bridge linking their older buildings by Durham Cathedral to a new site on the opposite bank of the River Wear. They anticipated this would be enough only for a bridge at the river valley floor, some 17m below the tops of the valley slopes. Arup convinced them that the same money could pay for a 107m long bridge running at height, if the structure could be made efficient.

The engineering of the bridge displays a masterful balance of structural requirements with construction requirements. The bridge has only two main support points, each on a piled base. From these, V-shaped "fingers" carry the bridge deck, reducing the deck spans. The arrangement also allowed for a highly economic method of construction, eliminating the need to construct reinforced concrete in-situ above the river. Each half of the bridge was built parallel to its river bank. The pier supports conceal an internal cone, which allowed both 150-ton halves of the bridge to be rotated 90 degrees into their final position.

This is certainly not a unique construction method, but it is relatively rare and I'm not sure whether there were any precursors in the UK at this time.

At first sight, the bridge is not an immediately loveable design, notwithstanding its status as a Grade I Listed Building. The supports are stiff and angular, and the bridge deck, apparently envisaged as a "thin, taut, white band stretching horizontally across the valley", is blank and lumpen. As the river banks have become considerably overgrown, it's also almost impossible to see the entire bridge as the single composition which was originally built.

Even in detail, the bridge has its flaws, and I find the "fingertips" which connect the finger piers to the deck to be particularly awkward. There's little sense of how the forces are transmitted (the deck is in tension between the fingers, and that considerable force has to be transmitted through these tiny fingertips). There's no shaping of the concrete to respond to those forces either: the band-like concrete side-beams are the same depth at the ends as at the middle, although the stresses in each section are very different. Arup was certainly not a master of concrete on the level of Maillart, Torroja, Candela or their ilk.

Nonetheless, it's an admirable bridge in many ways. The detailing of the finger piers is interesting, minimising material while providing stiffness by means of a folded cross-section that constantly transforms from top to bottom. And the bronze expansion joints where the two bridge halves connect are quite exquisite. These allow horizontal movement while locking the bridge decks together vertically.

I am not generally a fan of "half-through" footbridges, where solid beams on either side of the walkway double as the parapets. Generally, visual transparency seems preferable. However, for a footbridge at height, as is the case here, solid parapets do offer a much greater sense of security for the bridge users.

Shortly after the bridge was built, Concrete Quarterly praised it for having a "subtlety without chi-chi or softness, but keeping in its fineness a certain strength of plane". Its angularity closely recalls the great Italian engineer Riccardo Morandi, whose structures are equally hard to love. Today, I think it would be improved considerably simply by cutting down a few trees and giving it the chance to be seen properly as a sculptural object within the landscape.

Further information

• Google maps / Bing maps
• Wikipedia
• Structurae
• Arup
• Engineering Timelines (+ biography)
• Bridges on the Tyne
• British Listed Buildings
• Archive construction video
• Footbridge at Durham (Concrete Quarterly, No. 60, 1964)
• Modern British Bridges (Henry and Jerome, CR Books Ltd, 1965)
• A Celebration of Bridges between the Tweed and the Tees (Hartley and Brown, ICE, 1995)
• Civil Engineering Heritage: Northern England (Rennison, Thomas Telford Publishing, 1996)
• Footbridges (Birkhauser Verlag, Baus and Shlaich, 2008)
• An Encyclopaedia of Britain's Bridges (McFetrich, Priory Ash, 2010)

Teesside Bridges: 4. Infinity Bridge

Okay, it’s white elephant time.

A design competition was launched in 2003 for a new footbridge across the River Tees at Stockton. The intention, as in so many instances, was to use a landmark structure to signpost the local authority’s commitment to developing a new area, on the north bank of the Tees, and attract in outside investment. A budget for the bridge of £4.5m was stated.

The competition was won by Stephen Spence Associates with Expedition Engineering, who developed the concept for a twin arch bridge, with one arch twice the span of its neighbour. The asymmetric arrangement kept the central part of the river free from obstacles, this part of the river being used regularly for rowing training. There was to be some controversy over authorship of the design.

The bridge was completed in 2009, at a cost of £15m, an enormous increase over the original budget, and one which presumably involved the promoter having to plunge their hands deep into their pockets. This is above average for a landmark bridge, but not unreasonably so given the spans involved. The original budget was never enough for a truly iconic structure, and the Infinity Bridge is certainly that.

Visiting four years later, all that our tour group could find on the north bank was mud and grass – none of the hoped-for development has materialised, perhaps unsurprisingly given events in the wider economy. It’s in this sense that the bridge is a white elephant, and it remains to be seen whether the money was well spent.

Nonetheless, the bridge is both spectacular and elegant, and its architectural impact is largely a consequence of engineering requirements rather than the driver for the design. The “reflex curve”, or sag arch, which links the two main arches, originates as a gesture on a sketch. Expedition can make the claim that it is a functional element, as it serves to transfer bending between the two arch ribs, making them both much stiffer and improving behaviour overall. This is, however, probably rationalisation after the fact, a happy but not inevitable outcome.

Several things strike me about the structure. The deck is quite exceptionally slender, comprising a series of precast concrete slabs, prestressed together by the action of the arch’s horizontal bowstring cables.

There is very little stiffness in the deck, and although probably heavy enough to be difficult to excite, it still requires the presence of five enormous tuned mass dampers tucked below its soffit to ensure that vibrations are acceptable. The dampers are impressive but slightly odd – one above the south bank is set horizontally at a location where the deck has a longitudinal gradient, making it look as if the damper is peeling off the soffit and ready to fall.

The arch is tall yet narrow. Most designers, I think, would have adopted a wider arch to provide greater transverse stability. This narrowness is enhanced by the tripod form of each arch, where it bifurcates into two ribs at the central support, but comes to a single rib at the end support. The central support is therefore required to provide all the resistance to wind or eccentric loading. The effect is visually striking, but as an engineer I also found it a little disconcerting. It also means that the deck has to incorporate significant transfer elements to brace between the end supports of the arch (with an outward thrust) and the tie cables along the deck edges (with their inward tension).

The arches are formed from painted structural steel box sections, and generally are shaped attractively. In elevation, their shape was determined by form-finding, to minimise bending moments under dead load. However, the bracing between the twin arch ribs near the main support, which include huge circular sections resembling offcuts from a pipe factory, is much less attractive.


There’s also an odd disjuncture between the plain, minimal profile of the arch ribs, and the centre-pier struts which support them. Both above and below deck, these struts have a “fluted” form, which contrasts markedly with the arch ribs. I don’t particularly like the shape of these elements, although I do think that the high contrast was the visually appropriate decision. It is the shape of the arches including the reflex curve which needs to be highlighted and given visual continuity, even if this is structurally dishonest, in the sense that the primary flow of forces is from the arch rib down towards the point of support. I think this is therefore an instance where the popular maxim that form follows function would have it wrong.

Further information

• Google maps / Bing maps
• Wikipedia
• Structurae
• Bridges on the Tyne
• Expedition Engineering
• A Critical Analysis of North Shore Footbridge, Stockton-on-Tees, UK (Maskell, Uni of Bath student conference, 2009)
• Infinity Crosses the Tees (Wise and Harris, Ingenia, June 2010)
• An Encyclopaedia of Britain's Bridges (McFetrich, 2010)