Derbyshire Bridges: 1. Cathedral Green Footbridge

As with my last post, the title of this post is not an indication to expect lots of posts about bridges in Derbyshire, it's just a placeholder for the future. If you have suggestions of interesting bridges to visit in Derbyshire, let me know via the comments.

Derby's Cathedral Green footbridge was opened in 2009, following a design competition held in 2007. The design, by Whitbybird (team members now dispersed into Move Architecture and Ramboll), was for a moveable cable-stayed pedestrian bridge. Mechanical engineering design was undertaken by M G Bennett and Associates Ltd (team members now in Eadon Consulting).

Spanning the River Derwent, the bridge is around 57m long with a 22m tall central mast. As well as creating a new river crossing, the intention was to contribute to revitalising the Cathedral Green open space.

The bridge deck is cranked in plan, with an 19m back-span and 38m main-span, supported by stays from a tilted mast. The back span crosses a historic mill race. The reason the bridge is moveable is not for navigation, but to allow the bridge to be moved clear of the main river in times of flood.

In this arrangement, the layout is intended to allow pedestrians to continue to use both riverside walkways, an arrangement that is best understood either from the aerial view on Google Maps, or from this video:

The ability to retain the riverside walkway in an open position seems slightly unnecessary, as the mill race is covered over and hardly an obstacle any more. When I visited, the riverside path was also closed in both directions for construction work.

The asymmetry of the bridge led to the mechanical engineering being relatively unusual. The bridge rotates about a central pintle bearing, but its main supports are a wheel under the central area, and a second wheel at the rear of the back-span, which resists uplift and runs in a curved track. A hydraulic motor drives a pinion against a rack at the rear of the back-span when the bridge is required to rotate.

A further pair of wheel-bearings lock the other-end of the main span in place, the load in these being highly dependent on the thermal state of the bridge. It's interesting to think while crossing the bridge that it is so carefully balanced on just a few individual wheels.

The site looked so neglected and overgrown when I visited that I doubted the bridge still opened, but it seems that it does. Indeed, the impression of neglect was considerable, as will be obvious from some of my photos. The benefits of a stainless steel parapet are clear, although the mesh infill has been severely dented in several places, and adorned with love-locks.

So many "designer" bridges of the millennium years share the same conditions of grime and disrepair now. It was easier for aspirational local government bodies to secure capital funding for ambitious new infrastructure than to provide the funding for their proper maintenance.

This bridge is a case in point. I visited on a grey, overcast day at a time when the city centre was largely devoid of activity. It took a feat of imagination to see how this bridge may have looked soon after opening, well-used and glowing in the sunshine.

The bridge design is impressive and well-considered (there's a technical paper referenced below which is well worth reading if you can get hold of it), but on the day I visited it felt very much like seeing an expensive sports car covered in dust and with insulting messages finger-painted in the dirt.

Further information:

• Google maps
• Wikipedia
• Structurae
• Movable Bridges in the British Isles
• LUSAS
• The Design of Derby Cathedral Bridge, a Unique Cable-Stayed Swing Bridge (Wilson, IABSE / IASS Conference, London, 2011)

"The Queensferry Crossing - Vision to Reality" by David Watt

This the second of three "souvenir" books about recent major bridges in the UK that I'm going to feature here.

The Queensferry Crossing - Vision to Reality (Lily Publications, 224 pp, 2017) is a hefty, coffee-table tome, documenting Scotland's latest big bridge in exquisite detail. Grab the hardback if you get the chance, but the reprint softback is good value at £18.95 (both editions can currently be found on amazon.co.uk at reduced prices).

The author, David Watt, was for six years the Communications Manager for Forth Crossing Bridge Constructors, and brings to the book a combination of thoroughness and clarity. Compared to the Mersey Gateway book that I discussed last time, this is a much more in-depth document, diving much further into the detail of how the project was delivered.

As with the Mersey Gateway, the Queensferry Crossing is a three-tower cable-stayed bridge, but at a significantly larger scale, with a longest span of 650m compared to 300m. The towers at Queensferry are some 207m tall, and the bridge deck sits much higher above much deeper water. The scale and location mean that several aspects of design and construction were very difficult.

