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)

"Tower Bridge: 1894 to date. Operations Manual"

Haynes Publishing must be best known for their car and motorcycle maintenance manuals, but they have increasingly branched out into other territories, with recent publications including "The Human DNA Manual" and the "Milky Way Owner's Workshop Manual". In the areas of architecture and infrastructure they have published Manuals for "London Underground", "The Great Pyramid", "Hadrian's Wall" and now "Tower Bridge" (188pp, 2019, ISBN 978-1-78521-649-7).

It is, of course, Tower Bridge's 125th anniversary this year, and this new book by engineer John Smith joins books by Kenneth Powell and Harry Cory Wright published to mark the occasion. A comparison against the Powell book is inevitable, and although there is plenty of overlap between the two, there are some very clear differences.

Powell's book has, on the whole, the better photographs, and is a much easier read for a non-engineer, with much more detail on the context and a strong narrative surrounding those who designed and built the structure. As befits its publication by Haynes, Smith's book has far more detail on the construction work, the bridge components, and its operating technology.

The early sections of the book give a fairly comprehensive account of the somewhat tortuous process by which the bridge was eventually conceived, including the sometimes ingenious and sometimes monstrous alternative designs put forward.

The real dive into detail begins in the third chapter, documenting the eight separate contracts which were let for construction of the bridge, dividing up the works required for the piers and abutments, approach structures, metal superstructure, masonry superstructure, hydraulic machinery, paving and lighting. No client today would take this approach, retaining the entire liability for integrating a complex construction process on their own, but when the bridge was built there would have been no single contractor with the capability to do it all.

It's interesting here to see the extent to which the contract conditions used in the 1890s are very similar to those still in widespread use at the end of the 20th century. Extracts from the very first contract (for the piers and abutments) make this clear: the power of the resident engineer, payment retention, liquidated damages etc. The unrealistic timescales demanded by the client, and unrealistic prices submitted to win the work, also remain familiar today.

The core of the book consists of four chapters which itemise every single element of the bridge, describing them in exquisite detail and explaining just how every piece fits together. At times, the level of detail presented, with dimensions, plate thicknesses, etc, is numbing rather than interesting. For the engineering reader, there are several interesting extracts from drawings included, and the comprehensive nature of the text does mean that there appear to be no significant details left unmentioned.

There are many aspects of the bridge explained here which are essentially absent from the account in Powell's book. One example is the presence of stiffening girders concealed within the balustrades of the southern span, which ensure that water pipes carried across this span were protected against excessive movement. Another is the explanation of the arrangement of the high-level footways, the suspension bridge ties which pass through these, and the additional suspension cable added in 1960 to relieve the footway girders of the weight of those ties. These elements of the bridge are not immediately apparent to the casual visitor, but Smith's text, photographs and drawings make everything clear.

The book contains one excellent cutaway diagram showing how the components of the bridge fit together, and it's a shame there weren't more. My over-riding impression, after reading this book, is quite how complex Tower Bridge really is, and how well it merits this wealth of information. It really is an engineering masterpiece, whatever anyone may think of its architectural merits.

The book concludes with biographies of the main participants in the bridge's design and construction, and a detailed timeline of alterations and maintenance work in the period from 1894 to date. One of three appendices gives a detailed breakdown of the author's calculations of loads and forces in the bridge's key structural elements.

I couldn't, with any honesty, recommend this book to anyone who is not an engineer, but it is so detailed that it will probably remain a key reference work for Tower Bridge for the indefinite future. It is clear, thorough (sometimes too much so!) and well-illustrated throughout.

Merseyside Bridges: 13. Bradley Swing Bridge

I crossed this bridge en route to its much bigger and better known neighbour, the Sankey Viaduct. It is a rod-stayed pedestrian bridge spanning the Sankey Canal, and although there may have been several bridges like this in the canal's heyday, I believe this is the only one of this type that is left.

The Canal dates all the way back to 1757, but Historic England suggest that the Grade II Listed bridge dates from around 1857. The Listing states that the turning gear and pivot remain in place, although clearly the bridge is no longer operational, and the canal reaches a dead-end a short distance to the north of here.

If the 1857 date is correct, it must be one of the oldest surviving stayed bridges in England (there are certainly older examples in Scotland). It's not clear how much of the bridge is original - Historic England date the parapets to the 20th century, and there are turnbuckles in the main rods which are clearly an alteration.

