17 March 2018

The collapse of the FIU Sweetwater Pedestrian Bridge

I am reluctant to add to the media blizzard surrounding the tragic collapse of the Florida International University (FIU) pedestrian bridge in Miami. As I am typing this, the recovery operation is not yet complete, and I think it is both difficult and inappropriate to speculate in too much detail on why the bridge failed with such awful consequences.

I will therefore try to be cautious and factual in what I say, as it seems clear that the reasons for the bridge collapse will be better identified and shared by those with full access to the facts. The desire to rapidly identify causation (and to lay blame) is understandable, but I would like to minimise speculation.

Media coverage
Much of the coverage in the press has been ill-informed guesswork, attempting to draw together whatever half-truths have emerged in order to flag issues which may or may not ultimately prove to be meaningful.

Prime suspects identified by the media include past failures attributed to the two main design-and-build companies involved in the FIU bridge project, MCM (the contractor) and Figg (the designer). Repeated quality failings are a possible issue, but my experience is that there are almost always many contributing causes to any serious failure.

There are even less likely culprits put forward on Twitter: Trump (of course), a false-flag conspiracy, immigrant labour, and most egregiously of all, "diversity-hiring". For the sake of our sanity (often difficult when reading Twitter), I'll say nothing more about these and return to the suspects fingered in the mainstream media.

"Innovation" is linked by one engineering professor to "unexpected failure", as if to imply that innovation is always too risky an approach to take. He may have been misquoted, but this criticism is repeated elsewhere, giving the impression that 'doing new stuff' is so dangerous that it should never be attempted. Says the prof: "Innovations always bring potential 'failure modes' that have not been previously experienced".

There's no doubt that innovation can introduce new risks, but these are normally managed through appropriate review and risk management. I've seen nothing to suggest that the designer, checker, contractor, highway authority (Florida Department of Transportation, FDOT), or owner's engineer (TY Lin) had any doubt about the safety of any of this bridge's innovations in advance. In any event, it is clear from TY Lin's project specification that innovation was something their client would evaluate positively: they actively sought it out.

What innovation is at issue anyway? Many of the news reports point the finger at Accelerated Bridge Construction (ABC), the method adopted by the contractor to install the bridge span across a busy highway with as little disruption to traffic as possible. Ironically, ABC is something that FIU have a keen interest in, and in promoting their bid to build the bridge, the MCM-Figg team enthusiastically drew attention to the connection.

ABC refers to a family of methods for building bridges faster, usually more safely, and often cheaper. The common elements are the use of offsite or modular pre-construction techniques, so that bridges are assembled in-situ as quickly as possible, rather than built entirely in place. Engineers promoting ABC techniques in the US have come up with some excellent ideas, but it isn't fundamentally anything special, and rapid-installation techniques are widely used around the world. For a bridge such as the FIU Pedestrian Bridge, spanning a busy highway, you'd have to be asking serious questions of anyone who didn't adopt an ABC approach. Again, FIU's project specification made clear that ABC techniques would be acceptable, setting out associated construction requirements.

The span which collapsed was the first of two spans due to be installed, and is a simply supported concrete truss bridge designed to sit on its end-supports without any further temporary support (or indeed, permanent support - more on that later). It was built nearby and then wheeled into place on self-propelled modular transporters (SPMTs), an increasingly common way to build a bridge. In addition to reduced traffic disruption, a key driver for this was the presence of overhead power lines at one end of the bridge, which made craneage a less attractive approach. You can see the power lines at the left hand edge of a general arrangement drawing shared on Twitter:

Accelerated bridge construction is cited often in the initial news coverage of this disaster, but it is not in itself relevant, given that the bridge span was designed to span between its piers in both the temporary and permanent cases. More on this below.

Another 'issue' cited often in coverage is simply why the span was allowed to remain in place above live traffic. The highway authority, FDOT, have been at pains to rapidly disassociate themselves from the project, but have stated that it was their role to "authoriz[e] FIU to utilize the aerial space above the state road to build a structure".

My personal experience of building new bridges above existing highway or railway infrastructure is that the infrastructure owner takes a keen interest in the safety of the construction work, especially where the infrastructure will remain open to traffic prior to completion of the bridge. In the UK, they would undertake a full technical approval process, not checking the design, but assuring themselves that the teams involved are competent, that the processes in place are appropriate, and that risks have been properly identified and managed. Where a bridge will be in a temporary state with traffic running below, my experience is they take this very seriously.