A key design challenge for a multi-span cable-stay bridge is how to provide adequate stiffness, as only a limited number of cables are effectively anchored to the ground through end piers or abutments. The Mersey Gateway relies on the stiffness of the bridge deck to reduce demand on the towers, while the Queensferry Crossing adopts a more innovative solution, with crossover of cables at midspan providing the necessary rigidity. Queensferry is not unique (see for example the Viaduc sur la Rocade Sud in Rennes, France), but I think it pioneered this solution at such a large scale.

The Mersey Gateway was constructed with the help of a temporary access bridge, but this was not feasible for its Scottish cousin. Accordingly, one of the bridge's biggest challenges was logistical: where to fabricate and assemble the components, and access to the mast positions via boat. Many of the components were produced overseas (in Poland and in China, for example), and fortunately the project benefited from the proximity to the bridge of Rosyth Docks, a large area of which was taken over by the contractor and used as a storage, assembly, and casting yard.

The book is split into nine chapters, topped and tailed with discussion of the need for a bridge, project development and procurement, and sections on community engagement, environmental issues, and the bridge opening celebrations. There's a lengthy credits list for firms involved in the project, but I did find it extremely odd that the book nowhere mentions the bridge's architect, Dissing + Weitling, who seem to have been largely written out of the project's history.

The core of the book covers the bridge's design and construction, although the challenges of writing about design are apparent in that it gets only 10 pages while construction gets 118 pages. Every section of the book is well illustrated, often with spectacular photographs, and the construction section also has plenty of detail - even down to an image of the cable strand wedges!

As a record of the project, it's fantastic, but after a while the level of detail does get a little numbing, and I can't admit to having absorbed it all.

The book goes out of its way to recognise some of the many people involved in the project, with lots of images of members of the project team, and interviews with five of them.

Further information:

• Official website
• Queensferry Crossing ARChive - online repository of project information

"The Mersey Gateway - A Bridge To Prosperity"

It's not unusual for major bridge projects to mark their completion with a souvenir booklet of some kind. I've recently got hold of three examples where they have gone a bit further and an entire book has been published to celebrate the occasion.

The first of the three is Mersey Gateway – A Bridge to Prosperity (118pp, 2018), published by the project's client and bridge owner Halton Borough Council. The Mersey Gateway connects the towns of Runcorn and Widnes across the River Mersey and Manchester Ship Canal, providing a new route for highway traffic, bypassing the Silver Jubilee Bridge built in 1961.

The new river crossing is a three-tower cable-stayed bridge with an unusual configuration, having a central tower visibly shorter than the two other towers. The new structure has been built to provide improved traffic capacity, reduce reliance on ageing infrastructure, and improve travel times not just locally but for nearby areas, particularly Liverpool and its surroundings.

The book concentrates on the client's view of the scheme. There is very little information on the engineering or architectural design of the bridge, not even an explanation for why the central tower is shorter, to give just one example. However, the book makes up for this with its strong coverage of the bridge's economic and social benefits.

The structure of the book is not chronological, but instead sets out to grab attention from the start. A description of the project's history and development doesn't start until page 77. Instead, the first chapter documents key features of the construction phase: the temporary trestle bridge, foundation cofferdams, the moveable scaffold systems (MSS), deck construction, cable stays, traffic management, and the opening ceremony. It's a well-chosen selection of highlights from the project's construction timeline.

There are short interviews with two people involved in the project: tower crane operator Peter McDonough and visitor centre volunteer Evelyn Edwards. I'd like to have read more of these, as my own experience of major projects is that a huge number of people contribute, and they often have plenty to say.

The second chapter emphasises community and people, including education outreach, local training and apprenticeships, recognition of women in engineering, community volunteers, and also the work of the Mersey Gateway Environmental Trust. This really shows what matters to the project client on a project such as this: it's not just all about quantities of steel or traffic statistics.

