20 February 2018

"The Bridges of Medieval England: Transport and Society 400-1800" by David Harrison

This blog normally focuses on contemporary bridges and their designs, although I do also often cover historic bridges. I have featured a few medieval structures over the last few years: PostbridgeTwizel Bridge, Old Powick Bridge, Devil's Bridge, Framwellgate BridgePont Saint-BénézetPont Neuf, as well as the Roman Pont Du Gard.

David Harrison's book The Bridges of Medieval England: Transport and Society 400-1800 (Oxford University Press, 2004, 269pp) [amazon.co.uk] is an in-depth investigation of a period which is sometimes sidelined in histories of bridge building. Unlike more recent periods of history, there is simply less written evidence from the medieval period, less knowledge about what was built, by who, when, or for what reason.

Harrison's book is a conscious attempt to redress the balance, and to correct misconceptions about a period where the lack of evidence often leads to mistaken assumptions. The book is meticulously researched and referenced, often from primary sources, and builds a surprisingly detailed picture of the extent of bridge-building in medieval England.

The book is divided into three broad sections.

The first is a quantitative survey, considering how many bridges were built, where and when. Harrison's core argument is that historians have grossly underestimated the sheer quantity of bridges built in medieval times, and that in doing so they have misunderstood the organisational and economic capacity of society in that period.

Surveying a range of historical records, the author deduces that there were almost as many bridges in existence in 1250 as there were some five centuries later, and that many of these bridges dated back to Anglo-Saxon times. He also makes the case that most of these bridges were on roads freshly introduced after Roman times, rather than relics from the Roman occupation. If some of the evidence for earlier periods is somewhat tentative, for later centuries Harrison is able to tabulate numbers of bridges in impressive (albeit often laborious) detail.

The story is one of slow changes in transportation in early medieval centuries, with fords gradually giving way to bridge construction, generally in timber (or timber on stone piers). By the later medieval period, the picture changed to consolidation, with fewer new sites for bridges as most rivers were already bridged at key points, and a gradual conversion of timber structures into stone arch bridges. With written sources often sparse or incomplete, there is a great deal of detective work e.g. by considering place-names such as Bristol, first mentioned as Brycgstow in 1063 ("the site of the bridge").

The books's second part deals with the bridges as structures: their form and materials, and methods of construction. Here the evidence is much stronger, as over 200 medieval English bridges survive today. Bridges built elsewhere in Europe often have the same forms from the same periods.

Little information has survived on how medieval bridge-builders designed their structures: writings on the subject only really started in the 16th century, with the first book in English not published until 1772. Nonetheless, the key concern of the medieval bridge builder was clear: how to survive the elements. London Bridge, completed in 1209, was well-known for how it restricted the river flow, resulting in a huge difference in river levels above and below the river during peak tidal changes. Many early bridges had wide piers and narrow openings, and the history of many medieval bridges is one of occasional collapse, damage, and repeated repair and rebuilding.

Vulnerability to hydraulic forces, and to scour, meant that much of the art of medieval bridge building lay in selecting suitable places for the foundations, and suitable foundation construction. Harrison distinguishes between the large spans of the northern uplands (such as Twizel Bridge and Devil's Bridge), relying on rocky ground for support and providing clear flow for torrential rivers, and the often multi-span viaducts of the southern lowlands, often founded on poor ground and supported on timber "sole-plates" or timber piles driven into the earth. The invention of pile-driving was evidently one of the most significant achievements in the development of medieval bridge building.

There is plenty of interesting material here for anyone involved in conservation of old bridges, or just interested in understanding the challenges the bridge-builders faced, and how they were resolved. As well as presenting the factual details, Harrison is keen to address another misconception - that the many reports of problems with bridges indicate widespread problems with the condition of structures. Instead, he argues that bridges were generally well-maintained, with very few out of service at any given time.

The final section of the book is titled "Economics and society", considering costs of construction, and how bridge building and maintenance were funded. The picture here is of a society willing and ready to spend significant sums of money on a bridge and road network which was critical to trade. However, the mechanisms for funding were very different from today.

Funding for construction was generally private, through wealthy lords and landowners. Maintenance was funded in the same way, as well as from charitable donations (including religious indulgences), from tolls, and through legal obligations. The last of these dated to the Anglo-Saxon period, when "bridgework" was a duty required of certain local communities, along with duties to maintain fortifications and to participate in military service. The "bridgework" obligation fell away only slowly, with a gradual process of lifting of these duties as town charters were renewed or legal challenges made. There was an increasing tendency to place obligations onto bodies with a greater longevity than landowners and residents, with a shift towards the English counties following the 1530 Statute of Bridges.

