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:

.@NTSB_Newsroom officials say workers were working on strengthening the north side of the #fiubridge during the collapse. @MiamiHerald pic.twitter.com/nspRErAk7f

— Monique O. Madan (@MoniqueOMadan) March 17, 2018

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:

Breaking Video: Dramatic dash cam footage shows the moment of the bridge collapse at FIU. pic.twitter.com/bm63EncDTj

— PM Breaking News (@PMBreakingNews) March 17, 2018

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.

"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.

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.