The third section, on design, environmental assessment and other aspects of the development phase, is weak by comparison – perhaps it's because I'm a designer by trade, but I feel this was a lost opportunity to tell this part of the story. It reads pretty much as if nobody involved in producing the book had actually spoken to anyone involved in design. The weakness continues into the fourth and final section, which attempts to credit the main project participants but which I think will leave most readers little wiser about what some of those involved actually did.

This reflects the nature of the book as the celebratory record of a very substantial achievement: Halton is a small local authority now responsible for two of the UK's largest bridges – they are right to be proud and to emphasise the benefits of the scheme beyond the bridge as a structure. This also means that the book (like most souvenir publications) has a uniformly optimistic, positive narrative – if anything went wrong along the way, if difficult challenges were overcome, those stories are absent.

The bridge's graphic layout is excellent, and there is a wide range of excellent photographs of the bridge and its construction (taken by David Hunter, and some of them reproduced here). The author or authors are uncredited, but the writing is very clear, and particularly good at explaining engineering and construction issues in lay terms – much better at this than I would normally expect.

It isn't the easiest book to get hold of – I wasn't able to locate anyone selling it online, and I got my copy by contacting Curiosity Bookshop in Runcorn directly – if they still have it in stock, they can take an order over the phone and post the book out (it costs £9.99 plus postage).

Further reading:

• The Mersey Gateway - Lasting Infrastructure Connecting Societies (Ramboll, 2017)
• Official website for the Mersey Gateway
• Project photos by David Hunter

Separated at birth?

Glasgow City Council has published the design for a new bridge across the River Clyde connecting Govan and Partick, next to the Riverside Museum. The bridge is to be 110m long, with an opening span of 63m, and a swing bridge has been chosen so that the bridge can operate in high winds.

The bridge has been designed by CH2M Hill (now part of Jacobs), who in their previous incarnation Halcrow have previously delivered quite a few Clyde crossings. It's expected that a planning application will be made this summer, with the intention to complete construction in 2021.

The budget has been previously stated as £10m, but I don't know if that's accurate.

So ... spot the difference.

The top picture is the new Govan-Partick bridge. I'm sure any similarity to the Puente de la Mujer in Buenos Aires, shown in the second picture, is entirely coincidental.

I was especially amused to see that the artist tasked with visualising Glasgow's new design adhered so slavishly to their brief (presumably "copy this one please, pal") that they even included a sailing ship.

Or maybe this was their source material:

That one, of course, is in Dublin, and does at least have some back-stays unlike the Buenos Aires span or its Glaswegian clone. What is it about these bridges and sailing ships, anyway?

I don't think there's anything else I really want to say about the Govan-Partick bridge right now, so here are some more visualisations, and a rendered video:

London bridges series: 52. London Wall Place Highwalks

Look, up in the sky! Is it a bridge? Is it a walkway? Is it a skywalk? No, apparently it's the return of London's pedways.

The original pedway scheme was the idea of London's town planners in the 1950s and 1960s, an attempt to elevate pedestrians above increasingly car-dominated city streets. Several new buildings were required to include provision for pedway access at first-floor level, but few of the pedways  planned were actually built.

They were perhaps most fully realised in the vicinity of the Barbican development, spanning across roads, and connecting the new Barbican cultural centre to the residential blocks which surrounded it.


However, with most foot journeys originating and ending at ground level, pedways were often as much of an inconvenience as a boon, and the idea soon died away. Despite their lack of success, pedways continue to exert a strong fascination for design and architecture critics and bloggers, and I've included various relevant links below providing more information.

With the construction of a massive new office development at London Wall Place, the idea has been revived. This was an original pedway location, so the new development is essentially reinstating what had once been there, albeit with a 21st century sense of style.

The new walkways span one busy road (and others that are less busy), and at least do serve a useful function in connecting into the Barbican's pedways, which do still form a useful connection. However, the walk below the pedways at ground level also an attractive route with extensive public realm enhancements in the St Alphage Garden section.

The walkways were designed by the team also responsible for the office building, Make Architects and WSP. Spacehub contributed to the landscape architecture.

The walkways are accessible from several staircases, and there's a new lift, although that wasn't in operation when I visited. The detailing on the main staircase is particularly nice.