Some bridges benefited from the establishment of an endowment, lands from which the income was to be used for the management and maintenance of the bridge. Two well known examples survive today: the Rochester Bridge Trust, and London's Bridge House Estates.

This book's main flaw is a lack of illustration. There are a few maps, and 27 historic photographs and illustrations of relevant bridges, but it would have benefited greatly from pictorial explanations of the different forms of bridge construction, and from having some of the more detailed information presented more accessibly (e.g. timelines, tables, and charts).

It is not a book aimed at the casual pontist or perhaps even the armchair historian. It is thorough and detailed, and principally addressed to specialists in medieval history, or the history of transport and economics. However, it is a very impressive piece of work, and I think it should certainly be on the bookshelf of anyone who is serious about the history of bridges in England.

26 January 2018

"History of the Modern Suspension Bridge" by Tadaki Kawada

It feels like only yesterday, but apparently it was actually 2012 when I reviewed Richard Scott's book "In the Wake of Tacoma", a wide-ranging, very thorough history of 20th century suspension bridges. I praised Scott's wealth of detail, while noting the book's hesitance on key aerodynamic concepts, obsession with factual minutiae, and lack of diagrams and photographs.

Scott's book was published by ASCE Press in 2001. The following year, Tadaka Kawada published a similar book in Japan, tackling essentially the same history from a slightly different angle. In 2010, ASCE Press published this English translation of Kawada's book, edited by Richard Scott. The two books make for a very interesting comparison.

Kawada's book's full title is "History of the Modern Suspension Bridge: Solving the Dilemma between Economy and Stiffness" (ASCE Press, 2001, 246pp) [amazon.co.uk]. Despite the title, it also has very good coverage of earlier suspension bridges, with the first three chapters covering early suspended bridges (up to and including James Finley's structures), 19th century spans in Britain and France, and 19th century American spans. These, and later chapters, are very well illustrated, with hardly a two-page spread going by without some kind of image, whether a diagram, photograph, or historic paintings and engravings.

Much of the early history of suspension bridges is a history of failure. Finley invented the modern form of suspension bridge, with towers and a level deck, and around 40 bridges were built using his patent. However, his first bridge, at Jacob's Creek, Pennsylvania, survived only from 1801 until 1825, when it failed under load, and by that time many of his other bridges had also already collapsed for a variety of reasons.

British engineers lagged a few years behind, with various early cable supported bridges built from 1816 onwards. Captain Samuel Brown pioneered modern suspension bridges in Britain, with the Union Bridge completed in 1820. Thomas Telford's Conwy and Menai Bridges followed not long after, completed in 1826. As with Finley, understanding of the structural behaviour of these bridges was extremely limited, based on simple theory supplemented by experimental trials. Brown and Telford's bridges set new span records but were plagued by failures: several of Brown's structures collapsed or were damaged, caused variously by high winds, dynamic crowd loads, and an over-ambitious attempt to carry rail traffic on a structure initially designed only for highway loads. Telford's Menai Bridge oscillated severely and was seriously damaged by winds, and the bridge as seen today has been radically altered from the original design.

French engineer Louis Henri Navier studied the British designs and published an extensive report on the subject, but his single suspension bridge, the Pont des Invalides was never completed, after the anchorages were found to have moved during construction. A replacement design was built but lasted only 21 years. Other French engineers, such as the Seguin brothers and Louis-Joseph Vicat, were more successful, pioneering the use of wire instead of chains, and inventing aerial cable spinning. The French were better theorists than the British, and built far more suspension bridges in this period, but their understanding of how the bridges behaved was still extremely limited. Several French bridges suffered problems with vibrations, with the most notorious instance leading to the collapse of the Basse-Chaîne Bridge under marching troops, killing 226 people.

Kawada's writing is very clear and to-the-point, and accompanied by useful direct extracts from original literature and extensive referencing. Reading these early chapters, a trend of ever-more ambitious bridges being built under conditions of significant ignorance emerges. As the Americans regained the lead in suspension bridge construction, the same theme continued, with a notable disaster befalling Charles Ellet Jr's Wheeling Bridge in 1854. This 308m span, the longest ever built, collapsed under wind loading, with torsional undulations reported as rising nearly to the height of the support towers.

John Roebling's hybrid stayed suspension bridges, most famously including the Brooklyn Bridge, were significantly more successful. Kawada states that Roebling "understood the meaning of 'stiffness' in modern suspension bridges". Several earlier structures had incorporated stiffening trusses, largely as a pragmatic measure, but Roebling's adoption of measures to ensure significant stiffness resulted in bridges far less prone to problems under either live or wind loads.