The bridge is currently painted black and white but was previously painted green, as can be seen in photos at the Towpath Talk website, and on Wikimedia Commons, so the repainting is fairly recent.

The bridge's most unusual feature is the way in which the main span stays split into two. The stays are flattened locally to allow a pin to pass through. Combined with the bending of the rod over the narrow width of the cast iron posts, this is not an arrangement which could carry substantial loads, it would too easily be prone to fracture.

At deck level, there is a short linking piece connecting the main stay to the floor beams, which don't look original to me.

Further information:

• Google maps
• British Listed Buildings
• Canal Plan

Crowdfunding for Cody Dock Rolling Bridge

I thought I'd give a quick mention to this unusual bridge proposal, in case anyone has missed it.

Designed by Thomas Randall-Page, with support from engineer Tim Lucas at Price and Myers, it's a highly unconventional moveable pedestrian bridge, proposed to span Cody Dock at Canning Town in London.

There are three standard types of movable bridge: bascule bridges, which rotate about an axis parallel to the centreline of the river (or other obstacle crossed); swing bridges, which rotate about a vertical axis; and lift bridges, which move vertically without rotating.

There are also examples of retractable bridges which move horizontally, and a handful of bridges which move by transforming their shape, but very few examples of bridges that rotate about a horizontal axis perpendicular to the river centreline. The best known example of the latter is Gateshead Millennium Bridge, which like the Cody Dock design rotates to lift its deck sufficiently clear of the water to permit navigation. There are also a couple of examples in Belgium.

The Gateshead bridge pivots about its base, and so its weight is unbalanced in almost any position, requiring substantial machinery to operate.

The Cody Dock rolling bridge is instead proposed to be operated using a simple hand-crank, requiring its weight to be well-balanced in every position it rotates into. This is achieved by placing its centre of gravity exactly halfway between the floor of the bridge, and its overhead portals. When operated, the bridge rolls sideways like a giant pinion on a specially arranged rack; cog-teeth control the bridge's position.

This could be achieved by enclosing each end of the bridge in a large circular ring, which would roll along a horizontal rack somewhere below the floor level of the bridge. The half of the circular ring which is normally above the bridge deck would need to contain sufficient ballast to exactly balance the weight of the deck.

The Cody Dock proposal uses square portals at the end of the bridge, but rotating about their centre in the same manner as the circular ring. The centre-of-gravity remains horizontal, which means that the support rack has to be shaped to match the path which the rotating square traces in space. I think there will be quite a bit of ballast to pack into the upper part of the portal frames to make this work!

The cost of the bridge is estimated at £197,848, with a detailed budget cited which, perhaps worryingly, includes no contingency or risk allowance. There is currently no public funding for the project, nor any corporate sponsorship, so the project team are resorting to a crowdfunding web page to try and fund the scheme. This is currently sitting at around £62,000 pledged, with only 10 days left until their self-imposed deadline.

It's a very imaginative idea, and I wish them well.

"Tower Bridge" by Harry Cory Wright

This lovely new book is part of a series of "pocket photo books", and is published alongside books on the Barbican Centre and Trinity College Library, Dublin. Harry Cory Wright is a photographer better known for his landscape photographs, who applies his sensibility to buildings for these small-but-sweet volumes.

Tower Bridge (Thames and Hudson, 176pp, 2019, amazon.co.uk) measures 17cm x 12cm and features 120 images of this iconic London bridge. There are some similar photographs in Tower Bridge: History - Engineering - Design, which I reviewed recently, displayed there at larger size, but I don't think the pocket format detracts from Wright's images at all.

Unsurprisingly, the pictures have something of a landscape sensibility. Very few show the bridge in its surroundings, but several show the surroundings viewed from within the bridge towers or walkways. Most of the photos show details of various sorts, viewed in a way which emphasises shape, colour and texture.

People are notable by their complete absence, even in images of the control cabin and visitor areas.

This makes an opening interview with Chief Technical Officer Glen Ellis feel like a stray presence from another book, some kind of ghost in the machine (although not in the Cartesian sense). Other than this interview, Wright's book is all machine and no ghost.