In two places where the walkways span across roads, they are cantilevered from the office buildings, with the aid of stainless steel masts and stay bars. The walkway is suspended in an attractively-shaped weathering steel trough.

I found the mast-and-stay system visually awkward - it gives something of the impression that these are independent structures, although they obviously depend on the buildings for support. There's something about the arrangement that is out of place with the rest of the scheme.

The "centrepiece" of the walkway is another trough structure which snakes across the public space, skirting the ruins of an old church. This structure is a continuous beam cantilevering from each end, and it gives the impression that it's floating in space, thanks to the lack of intermediate supports.

It would have been so easy simply to have a multi-span walkway across this stretch, in the same style as other spans, but it's definitely this curved span that puts the whole scheme at a higher level of quality.

The various spans have spaces for seating, ensuring the walkways feel part of the public realm as a place to stop and breathe rather than just another way of traversing the city as rapidly as possible.

The whole scheme has been very well designed, and is a great addition to this one small corner of central London. I certainly can't see it inspiring yet more elevated pedways - the city streets are already too constrained and difficult an environment, but it's good to see it resurrected here.

Further information:

• Google maps
• Steelconstruction.info
• The Guardian
• Ian Visits blog
• Diamond Geezer blog
• Londonist
• The Pedway: Elevating London
• City of London Pedway scheme (Wikipedia)
• High Lines: Barbican Highwalks

Collapse of the Polcevera Viaduct

I'm sure most of my readers will have seen yesterday's tragic news that the Polcevera Viaduct in Genoa, Italy, collapsed, with at least 39 fatalities reported. At the time of writing, rescue and recovery efforts are ongoing. A state of emergency has been declared in the local region.

The 1.1km long viaduct carried a major toll road through Genoa, and was a key connection in the route from central and southern Italy to the south coast of France. It was completed in 1967 to a design by the famous Italian engineer Riccardo Morandi, one of a series of innovative concrete stayed bridges that he created starting with the Lake Maracaibo bridge in 1962.

These were distinguished by the use of very simple stay arrangements, using prestressed concrete stays rather than the steel cables already in wide use at the time (e.g. Strömsund Bridge in Sweden, 1955, and the Nordbrücke in Düsseldorf, 1957).

Eduardo Torroja's Tempul Aqueduct, built in 1926, is one of the few predecessors, although Torroja used concrete only as a protective material, it was not prestressed. Concrete-encased stays were also later used on the Prins Willem-Alexanderbrug in the Netherlands (1972) and the Metten Danube Bridge (1981). Morandi's tower arrangement was also used (without the concrete stays) on the Chaco Corrientes Bridge in Argentina (1973).

For more on Morandi's bridges and their relatives, see Walter Podolny's 1973 paper Cable-stayed Bridges of Prestressed Concrete.

Morandi's designs, although highly innovative, were a dead end. They required extensive temporary works to support the bridge deck until the stays were completed.

At Polcevera, temporary prestressing was used in the cantilevering deck sections, only to be removed once the stays had been installed. For his bridge at Wadi el Kuf, in Libya (1972), an array of temporary stay cables was used to support the longer spans, with all these cables then removed, rather than left in place as the permanent support system, which was the more logical and much more widespread solution. The Libyan bridge, incidentally, was recently closed for safety reasons.

Possibly relevant to the Genoa disaster, these designs also lack structural redundancy. The failure of any one key structural member of the bridge can lead to disproportionate collapse.

The Polcevera Viaduct, and its cousins, have been much admired by engineers and architects.

Michel Virlogeux, in his paper Bridges with Multiple Cable-stayed Spans, notes that the Lake Maracaibo Bridge was "much admired by architects who understand the evident flow of forces and who are sensitive to the impression of strength that emanates from the mass and shapes of the structure". Leonardo Fernandez Troyano described the same structure as "one of the great works in the recent history of bridges" in his book Bridge Engineering - A Global Perspective. In a 2010 paper summarising Morandi's work, Luca Sampo claimed that the Polcevera bridge's "technical features may still today be considered unsurpassed".