As the 19th century came to an end, suspension bridge theory began to mature significantly. Joseph Melan's Theory of Steel Arches and Suspension Bridges, published in 1888, became for some time a definitive text, setting out the so-called Elastic Theory. Kawada is conscientious in explaining both the Elastic Theory, and its later successor, the Deflection Theory, with diagrams and equations. The Elastic Theory ignores deflection of the suspension cable under live load, treating it simply as a means of support which relieves load in the bridge deck's stiffening girder or truss. The Deflection Theory, popularised by Leon Moisseiff, also takes account of the deflection of the main cable under live load. This increases the overall calculated stiffness of the system (by adding the cable stiffness to the deck stiffness), giving both a more accurate result and also a more economic design.

Moisseiff used the Deflection Theory to design the Manhattan Bridge, completed in 1909. Kawada compares it to Leffert Buck's Williamsburg Bridge, complete in 1903 using the older theory. The Manhattan Bridge has a much shallower truss.

The rapidly improving understanding of bridge behaviour opened the way to steadily larger and less expensive structures. In 1931, the George Washington Bridge nearly doubled the world record span, at 1067m. Initially, this was built without any stiffening truss at all, reliant on its massive weight for stiffness (the bridge was only stiffened in 1962 to add a second deck and accommodate more traffic). Other large spans were also under construction, the largest being the 1280m Golden Gate Bridge in 1937.

Any student of bridge design will know what happened next. In 1939, work was completed on Othmar Ammann's Bronx-Whitestone Bridge, again with no stiffening truss, but relying largely on weight for stiffness. Leon Moisseiff took the same approach for the Tacoma Narrows Bridge, completed the following year: as with its immediate predecessors, the road deck was carried by two simple edge girders. The decision was disastrous, with the bridge collapsing when subject to moderate winds only four months later.


The bluff profile of the edge girders led to the creation of wind vortices, which induced oscillation of the bridge deck. Wind tunnel tests were rapidly undertaken on the bridge's cross-section in the months between completion and collapse, confirming the section to be highly unstable under wind effects, and plans were made to install fairings on the girders to reduce the vortex shedding. The bridge failed before the fairings could be installed.

Kawada explains the aerodynamic issues with clear diagrams, including charts and graphs taken from the contemporaneous studies. These are particularly helpful in seeing how an understanding of the critical wind phenomena emerged and then developed further. A major report into the bridge failure was completed in 1941 by Ammann and others, largely exonerating Moisseiff on the grounds that he had simply followed the general trends in suspension bridge design.

However, the trend towards narrower bridges with less stiffness had brought designers back to the types of structure which had repeatedly failed in the 19th century, any lessons from the past having been forgotten or ignored. It is perhaps no surprise that Ammann's investigation report held Moisseiff largely blameless, when it is noted that Ammann's own Bronx-Whitestone Bridge had suffered from wind oscillation problems of its own, although less dramatic in magnitude.

Indeed, there were further lessons to be found in other contemporary bridges: the Thousand Islands and Deer Isle suspension bridges had been completed in 1937 and 1939 respectively, and both shallow-girder designs had encountered serious wind-induced vibration soon after completion. Both these bridges were stiffened by the addition of cable-stays, sufficiently to resolve the problems. Designer David Steinman had reported the problems to other engineers, but it seems that Moisseiff and Ammann had paid little attention.

The Tacoma Narrows disaster led to a huge retrenchment in American suspension bridge design, with deep trusses rapidly returned to favour. Some of these adopted new approaches to providing aerodynamic stability, introducing grids and gaps in the bridge deck, which greatly reduced instability. This trend also continued in the majority of Japanese suspension bridges built in the later parts of the 20th century.

Back in Europe, designers retained a degree of boldness. The Forth Road Bridge (1964) and Tagus River Bridge (1966) largely followed the safe truss-stiffened philosophy, although the latter was designed by Americans, including Steinman. However, Fritz Leonhardt had proposed a radical innovation for the Tagus design competition, an aerofoil box girder design, and this idea was taken up by the British for the Severn Bridge (1966).

The idea of eliminating aerodynamic disturbance, rather than resisting it, was not entirely new, as was clear from the proposal to add fairings to the Tacoma Narrows bridge. However, the Severn Bridge was revolutionary in the completeness of its design conception, using its aerodynamically sleek profile to achieve substantial economies in the amount of material required. Compare, for example, the American Verrazano-Narrows Bridge, completed in 1964. At a span of 1298m, it was significantly longer than the Forth Road Bridge (1006m) or Severn Bridge (988m). However, the weight of the bridge deck is many times higher: 45200 tonnes for Verrazano-Narrows, as against 16300 tonnes on the Forth, and 11400 tonnes on the Severn.