There are relatively few photos of the exterior of the bridge, and especially few of the stonework cladding, which is perhaps a shame as many parts of it are exquisitely detailed. However, Tower Bridge makes up for it with everything else that is included.

There are some fascinating images of the interior of the towers, glimpses of steel framework peeking out between stone and staircases. Rivets appear, and then reappear repeatedly.

There are some great images of well-preserved control gear within the bridge operator's cabin, and of the bridge machinery, both operational parts as well as the preserved but now motionless steam engines. The epic bascule chambers appear, but so also does the inside of the accumulator tower, which is not something I've often seen photos of.

Towards the end, there are some particularly nice photographs of small machinery parts, valves, cogs, regulators and the like, as well as workers' tools and shelves full of spare nuts and bolts.

The reader is left to make of it all what they will. Some brief information is given for each photograph in a section at the end, for the curious to pursue.

What stood out for me is the extent to which Tower Bridge really is one of our greatest surviving examples of Victorian engineering, notable for assembling in one place such a variety of interesting parts. It is extremely well cared for, and if it has been substantially altered then that is generally very well hidden.

Obviously, this is a book which should appeal to anyone interested in architecture and engineering, but also admirers of fine photography. The price and size also make it an affordable gift. I very much enjoyed it.

"Tower Bridge: History - Engineering - Design" by Kenneth Powell

Tower Bridge: History - Engineering - Design (Thames and Hudson, 192pp hardback, amazon.co.uk) has recently been published to mark the occasion of the bridge's 125th anniversary (which falls on 30th June this year).

The bridge is one of the most iconic and memorable in the world, and it certainly merits this excellently researched and beautifully presented volume.

The title is a good guide to the contents, which emphasise the achievement of the engineers involved and avoid the temptation to focus on the bridge as an architectural artefact.

The book starts out not with the bridge but with the River Thames itself, the earlier river crossings, and the role of the Pool of London as a key part of the city's port facilities prior to the construction of new docks at the Isle of Dogs in the early 19th century. Setting the tone for the rest of the book, this section is copiously illustrated with old photographs, plans and paintings.

Until the Dartford Crossing was finished in 1991, Tower Bridge was the last bridge across the Thames before the sea. Prior to its completion in 1894, that honour had been held by London Bridge for over 600 years.

As well as discussing the history of several of the key Thames crossings (both bridges and tunnels), the book features several that were never built, such as George Dance the Younger's 1800 idea to replace London Bridge with two parallel bridges, and Thomas Telford's 1801 proposal for a huge cast iron arch.

This is all interesting, but the story of Tower Bridge really began in the 1870s. London Bridge was increasingly congested and a committee was established by the City Corporation to investigate the possibility of a new bridge or subway. Numerous schemes were developed, some by well known engineers such as Joseph Bazalgette and Rowland Mason Ordish, and Powell's book covers these well, including some great illustrations of what now look like quite absurd ideas.

The twin bascule solution was originally proposed by City Architect Horace Jones in 1878, and is shown in detailed original drawings included in the book. This was a drawbridge design, with ornate towers helping to support an arch from which, in turn, the bascules were slung.

The involvement of engineer John Wolfe Barry in the project from 1884 led to the change to a design essentially similar to what was eventually built, with suspended side spans anchored to two elevated, horizontal tie girders, which eliminated the risk of the original arch being struck by tall ships and also served as walkways.

Powell's book does well to cover the many key individuals who were responsible for the bridge's design and construction. These include John Jackson, the lead contractor; William Arrol, whose firm fabricated the steelwork (most of it hidden behind the tower cladding); and William Armstrong, whose firm supplied the hydraulic operating machinery.

Horace Jones died in 1887, a year after the laying of the bridge's foundation stone, and the task of preparing the architectural detail drawings fell to George Daniel Stevenson, who later established his own architectural practice. Several of Stevenson's beautiful drawings are reproduced in the book, along with a handful of engineering drawings and plans. I'd happily have seen more of these, but the book is judicious in its choices.

After describing the architecture, structural engineering and construction work, the book devotes a full chapter to the machinery and operation of the bridge, with more excellent photographs and diagrams. The engineering aspects are well-explained, and I think the level of detail given is about right for a lay-readership - informative without being too extensive.