Perhaps in the US it is different, but I would have thought that the primary responsibility for the safety of highway users lies with the highway authority, and that in agreeing to "authorize utilization of aerial space above the state road", they would take a keen interest in the details of what was proposed. Presumably they have the power not to permit the work to go ahead if they have any concerns.

In this case, however, there should have been no great concern about running traffic below the bridge: it was, as we will see, designed to span the highway without additional support, and to be able to carry full live loading in the same configuration. The design load required in FIU's specifications is 90 psf (4 kPa), on a span 31-feet (9.4m) wide by 175-feet (53.3m) long; a total live load of roughly 200 tonnes. It was clearly carrying nowhere near this load at the time of collapse.

Much of the initial commentary has noted the obvious disparity between the bridge's temporary condition (a concrete truss spanning simply supported), and the final cable-stayed arrangement shown in design visualisations (and on the drawings):

The suggestion is made that the bridge could not be expected to stand up without the stays in place, which would of course also require the tower to be complete, and the back-span, and the back-span abutment. All of these can be seen on the general arrangement drawing shown above (and on what you will see below).

Tender-stage design
However, the bridge was not designed to rely on the stay system. There are quite a few documents relating to the project online at the FIU's project website. For details of what was being proposed at tender stage, refer to the technical proposal from MCM and Figg. The images and drawings that follow are taken directly from that document. It must be emphasised that the final construction design may have been different, although I have not seen anything in photographs of the bridge which differs from these early drawings.

The proposal is a sales-pitch, and much of it reads very badly with hindsight, but there is no blame or shame in that. The picture above summarises some of the salient features of the design. The 5.5m tall concrete truss is conceptualised as a giant "I-girder", with the canopy overhead forming the top flange, the floor forming the bottom flange, and the diagonal truss members the web. The centre-to-centre distance of the flanges is around 5m, which is ample for a pedestrian bridge of this span. Here's the cross-section drawing from the proposal document:

Selection of a truss is in line with FIU's expectations: their own project specification identifies it as the most likely solution.

I've not found a clear explanation as to why concrete was preferred over the much more obvious use of steel for a trussed footbridge. MCM and Figg's proposal notes concrete's good vibration damping and thermal mass. The client specification permits use of both concrete and steel, although it does include a "Buy America" clause, which might make purchase of less expensive imported steel an issue.

The structure is all in post-tensioned concrete. The bottom slab is prestressed both longitudinally and transversely. The top slab is prestressed longitudinally. Most of the diagonal members are also shown as prestressed. A series of design drawings on pages 109-115 of the design-build technical proposal show the prestressing details proposed at the time of tender, and one of these is discussed further below.

Here is the general arrangement drawing from the technical proposal:

Diagrams in the proposal make clear that the structure did not require erection of the tower or stays during construction:

The explanation for the tower and stay system is twofold. Much is said about its relevance as a visual statement, the provision of a landmark structure. It can be seen that the truss arrangement has been adapted to suit the angle of the stays - this appears to be entirely for visual reasons, as you'll see shortly that the stays are not strongly connected to either the deck or the tower.

The diagram below makes clear the second reason for the stays, that they are there to alter the stiffness of the main span, bringing its vertical frequency above 3 Hz and hence out of the range for pedestrian excitation. This is a simplistic approach - I believe most pedestrian bridge designers would have accepted a lower frequency and dealt with the issue by more detailed analysis or by use of damping devices if necessary.

This diagram above states clearly that "the structure meets strength design criteria without the stays". The truss was designed to be strong enough on its own to carry its self-weight plus pedestrian loading. The stays are only there to control vibration, and for visual effect.

Some of this was evident from the photographs of the collapsed bridge. There are no conventional cable connections on the top of the truss structure, only concrete blisters with protruding bolt heads. These could not possibly carry the tension forces required in stays carrying significant loads. Here's the detail shown on the tender drawings:

Note that the stays are not shown as cables, but steel pipes. Even with pipes, it's doubtful whether with a truss as stiff as this, the stays would have sufficient axial stiffness to carry any significant share of imposed load. Reducing vibrations is the best that they can do.