The Severn Bridge was bold, but problems with the structure were rapidly discovered, including issues with hanger vibration, the strength of the towers, and fatigue in the deck box girder. Kawada analyses these bridges in detail, concluding that the Severn Bridge's problems can be attributed directly to its economy, specifically its lightness of weight. He argues that engineers had forgotten that the stiffening effects of mass could be a virtue. In this sense, the pursuit of slenderness had again led to failure. His basic point is well made, but I think it is not entirely fair in the case of the Severn Bridge, with many of the problems resulting mainly from rapidly growing traffic volumes, well in excess of the original design specification.

Kawada's book comes up-to-date with examinations of the world record holding Akashi Kaikyō Bridge (designed on the American heavy-truss principle), the Storebaelt Bridge (designed on the European aerofoil principle), and the London Millennium Bridge (designed on the "we-know-nothing-about-suspension-bridges principle"). He ends by looking at possible future bridges, such as the Messina Strait Crossing.

In concluding, Kawada quotes with approval the American professor David Billington:
"History, for structural engineers, is of an importance equal to science".
This is undeniably the value of this excellent book. I don't think you have to be a designer of enormous suspension bridges to grasp the significance of the history which is recounted here: it is a story of ignorance and complacency, and of the unavoidable surprises which await pioneers of any stripe. These issues appear in many guises in other areas of structural engineering, but are seldom recounted with such thoroughness and clarity.

"History of the Modern Suspension Bridge" is clearly worth reading for any bridge engineer. If you haven't already read Richard Scott's "In the Wake of Tacoma", I would recommend it just as much, although for different reasons - the two books are complementary. Kawada is good on the engineering, the diagrams, and has commendable brevity. Scott is better on the personalities, and has a level of detail that Kawada doesn't match. I enjoyed both books, very much.

21 January 2018

Johnson Street Bridge shenanigans

I can only scratch the surface of this exceptionally complex and sorry saga. Long-time readers may recall that I wrote about the Johnson Street Bridge project in Victoria, Canada, back in October and November 2009. I haven't really followed it in any detail since then, but I've been missing out on a fascinating story.


Back in 2009, the controversy was over Victoria's decision to replace a highly historic Strauss heel trunnion bascule bridge, a rare and complex structure (pictured, courtesy Cacophony via Wikipedia). In April 2009, the city had identified that the bridge was in poor condition and vulnerable to potential collapse in a seismic event. They commissioned engineers MMM and architect Wilkinson Eyre to develop options for a replacement span. An unusual and very interesting design was selected, a $63m rolling bascule bridge with a ring girder which rotates about its centre point (visualisation from WEA shown below).


Controversy centred on whether the new bridge was actually necessary, or whether the historic structure could have been refurbished rather than lost. There was plenty of discussion, and the debate was well publicised and well informed.

Things have moved on considerably. The span replacement project has continued, with PCL appointed as contractor and Hardesty and Hanover added to the design team. The structure is currently on site, due for completion in March. The budget for the project is now reported to be $105m.

Before you read any further, you may wish to find a comfortable chair, and pour a very large glass of whisky. Or two.

The people at johnsonstreetbridge.org were key in opposing the original intention for a bridge replacement, and have since kept a close eye on the project. They have links to a whole series of key documents which describe the scheme's often shambolic progress, which taken together make for very painful reading.

A particular classic is MMM's letter to the City of Victoria in May 2014. MMM were (and still are) appointed as Victoria's bridge design consultant; this is not a design-and-build project. PCL had a peculiar contract which covers construction but which obliges them to propose value engineering ideas with a view to reducing the cost of the project (clearly, this strategy has been a big fat failure!) Despite this glorious aspiration, PCL had written to Victoria in March 2014 requesting both an extension of time and an increase in their payment for the works. They argued that it was impossible to stay within their original price, and that they were entitled to more.

MMM's letter advised Victoria on whether to accept PCL's claim. It cannot be considered in any respect objective, as MMM were also defending themselves against a series of faults alleged by PCL. Read all 30 pages of it with a very large pinch of salt. However, it's a frankly terrifying read: an appalling saga of poor performance, buck-passing, and what must be one of the worst procurement arrangements I've ever seen.

I can't bear to give even the edited highlights of it here: go and read it (only if you really did pour that big glass of whisky) to see how this project was spiralling rapidly downhill. The real root of the problems is never mentioned, however, which is the absurdity of the procurement arrangement.

MMM had prepared a reference design for the City, although this was only quite preliminary in scope at the time when PCL were appointed (on what was supposedly a lump-sum contract). PCL were obliged to develop a value-engineered design as part of their bid, and appointed Hardesty and Hanover to assist with this. This was also not undertaken to any great level of detail. Despite this, PCL were obliged to stick to their agreed lump-sum, while Victoria and MMM retained full responsibility for the detailed design.