A chapter towards the end of the book addresses Tower Bridge's life following construction and its status as an icon, although I think there was more that could have been said here (compare, for example, Peter Spearritt's book on Sydney Harbour Bridge). The Tower Bridge clone in Suzhou, China, doesn't even get a mention!

The final chapter discusses a few of the people involved in the bridge during its operational life, such as the various Bridge Masters, and although this is very welcome I think it would have been better to integrate these tales into the body of the book. There are also chapter notes, a bibliography and helpful index.

Overall, this is a genuinely excellent book about a bridge which certainly merits this level of in-depth attention. In addition to the historic images, there are many very high quality photographs of the bridge today, which make the book a joy to look through. It should certainly appeal to bridge enthusiasts, but more widely to readers interested in London, architecture and engineering.

Related posts:

• Tower Bridge (with links to other resources)
• Tower Bridge ... some musings (on the rarity of stiffened suspension "cables")

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:

Bridge competition debris part 28: Upper Orwell Crossing

In the wake of the decision to cancel the Upper Orwell Crossing project, I realised that I've never taken the opportunity to feature the losing designs in the original design competition.

All these images are taken from an article in the Architects' Journal, as only William Matthews Associates have shared details of their design on their own website.

My conclusion on looking back through these is that the Foster and Partners designs were pretty well-deserved winners. It would be interesting to see how the project budget would have turned out had some of these submissions been chosen!

I've provided further comments below where appropriate.

Adamson Associates with William Matthews Associates and Ney and Partners

These are my favourites of the losing designs - they have a real sense of style even if it's accompanied by a huge slice of impracticality.

The first of two alternative designs for the main river crossing is an unusual cable-stayed bridge, with an inverted-V tower straddling the carriageway, and supporting the two approach decks. It is a bit like two Alamillo Bridges placed back-to-back. It has the effect that the central twin bascule span appears inconsequential compared to the approaches, which I find a little disconcerting. It also feels somewhat out-of-proportion to the site - possibly something more appropriate in a larger city.

The second alternative for the main river crossing consists of a series of huge box-girder structures, like inverted pyramids, supporting a twin bascule arrangement in the middle. This feels less odd in its relationship to the river, but also overpowered by formalism. At first sight the river supports look unstable, but if there's enough weight in the system and it's well enough held down by approach spans, it is in theory stable.

The design for the shorter opening span is conventional in form (a Dutch drawbridge, with the deck suspended from overhead booms), but with a visually attractive box form for the upper boom. Again, the design does not look very stable.

Knight Architects

It's difficult to judge the Knight Architects design based only on these two images. The structure illustrated appears to be a vertical lift span, with a single tower at each end rather than the more normal pair of towers. It's not clear where the counterweights would be, and the towers don't look substantial enough to deal with any overturning loads due to wind on the raised bridge deck.

The notable feature of the design is the presence of people on the bridge while it is lifted, as much a fairground ride as a bridge in this situation. It isn't normal to permit people on a moving bridge deck while it's in operation (the Scale Lane Bridge in Hull is an exception), but it's entirely feasible. It means the bridge can't be perfectly counterweighted, but that's true of many moveable bridges today anyway.

Marc Mimram

I think there's some head-scratching required to work out what is going on with the main Mimram design here. The highway bridge deck is supported from a forest of poles, and I think the central deck sections are lifted using draw-cables, pivoting on struts below so that they don't tilt. It's somewhere between a bascule drawbridge and a lift bridge. I don't like the look of it.

The second span is supported from two arch ribs tilted at different angles to give the impression of a ladder or railway track twisting through space. It's not a straightforward design and would require foundations at a scale disproportionate to what is actually merited. To add to the fairground feel of the contest, it reminds me of a rollercoaster track.

Wilkinson Eyre with FHECOR and Eadon Consulting

Wilkinson Eyre and their partners offered two variants on the same theme for the main crossing, both with twin bascules. One has two rectangular bascule leafs with a minimal counterweight. The other uses diamond shaped leafs, in an interesting arrangement which allows the rear part of the deck to act as a counterweight. I think the axle or pivots supporting these decks would be working very hard.

The design is obviously reminiscent of the same designer's bridge at Poole Harbour, the Twin Sails Bridge. That has had a history of problems, with the bridge closed in 2012 (soon after completion), and again in December 2018. Movable bridges can be troublesome at the best of times, and adding complexity to the more normal geometry may not be the best decision.