Also note that the connection between the main span and the back span is nothing substantial. In a true stayed bridge, there would be a substantial connection at this point, to carry the longitudinal compressive forces in the bridge deck which balance the tension forces in the stays:

Probably the most interesting detail in the tender-stage drawings is one which shows the prestressing in the diagonal truss members:

In any truss node, quite a lot is happening structurally. The vertical forces in the diagonals will be in balance: in the drawing above, if the left-hand diagonal at the node is in compression, the vertical component of that compression will be matched by a vertical component of tension in the right-hand diagonal. The sum of the horizontal components of force in the two diagonals is balanced by a change in horizontal force between the left-hand and right-hand elements of the horizontal member, which on the drawing represents the roof slab.

As this is a prestressed structure, there will significant compressive forces in the node, with high localised stresses due to the proximity of the stressing bar anchorages. Taken together with the change in forces to be accommodated through the node, this is a highly complex design element, and one which would have been much easier to design in steel rather than in concrete.

Bridge collapse
It is also the exact location where work was taking place immediately prior to the collapse. The news reports make reference to "stress tests" being undertaken at the time. One engineer speculates about adjustments to precamber, although this would not be possible in such a stiff truss structure.

Two days prior to the collapse, the lead bridge design engineer phoned the Florida Department of Transportation (FDOT) to advise that cracks had been found in the bridge. In a statement, FDOT make clear that this message was left as a voicemail, and not listened to until after the bridge had collapsed. This does not seem very relevant, given that in the same statement FDOT acknowledge that their representative did attend a meeting with the project team early on the day of the bridge collapse.

A statement from FIU confirms that this meeting involved the contractor, designer, FIU and FDOT, and that a detailed technical presentation was made regarding the crack. The design engineer is reported by FIU as stating that there were no safety concerns regarding the crack.

Later the same day, work was taking place on the bridge directly above one of the truss nodes. A crane can be seen to be in place, and appears to have been supporting equipment, in two videos which show the bridge collapsing. The first is taken from surveillance camera footage, the second from a vehicle's dashboard camera. The best-quality version of the footage that I've seen can be found on Twitter:

As I write, it isn't clear what work was taking place, nor what the various organisations involved had been told about that work. The preliminary drawings indicate this to be the position of dead-end anchorages for the web prestressing, not stressing anchorages, but it's possible that was changed during detailed design.

The designers, Figg, and the contractor, MCM, have said little at this point of time (e.g. see Figg's statement). They probably have little choice: it is very likely to be a condition of their insurance that in the event of a legal claim arising the insurer takes control of what is communicated.

In the video, it can be seen that if the truss is conceptualised like a girder, a global shear failure occurs around the position where work is taking place. Shear in a truss is carried by alternating compression and tension in the web members, so it is possible that the overall failure was caused by failure of a single web member, or by failure of the connecting node.

It appears from the videos that the second triangular frame from the left (upward-pointing, directly below the crane) deforms, with all other triangles retaining their shape. The very first (downward-pointing) triangle on the left is largely non-structural: the vertical on the end is just there to support the future bridge pylon, while the horizontal upper member in this triangle is just there to carry the upper prestressing tendons to their anchorage.

This is as far as I will go in commenting; it is tempting to speculate further, but it can only be speculation. No doubt more information will emerge soon, possibly between my typing this and you reading it.

I am sure there will be more to discuss once further facts come to light. Only then will it be possible to consider what lessons there may be for others working in the bridge design and construction industry.

10 March 2018

"Danube-bridges" by Péter Gyukics

I was delighted recently to pick up a copy of "Danube-bridges: from the Black Forest to the Black Sea" (Yuki Studio, 330pp, 2010) by photographer Péter Gyukics. This is the English edition of a book also available in Hungarian and in German, which depicts every single bridge along the River Danube from source to sea.

Gyukics has two previous books of bridge photography, 2005's "Hidak Magyarországon" ("Bridges of Hungary"), and 2007's "Hidak mentén a Tiszán" ("Bridges along the Tisza"). I haven't had the good fortune to see either of those, but I am impressed by "Danube-bridges" and I would certainly like to do so.

The Danube is 2860km long, passing through or along the border of ten European countries. It winds through four capital cities: Vienna, Bratislava, Budapest and Belgrade. It has been significant both as a boundary and as a transport corridor, and today it is also an important source of hydroelectric power.

Gyukics took two years to photograph every single bridge on the main river, and the book features them in sequence from the confluence of the Brigach and Breg rivers where the Danube begins, down to the river delta where it empties into the Black Sea. He also includes bridges on the navigable side-channels of the river (such as the Danube canal through Vienna), for a total of 342 bridges shown in 962 photos. All the bridges are those that carry traffic of some sort, which is a shame as it means that utility bridges such as the spectacular gas pipe suspension bridge at Smederevo are not included.