Yes, read that again, and weep if you wish. PCL could not properly control the design (and hence the extent of their construction work), yet were held to a fixed price. Indeed, PCL's designers, Hardesty and Hanover, were then novated across to join MMM's team in developing the detailed design. I don't think I've ever heard of such an arrangement before - the more obvious setup would have been to novate MMM to PCL so that the contractor could control the design-and-build process as an integrated exercise in order to mitigate their risks. This would at least have given Victoria complete clarity as to where any further problems lay. The whole arrangement is utterly bizarre.

Beginning to recognise that the project was in serious difficulty, Victoria appointed independent consultant Jonathan Huggett to report on what was going wrong, and recommend how it should be put right. His report was issued in July 2014, and sidesteps the obviously flawed contractual arrangements but highlights a complete lack of project leadership, the complete lack of collaborative behaviours, failure to properly identify and address key risks, and lack of independent dispute resolution, amongst other problems.

Huggett's report was clearly taken quickly to heart by the City of Victoria - even before he had finished writing it, he had already been appointed to take charge of the project going forward.

Somewhere in all this, it's worth noting that Wilkinson Eyre's original design has been substantially watered down in the ongoing effort to contain the ever-escalating costs. We can only guess how much higher the project budget would now be if the original design was still being built!

There are a few more reports worth reading if you poured a second glass of whisky, not just one.

Online news site Focus on Victoria has been a dogged pursuer of the project's difficulties. An article earlier this year on issues with the bridge's fendering design is a splendid example of how easily the project seems to have blundered into difficulty.

The latest reports from Focus cover issues with the bridge's steel fabrication. They highlight the discovery of a problem with the steelwork, which appears to have been covered over with a truly awful looking bolted plate, a real bodge if ever you see one (photos are from Focus, with permission). The steel ring girders had to be cut open for repair work, although the reason has not been made public in any detail. The contractor's QA firm reportedly found a "design flaw" while steel was under fabrication in China.


This doesn't really make sense: QA firms are not there to validate design, they are there to ensure compliance with the design during construction. Indeed, Focus's lengthy article may well be making a mountain out of a molehill, suggesting a cover-up and conspiracy to the extent of malfeasance. It's difficult to judge the seriousness of the issue without further information being made available. However, Focus is quite write to criticise the detail. It's clear from the photographs that nothing this awful should be considered acceptable as part of the finished structure.


Like a dog with a bone, Focus won't let this one go, returning with a second article wondering quite why the (presumably exasperated) City Council won't make public all the details. What have they got to hide? Perhaps on a project so bedevilled with disaster they simply lack the energy for further exposure. Who could blame them? On the other hand, Victoria has been exhaustively open about what else it releases, down even to copies of supplier invoices.

This is a complex and innovative bridge, and it's hard not to think that further problems will occur before the project will be complete. Some of that is inevitable with such a bespoke, pioneering design. I certainly wouldn't bet against mechanical problems during the commissioning stage. Teething problems are, however, normal, and I hope that any further press coverage is balanced rather than sensational.

If all goes well, in a few months time we can look forward to seeing the completed bridge, and comparing it against the original vision. Hopefully it will be something that everyone involved can be proud of. However, the tale of woe that has bedevilled the project from the outset will continue to offer many lessons to be learned for others involved in bridge procurement, long after the dust eventually settles.

14 January 2018

Danish Bridges: 6. Cykelslangen, Copenhagen


This is the last in this series of posts about the bridges of Copenhagen. There are other interesting structures in the city, but I didn't have time to visit them on this occasion.

Opened in 2014, this 230m long bridge was designed by Dissing + Weitling with Ramboll. It is a bridge for cyclists only - no pedestrians are permitted. It forms a key link in a long cycle path, connecting the Bryggebroen at one end to Dybbølsbro at the other, allowing cyclists to pass across Copenhagen's inner harbour, bypass the Fisketorvet shopping mall, and cross over a major railway corridor. As well as making cyclists' journeys easier, it also helps keep them out of the way of pedestrians.

Nicknamed the "bicycle snake", the bridge curves between buildings and above a harbour inlet. I imagine it gives cyclists some great views, but I decided not to get in their way. I could only admire the bridge from below.

The bridge is minimalist in design, with a steel spine box girder below the deck supported on steel tubular columns. The curved layout of the bridge in plan is sufficient to ensure stability against overturning, with the girder restraining torsional movement. There is nothing extraneous in the design, just what's necessary and no more. The only concession to anything even slightly excessive is the adoption of an orange surface for the floor, although that is less evident under bright lighting at night.