There is plenty of speculation on the internet regarding the cause of the collapse, which I won't repeat here.

The bridge's brand-new Wikipedia article is a pretty good source of information. There's a paper from 1995 which discusses previous remedial works to the bridge's main stays. Probably the best read is a contemporary article from 1968 with lots of construction drawings and photographs. All the images I've used here are taken from that article.

Welsh Bridges: 10. Doldowlod Bridge

This is a very unusual pedestrian bridge hidden on a private estate, and I think it is not very well known. I discovered it thanks to one of the Civil Engineering Heritage books. Please be aware that it is on private land, not a public right of way, so there is no right of access to the bridge without the permission of the landowner.

The Doldowlod Estate was purchased and developed by the famous engineer James Watt near the beginning of the 19th century, and it remains owned by his descendants, the Gibson-Watt family. Doldowlod Hall was developed by Watt's family in the mid 19th-century, but it's not entirely clear when this bridge was built on the estate, carrying a path across the River Wye.

McQuillan includes this bridge in his paper on brewer-turned-bridge-engineer James Dredge, but refers to it as Llanwrthyl Bridge, or Ystrad Bridge, stating that it was built around 1867 by Dredge's son William. Doldowlod is around 3.5km from Llanwrthyl, and Ystrad is the name of a farmhouse on the west bank of the river close to the bridge.

In Civil Engineering Heritage, Roger Cragg is silent on the designer but indicates the bridge was built around 1880, with the ironwork cast at Llanidloes Railway Foundry. Alan Crow, in the excellent book Bridges on the River Wye, says the same. This date seems to have been estimated by the bridge owner when the structure was visited and added to the ICE Panel for Historical Engineering Works database.

James Dredge died in 1863, so if the dates are correct, then it seems likely that William Dredge did indeed complete the bridge using his father's patent design. William died in 1886.

The bridge is a classic James Dredge design, with the exception of the cast-iron A-frame towers. It spans either 120 feet or 42m across the river (depending which source you credit), supporting the deck from chained eyebar rods.

As in all Dredge bridges, the main "cables" diminish in size from support to the centre of the bridge, this being achieved by the expedient of diverting component bars downwards to support the bridge deck at each link in the chain.

There are five bars in each chain at the towers, reducing to two at midspan. At Doldowlod, two smaller bars are diverted downwards at each connection point.

The effect, in theory, is to closely match the amount of metal used in the main "cable" to the level of stress present at each point, and this works as a cantilever bridge, not a suspension bridge (where instead the force in the main "cable" is relatively equal all along its length). This should give an efficient use of material, but it is not a very robust design.

The diagonal hanger introduce a longitudinal force into the deck; this is balanced by forces in pairs of flat steel bars which run along each edge of the deck. Deck joists are held in place between these flat bars, and these carry timber plank flooring.

The bridge's appearance is visibly distorted, and it is certainly quite "lively" even under just one person walking across. However, it is otherwise in fair condition considering its age and likely fairly limited maintenance.

Some sources indicate that there are no more than 7 surviving Dredge bridges in the UK, although that figure does not include the bridge at Doldowlod. I've previously visited and reported on the surviving spans in Bath and Inverness.

Doldowlod Bridge appears to hold no protected heritage status, which while not unusual for an obscure engineering work, is very difficult to justify given the age and rarity of the structural form.

Alan Crow's book indicates that the bridge was repaired in 1962 after being struck by a floating tree, and that in 1989 it was dismantled, refurbished and reassembled by R.J. Hope and Son of Newbridge-on-Wye. He also indicates that there were previously two other suspension bridges on the Doldowlod Estate, of which only foundations now remain.

Further information:

• Google maps
• Panel for Historical Engineering Works
• Dredge's Suspension Bridge Explained Upon the Principles of the Lever (Turnbull, 1841)
• From brewer to bridge builder: reflections on the life and work of James Dredge (McQuillan, Proc. ICE, 1994)
• Bridges on the River Wye (Crow, 1995)
• Civil Engineering Heritage: Wales and West Central England (Cragg, 1997)