The book is one-of-a-kind, as a combination travelogue and encyclopaedia. It's not the only photographic record of a journey down the Danube, but it is the only complete record of the river bridges.

The photographs are accompanied by text from bridge experts Ernő Tóth and Herbert Träger, giving whatever factual information has been gleaned on each structure, such as year of construction, key dimensions, designer, contractor, and a description of interesting features or historical aspects. Ernő Tóth is a prolific writer on bridges in Hungary, and those interested should check out the Első Lánchíd website for more.

The book features useful maps, a detailed index, and a series of useful introductory sections, including a detailed and informative description of the river written jointly by a geologist and a hydraulic engineer.

The smallest bridge spans only 9m; the largest 351m. The bridges date from 1146 to the present day, although the majority have either been built or rebuilt within the last 75 years. This is a book of contemporary photography, so although older bridges on each site are described, there are no historic photos or images. Those can in some cases be found elsewhere, for example, the bridges in the Hungarian stretch are covered in more detail in the excellent book "Duna-hídjaink", which is freely available online. One of the oldest and longest bridges across the Danube, Constantine's Bridge, is an absentee (because it no longer exists), while the famous Trajan's Bridge is represented only by its remaining ruined foundation.

Gyukics has done well to find good vantage points to see the majority of the bridges, some of them photographed from the air or from boats on the river. The journey starts out slowly, with many pages of spans which are undistinguished although not entirely without interest. These are mostly fairly anonymous highway, rail and footway bridges in rural Germany. There are a few oddities to be found this high on the river, but what strikes me most is how much variety there is within the mundane. There are almost no two identical bridges, even where the same river crossing problem is solved again and again. Minor features of the context, and differences in approach by individual engineers, lead repeatedly to subtly different outcomes. There is plenty here for anyone who mistakenly thinks engineering is a science, rather than an art.

Although the photos focus upon the bridges, there is plenty to see in the countryside, as well as those who use the bridges. As the book proceeds, the possibilities in structural engineering steadily expand, while the scenery shifts gradually. Flip forward a few pages at a time and what is initially imperceptible becomes clearer, as the river increases in volume and comes steadily to dominate the landscape.

There are several bridges which are frankly dull, at least to begin with, but interspersed with cute little covered timber bridges, and more than a few interesting and unusual concrete and steel designs. Moving downstream, there is an increasing number of steel trusses, which accumulate until they become the Danube's dominant bridge form. It's tempting to try and pick out highlights, but there are so many structures that any bridge enthusiast should find something that delights or surprises. You can find thumbnail samples for most bridges at the publisher's website. Along its way, the Danube features some world-famous spans and well known designers, as well as a number of bridges which are structurally or architecturally remarkable.

There is a box girder bridge with a secret railway passing inside the box; a cable-stayed bridge with a cafe at its top; several arch bridges so thin they appear unstable; bridges with legs that look like inverted pyramids; an Austrian variation on Sergio Musmeci's thin-shell experiment in Basento; a twin-deck structure which lifts its lower deck when boats pass, like hitching up a skirt; and plenty more which are weird, wonderful and amazing.

It's impossible not to begin to spot one key reason behind the variety of structures, and the explanation why there are so few older structures. Many of the bridges are described as having been damaged or destroyed in the Second World War, and having been rebuilt since. The same is noted for bridges in Serbia, many of which were affected by NATO's air strikes in 1999. In times of war, access to river crossings is key, and the history of the Danube's bridges provides a reminder that bridges often play a  key role in peaceful trade and cooperation, and hence become particularly vulnerable in times of conflict.

The text is not always well translated into English, but it's quite good enough. Knowing that "permanent height" should instead be "constant depth" and that "belt" should be "chord" will resolve most of the more peculiar captions.

Copies of the book are available directly from the publisher, priced at €20 plus postage. For the UK, that worked out for me at €35 total, which is very good value for a full colour book of this size and length, although I had to pay bank transfer fees on top of this.

02 March 2018

Rotherhithe Bridge "controversy"

A couple of weeks ago, the architectural trade press reported a controversy on the procurement of Transport for London's proposed Canary Wharf to Rotherhithe crossing.