It's an admirable design, and much superior to its neighbour, Bryggebroen. It shows that even a functional and economical design can be greatly improved if treated with care and attention.






Further information:

11 January 2018

Danish Bridges: 5. Bryggebroen, Copenhagen


The previous four bridges in this series of posts have all been moveable bridges, and Bryggebroen ("Quay Bridge") doesn't break that trend.

Built in 2006, it connects two quaysides across the Copenhagen inner harbour, hence the imaginative name. The east side is mostly a residential area, with the Gemini Residence, apartments built around former grain silos, dominating the area immediately adjacent to the bridge. The west bank features offices, residences, and a large shopping mall.

The bridge was designed by Dissing + Weitling with Carl Bro as engineer. The contractor's engineer for the D&B phase was COWI. The 190m long bridge is a steel spine-beam structure, with the spine box dividing cycle and foot traffic. The structural form was a consequence both of the traffic segregation and the desire for a low construction depth from floor to soffit.

The opening element is an asymmetric swing span (44m navigation span and 23m back span). Deep boxes sag below the walkway floor to provide sufficient strength and stiffness, and the pivot is tucked away between two sides of the box. As with the similar arrangement on the Inderhavnbroen, it is visually massive, and out-of-keeping with the rest of the bridge.

The designers acknowledge that this was never meant to be an "iconic" structure, but given its prominent position, I think too little effort was made to render it visually attractive. The piers are thin tubular trestle legs, which feel awkward next to the width and scale of the bridge deck.

The spine beam, particularly seen up close, is lengthy and somewhat morose. I'm puzzled as to why nothing was done to prevent people walking, skating or riding along its top: perhaps these things simply don't happen in sober Copenhagen.


Further information:
  • Google maps
  • Wikipedia
  • Structurae
  • Non-iconic footbridges (Jensen, Footbridge 2008)
  • Rethinking Cities (Trojaborg, Jensen and Henriksen, Footbridge 2017)

09 January 2018

Danish Bridges: 4. Cirkelbroen, Copenhagen


The fourth bridge in this set of six Copenhagen spans is another moveable bridge, although of a very different type to the three previous examples.

Copenhagen appears to have had something of a bridge-building boom. Two of the bridges I've covered so far are recent: Inderhavnsbroen (2016) and the Butterfly Bridge (2015). The Cirkelbroen ("Circle Bridge") was also completed in 2015. Another bridge is in the process of being built across the inner harbour right now, designed by Buro Happold and Wilkinson Eyre.

The Cirkelbroen spans across the mouth of the Christianshavns canal, where it enters Copenhagen's inner harbour. It therefore eliminates the need for quite a long walking or cycling detour, and it's clearly a useful piece of infrastructure. Its construction was funded by a private donor, Nordea Fonden, who appointed artist Olafur Eliasson to come up with the design, working alongside engineers Rambøll.

The bridge consists of five overlapping circular platforms, each ornamented with a tall mast, stabilised with a series of cables, like five intersecting spectral Christmas trees. The designer's intent was to create a non-linear pathway, forcing bridge users to slow down, stop, and look around. Given the problems encountered by speeding cyclists on the highly linear Inderhavnsbroen, it feels like a smart move.

As with the Inderhavnsbroen, the Cirkelbroen's history has not been without incident. In 2013, the main contractor Pihl collapsed, followed shortly afterwards by the steelwork subcontractor VSB.  In the same year, the bridge was attacked in the courts. It had been granted a special dispensation from the municipal development plan in 2011, presumably to allow Copenhagen to take advantage of Nordea Fonden's generosity, but a lawsuit sought to have this declared illegal. In 2014, the lawsuit was successful, and work on the bridge was halted, with much of the substructure already complete, and the superstructure steelwork largely complete but not yet on site.

The project recovered fairly quickly, with a new development plan put in place, and a new contractor appointed. The shenanigans surrounding artist-led, privately-funded development pushed through a planning consent process for financial and political reasons reminds me a little of London's Garden Bridge, of the dangers in allowing private gifts to distort or bypass the proper democratic process, whatever the ultimate public benefit.

The finished bridge is, I think, mostly a success.

It is easy to criticise the masts as unnecessary adornment, on a bridge with tiny spans which simply doesn't need to express any height. I think the structure would have looked absolutely fine without these elements, and they are emblematic of how difficult it is for artists to understand and address sympathetically the arena of architecture and infrastructure. Dispensing with the tent-poles would have allowed more attention to be devoted to the bridge at floor-level. Alternatively, the tent-poles could have been made genuinely structural, and integrated into a lighter-weight bridge floor. In this respect, I think the design is a failure, as all the best bridge designs are better able to integrate their disparate elements.

The bridge at deck-level is an enjoyable structure. I think perhaps it should have been made even less linear, as cyclists are still tempted to cross too rapidly for comfort of nearby pedestrians. The parapet detailing is attractive, and the bridge platform becomes a series of interlinked belvederes.

When I visited the bridge, I was unaware of how it opened. There are clear joints in the deck which indicated some form of rotation, but I could only make sense of it after returning home and viewing videos.

The best understanding of how it works can be gleaned from a paper presented to the Nordic annual bridge conference in 2014. The two southern discs are fixed, while the three northern discs move when the bridge opens. The central disc forms a pivot, and the three moving discs rotate about it: this is a swing bridge, if a very unusual one. To a structural engineer, it's initially baffling, as you assume that the pillars below each disc provide permanent support.

Instead, the two northern pillars are supported on a giant steel box hidden below water level, shaped like a triangle in plan to support those two pillars while cantilevering from the rotating pivot pillar. At first sight, with the absence of any counterweight, it looks physically impossible, but the triangular support box is hollow and hence buoyant - its tendency to float counteracts its self-weight. I assume it cannot be perfectly balanced, as the buoyancy will vary with water level and fluctuations in salinity, but presumably it's balanced enough that the variation in loads on the pivot pillar are tolerable.

You can see the bridge opening here on YouTube (concept and reality):



It's not hard to see this system as being ripe for problems over the structure's design life: the rotating surface is below water level, vulnerable to corrosion, silting and other degradation. I trust that the city of Copenhagen has a good maintenance regime in place.

I struggle a little with the swing bridge concept for this structure, which feels counter-intuitive and unnatural. At the same time, I admire its audacity, and the ability of the bridge to subvert expectations. There's a little hint of magic to it.

Despite its oddities, I liked this bridge.


Further information:

07 January 2018

Danish Bridges: 3. Inderhavnsbroen, Copenhagen


All the bridges I visited in Copenhagen either span the city's inner harbour, or run close by it. The third in this set, the Inderhavnsbroen, is named simply for what it is: the Inner Harbour Bridge. It's the most recent bridge to span these waters, completed in August 2016.

The bridge was designed by COWI and Studio Bednarski, adopting a rare bridge typology and making it even more unique. It carries pedestrians and cyclists across the waterway, and is a double leaf retractable bridge. Structurae only lists seven retractable bridges worldwide, and I think only 4 of those are still in use, although that's certainly an incomplete list.

The Inderhavnsbroen is distinctive in a number of ways. The general arrangement is unusual, with two approaches each arranged in a tuning-fork layout, allowing the retractable decks to slide back and forth between the prongs of the fork. Plan curvature adds significant complexity, with the bridge overall adopting a gentle S-shaped curve.

The bridge had a troubled gestation. This is an understatement, to say the least! The contractor Pihl and Søn was appointed in 2011, with a price reported to be significantly below that of other bidders. As the project progressed, problems were reported with the steelwork not meeting requirements, and later with cracks and other defects in the concrete supports. Errors were reported in design drawings showing supports 0.6m higher than they should have done.

In August 2013, Pihl and Søn collapsed. I believe all the bridge steelwork already fabricated was scrapped, and during the wait for a new firm to be appointed, storm flooding damaged electric motors which had already been installed. Independent engineers were brought in to review the works, reporting inadequate reinforcement in some supports. The new contractor appointed in 2014, Valmont SM, had to undertake extensive remedial works to parts of the structure already completed.

Progress improved, but new problems were still encountered. The wire rope system which pulls the bridge open and closed was found to be faulty. Close to completion of the construction, it was found that the two arms of the bridge did not meet properly in the middle, with locking bolts misaligned. This was attributed to twisting under differential temperature effects, caused by temperature variations across the width of the bridge deck, not allowed for in design standards.

By the time the bridge was complete, it was claimed to have taken twice as long as planned, and cost 50% more than the original budget. The design team went on record to criticise their own client, the City of Copenhagen, who have also gone on record criticising the design engineer. For the most part, the design team's complaints related to the client's procurement and project management process, which led to a number of "unauthorised" or bodged changes to the original design.

Problems during design and construction are often forgotten and forgiven once a bridge is complete. Unfortunately, the Inderhavnsbroen has experienced considerable public criticism even once in service - more on that in a moment.

It's worth commenting on the way in which the bridge was funded, along with three smaller structures including the nearby Butterfly Bridge. It was not funded by the City of Copenhagen, but by the A.P. Møller Foundation, to the tune (in the end) of 241 million Danish Krone (about £29m, or €32m). These are not in my view excessive sums when compared to other expensive and complex spans such as the millennium bridges in Gateshead or London. However, I do wonder whether the lure of private money can subvert normal sound governance, public accountability, and good decision-making. Would the municipality have approached the project differently if it had been entirely their own money?

The basic concept for the bridge has a lot of merit. The arrangement chosen allows pedestrians to get much closer to "the action" when the bridge opens than is often the case with opening bridge. Bascule spans rise up to block the river from view. Swing bridges leave waiting users stranded. The Inderhavnenbroen concept uses the side spans as balconies so that those waiting can have a closer encounter both with the movement of the main span and with any ships that pass through.

The arrangement has some unfortunate visual side effects, however. Moving bridges often suffer from a loss of visual continuity associated with unavoidable breaks in the structure. Here, however, the offset from the side spans to the main spans creates a significant disjuncture in both form and scale, which I felt was quite jarring from many viewing positions.

This is then accentuated further by the design of the moving spans. The side spans are all girders below deck, while the moving spans have massive edge girders which not only sag below deck at the point of support, like the wings of a robotic manta ray, but also rise up above floor level at the same point. This leads to a very distinctive change in level of the edge of the deck at the position where the opening and fixed spans meet, and it has also introduced a complex twist into the opening span balustrades.

Generally, the moving spans seem far too deep to me, giving them a feel of heaviness rather than the lightness that a moveable bridge normally embodies. The design of the edge girders also determines too many other elements of the design. They are angled outwards, leading to some mildly gruesome complications along the join-line between moving and fixed spans: the bridge balustrades are forced to be tilted outwards on the fixed spans, as are gates which prevent access to the main spans when the bridge opens. However, the balustrades on the moving spans are vertical at this point, tilting inwards above the support girders, and then becoming vertical again at midspan. It's all extremely awkward.

The rising barrier gates reportedly became a problem late in the bridge's development, with short segments of triangular barrier added next to them as an afterthought.

The whole area of the ends of the moving deck is troublesome. A median barrier is used to separate pedestrian and cyclists, although they have no barrier on the rest of the main span. Cyclists are obliged to negotiate rapid chicanes to pass from the internal to the external spans, and this has been the source of much criticism, expressed most pitilessly in an article for ArchDaily, and physically embodied by the addition of red and white striped warning stickers on various parts of the bridge.

The bridge appears quite steep, and the chicanes are clearly too tight to negotiate on a bicycle without slowing down. A series of rubber skid-marks attests to the inevitable outcome: cyclists struggling to brake and avoid headfirst impact into the glazed balustrades which form the ends of the moveable span.

The slope of the deck was probably unavoidable given navigational requirements and the general topography of the harbour area. However, the dead-end facing oncoming cyclists was eminently avoidable: the chicanes could have been made less tight, and the ends of the deck angled so that errant cyclists are at least spared head-on impact.

It's hard not to wonder whether the twin-girder solution is also partly to blame. A spine-beam deck would have kept cyclists and pedestrians separated throughout, avoiding the risk of impacting the end of the median barrier. It may also have allowed the visual depth of the opening span to be better integrated with the side spans.

The separation of cyclists and pedestrians also generates a great deal of dead space on the bridge. The balcony on the pedestrian side of the bridge may get some use, but matching balconies on the cyclist side presumably see very little use most of the time: I certainly didn't want to risk crossing the cycle path to get to it.

Perhaps it's not the done thing in Copenhagen, which is criss-crossed with cycleways, but adopting a shared-space approach would have had the advantage of both slowing down cyclists tremendously as well as giving all bridge users better access to all parts of the bridge. With the current arrangement, pedestrians are essentially barred from the north half of the structure.

On the positive side, the bridge clearly fills an important gap in the city's connectivity; I visited both at night and in daytime, and it was always well used. It also escapes the greyness which is characteristic of too much Scandinavian infrastructure, even though only a little. The blue and yellow glazed panels contrast well with the rest of the bridge, and are a nice touch. One panel is currently cracked, but I don't know whether a cycling accident or vandalism is to blame.

The choice of design was ambitious from the outset. The numerous problems encountered during design development and construction are to some extent the result of misfortune and mismanagement. However, some are inherent in the design concept, and others are the sort of thing which is inevitable with any highly complex, pioneering structure, especially where bespoke mechanical engineering is involved. That's an inevitable consequence of a challenging, uncompromising design.

I was very glad to visit the Inderhavnsbroen. It's a very interesting structure, and everyone involved no doubt feels a great sense of accomplishment for its (eventual) completion. It will be interesting to see whether the Copenhagen municipality can find ways to address the design flaws other than simply sticking on red-and-white warning markers. I would hope that there are more creative solutions better in keeping with the rest of the design intent.






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