Following a detailed feasibility study, which looked at options for a ferry, tunnel, or opening bridge for pedestrians and cyclists at this location, TfL identified the opening bridge as their preferred solution. They undertook a public consultation, which ended in January. Meanwhile, they elected to push appointment of a design-and-build contractor to build the bridge back down the track. Instead, TfL are now looking to appoint a design team to develop a reference bridge design capable of securing consent under a Transport and Works Act Order.

The project has a history which in some ways brings to mind the Garden Bridge fiasco. It seems that the Rotherhithe scheme was not originally TfL's idea. Back in 2013, a bridge was proposed by reForm Architects, working with engineers Elliott Wood. ReForm's concept was for a double-leaf cable-stayed bascule structure (pictured), and they devoted quite considerable effort to promoting the concept and developing it further.

ReForm teamed up with cycling charity Sustrans to undertake a feasibility study, which demonstrated clearly that a bridge at this site could be highly valuable (although it is notable that the 2016 Sustrans report makes no mention of the reForm concept design). Ever since, reForm have remained tenacious in promoting the project, even setting up a "Save Rotherhithe Bridge" website, and asking Buro Happold to join their team to add credibility.

Recognising the merits of the proposed pedestrian and cycle crossing, TfL set out to investigate options for themselves, appointing consultant Arcadis from their engineering services framework. Arcadis teamed up with ubiquitous bridge specialists Knight Architects. The outcome of their study forms part of the public consultation material, with a clear and sensible report which evaluates the various issues without settling on a definitive solution.

The Arcadis/Knight report does however take against the reForm/Elliott Wood proposal, noting that a 150m span double-bascule bridge would be a world record-breaker (by some margin), while swing and lift bridges are more proven forms at this span length.

TfL are looking to appoint a team to produce a reference design (and take it through the Transport and Works Act process) from the same framework panel where they found Arcadis. At this point, reForm and Elliott Wood have cried foul, stirring up an utterly unmerited controversy in the press.

Construction News reports that there may be a "conflict of interest", on the grounds that Arcadis recommended against the bascule solution, and have therefore developed a brief which prevents competitors from participating.

It's hard to know where to begin with this nonsense. Reading the Arcadis report, it's clear that a bascule solution is not favoured, but nor is it ruled out. ReForm's problem is that they are not on the TfL framework. However, there is absolutely nothing preventing them from teaming up with one of the consultants who are on the framework. The real problem is their unwillingness to depart from their original design. TfL appear to be seeking a team who will properly evaluate the options and take forward the best one: not a team who are so committed to a single idea that they may tie TfL to an unsuitable outcome.

Indeed, the bascule design hardly seems like the best bet. 75m long bascule cantilevers will be a problem to operate in high wind conditions. The cable-stayed arrangement means that the centre of gravity is remote from the centre of rotation, with the result that energy requirements during operation are higher than for a better balanced design (such as a swing bridge or lift bridge).

Should TfL have opened up bidding beyond their framework? Again, this seems ridiculous: their framework consultants are generally large firms with plenty of expertise, and who are free to supplement their teams with niche specialists where they need to. Arcadis have no particular advantage as the incumbent, as the skills required for the previous phase may not be those needed going forward. TfL have made clear that they are looking at this stage for the right team, not a single design.

ReForm's complaint is that as a small firm, they are disadvantaged, but there is no reason whatsoever why TfL should not follow its existing procedure for procuring appropriate expertise. Presumably all the firms on their consultancy framework have already demonstrated that they can comply with TfL's general commercial and quality requirements, and will be providing further appropriate detail if they bid for the next stage.

Imagine if reForm had been appointed in place of all these firms to develop this project. Is it not overwhelmingly likely that those who had already bid for a place on the framework, jumping through multiple prequalification and tender hurdles to do so, would then be crying foul?

Indeed, if TfL were to bend over to create a way in for reForm Architects, they would be accused of exactly the same failures that allowed Heatherwick Studio and Arup to win the Garden Bridge contract in the face of better-scoring opposition. As with reForm, Heatherwick already had a concept design, and no real interest in evaluating alternatives to it.

If TfL genuinely want to do the best for their London taxpayers, then they should instead be applauded for the transparency and openness of their approach, carefully evaluating alternatives, gathering evidence, and making sure that the case for building the Canary Wharf to Rotherhithe Crossing is properly merited. Having done so, they are quite right to be considering all the options in more detail before settling on any specific design.

There is no conflict of interest here, and no real controversy.

Move along.

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: