In August 2017, I posted the 17-strong shortlist for a bridge design competition in Budapest, Hungary. The contest was seeking a design for a major new road bridge over the River Danube, to provide a long-planned transport connection on the south side of the city.
The competition winner was announced yesterday, and is a twin-mast cable-stayed bridge designed by UN Studio and Buro Happold. So far, I've not found much detail on the internet (and it's all in Hungarian), but I guess the span is around 250-300m.
The most obvious thing about the bridge (remarked on in several of the Hungarian news features), is its striking resemblance to the Erasmus Bridge in Rotterdam, also designed by UN Studio (but with different engineers). I guess if plagiarism is the highest form of flattery, plagiarising your own ideas shows who you respect the most.
It's basically two Erasmus bridges joined together, and has a figurative resemblance to two people kneeling down facing each other. The cranked tower legs are connected to massive edge girders in the back-spans.
Other than the doubling up, the other obvious difference to the Erasmus Bridge is that the towers are significantly chunkier in Budapest, which I guess is a consequence of carrying a heavier carriageway (the Danube Bridge contest specified four highway and two tramway lanes).
The Erasmus Bridge design was problematic - expensive to build; and the two edge girders were to a great extent over-sized fascias, sized for visual rather than structural effect. It was also greatly complicated by the details required to transfer axial loads between the main-span and back-span girders, as these are offset from one another. From what can be seen in these images, the same problem exists in the Budapest design.
There are two joint second-prize winners, Lavigne et Chéron Architectes, Bureau d’Etude Greisch, Közkekedés Engineers, Geovil; and Leonhardt Andrä und Partner, Beratende Ingenieure, Zaha Hadid Architects, WERNER Consult, Smoltzcyk and Partner. I've only found an image of the second of those:
This twin-arch design has the kind of gargantuan disregard for context that you would expect from ZHA. I'm struggling to understand why the judges would so highly praise a concept which puts a support pedestal slap-bang in the middle of the river, when this is so easily avoidable. The "hangers" connecting the arch to the deck are also so large that they give the impression they are holding up the arch, rather than suspending the deck.
I'm hoping that more of the entries will be made public, as it will be interesting to put the winners in context, and understand what made them the stand-out choices.
This is the newest of all the structures I saw on my trip to Leeds, having only opened to the public in October 2017. It carries a branch of the Trans Pennine Trail cycle/footpath across the River Aire, and it was installed as part of works for the first phase of the Leeds Flood Alleviation Scheme. The bridge was designed by Knight Architects and Mott MacDonald, and built by contractor BAM Nuttall.
Approaching this along the path from the nearby Thwaite Mills, my first thought is what a real oddity this bridge is. It feels a little like a space-age intruder in a semi-urban wilderness: super-sleek architectural design, the sort of thing that would feel right at a visitor "destination" but feels much more of a surprise on this out-of-the-way pathway. I found it disconcerting, at least to begin with.
The 70m long bridge spans across the new Knostrop weir, a series of three inflatable weir structures intended to be adjustable in times of flood. The weirs (and associated fish passes) are staggered in plan and are separated by slender concrete walls, which also form the supports for the bridge.
There seems to be nothing technical published online about the bridge design, but there are some images of construction at the website of steelwork contractor SH Structures, which provide a little illumination.
The YouTube video above is worth a look first, as the early sequences include some overhead views of the bridge layout.
The bridge crosses the river at what looks like roughly a 45-degree skew, and sits on a series of support legs which run parallel to the river (hence at 45 degrees to the bridge), plus a square abutment at one end. The bridge is straight in plan, but widens out with curved standing areas above each support position.
In cross-section, the bridge is a shallow multi-cellular steel box girder, with a smoothly curved soffit plate. The intersection of the plan geometry with the curved underside leads to a distinctive "scalloped" profile along both edges of the bridge.
The bridge legs are formed from simple steel plates, each only 50mm thick, which must be amongst the most slender footbridge supports anywhere in the UK. It's not quite down to Ney and Partners standards, but it's quite remarkable nonetheless. The legs are attractively shaped as well, holding the bridge delicately to one side of the weir so that bridge users get a good view of the site's main attraction.
The construction photos indicate that the pier legs also form diaphragm plates within the box girders, and that each box span is bolted through the diaphragm. I guess a cover plate was then welded across the finished joints.
Presumably the "leaf" piers are thin enough simply to flex under thermal expansion and contraction, although I think the effects must be complex due to the high skew. They must also be short enough to be safe against buckling against vertical load, so a careful balance of stiffness is required.
The parapets comprise stainless steel ribs with timber leaning rails. I like the simplicity of the main parapet section, but the upper rail, presumably to provide standard cycle parapet height, looks a little like an afterthought.
Nonetheless, this is a very high-quality bridge to find in such an unexpected place.
The next bridge I visited was the Urn Farm Bridge, which today carries the footpath Orion Walk across the M621 motorway. It was built as an accommodation bridge as part of the original M1 motorway circa 1967.
This is a three-hinged reinforced concrete arch, spanning 213 feet (65m) between springings and 320 feet (98m) long in total. The upper side spans are supported on half-joints. A photograph of the bridge under construction can be seen at the Motorway Archive online.
It is a moderately familiar form of construction, with other examples including Needle Eye Bridge, also on the M1, and Swanscombe Cutting Bridge. However, I doubt there are more than half-a-dozen in the UK. I always admire their clarity and elegance.
This example has been slightly marred by the addition of anti-climb guards to the legs, but it's still a lovely structure, a fine gateway on the motorway approach to Leeds, or a place for walkers to pause and admire the view.
From Stanley Ferry, I was heading north towards Leeds. My next stop was the Lofthouse Interchange, the magnificent junction between the M1 and M62 motorways, which can be viewed from Long Thorpe Lane on its northern side, for anyone so inclined.
The Lofthouse Interchange may be magnificent, but it's also notorious. Built in 1967, it is a three-level interchange, with the M62 crossing above the M1, and an 800-foot diameter roundabout built above both motorways to accommodate all interchanging traffic. It looks fantastic from above, but it creates a terrible bottleneck, due to the conflict between traffic streams entering the roundabout.
In 1999, major improvements were made by the introduction of new link roads connecting the western arm of the M62 to the northern arm of the M1. However, many conflicting movements remain on the roundabout, and it is known both for congestion and for collisions.
Highways England are consulting on a scheme to improve the junction, although there's no information yet on what this may actually involve. The best long-term solution would be to completely separate all slip road movements, but that is likely to be both hugely expensive and hugely disruptive during construction.
One casualty of any radical change could be the junction's distinctive "banana piers". Judging from the degraded state of the concrete on these, that may not be such a bad thing.
The designers of the Lofthouse Interchange were looking to solve two structural engineering problems, at a time when computer structural analysis was not as ubiquitous as it is today. The first was the concern that mineworkings in the area could lead to settlement of the bridges.
The common solution to this at the time was to introduce as much articulation as possible, so, for example, using a series of simply supported spans rather than continuous beams. The second concern for the engineers was thermal expansion and contraction of the curved bridge decks on the tall support piers.
The issue is not entirely clear: for short simply-supported spans, sliding could be accommodated by bearings on the top of each pier, and the piers designed accordingly. I think the issue here is that the designers wanted the decks to be connected together so that there was only one expansion joint at the end of each bridge: the decks are therefore connected to the pier via fixed bearings (permitting rotation under settlement), all expanding from one abutment.
In any event, the "banana piers" were the solution. The "banana" element is hinged at its base, and supported from a cruciform-section concrete strut hinged top and bottom. The effect of this is that the bridge deck can expand along the direction of its curvature while both deck and pier remain stable. It is a simplistic, statically determinate solution, which today would probably be dealt with by designing the piers to be flexible enough to accommodate thermal movement.
Working with RIBA's competitions office, SCC had set up a contest to select an architectural partner for their previously-appointed consulting engineer, WSP.
The prequalification conditions were set in such a way as to prevent smaller practices from entering (although this was later changed, after complaints), and engineers, however creative, were excluded, unless they were also registered as an architect. The contest was in essence a beauty parade to set up an arranged marriage between the winner and WSP, but was formatted as a design contest, with each competitor preparing designs to be judged, but not necessarily selected for further development.
Probably the contest's worst feature was that the evaluation was to be made on a quality/price split, with 60% of marks for quality of the design, and 40% on the basis of a lump sum fee quotation. This gave SCC some assurance that they would not be screwed in post-contest negotiations over the architect's fee, but it set up the possibility that a poor designer could be selected on the basis of being cheaper than better contestants, or that a good designer could lose out due to a slightly higher fee.
It was far from clear what SCC were really seeking, but the whole competition seemed to fly in the face of best practice.
My comments were echoed by others, with several criticisms reported in the Architects' Journal the following month. Much of the criticism focused on the arranged marriage element, which is not the way most successful collaborative teams normally come together.
The competition organisers selected five competitors in October 2016: Foster + Partners, WilkinsonEyre, Knight Architects, Marc Mimram, and a team of Adamson Associates / Ney and Partners / William Matthews Associates (WMA). The observant reader will note that three of the five (Foster, Mimram and Ney) have substantial engineering expertise of their own. At this time I commented: "The financial element will work against certain entrants, I think, as my experience is that architects' fee levels can vary significantly."
In March 2017, the winner was announced as Foster + Partners, which was interesting as they were one of the competitors I would have expected to have been disadvantaged by the price element of the evaluation. I doubt that many clients select Foster because they are cheaper than the alternatives.
Local MP Ben Gummer was quoted:
"The fact that we will have what will be a globally recognised bridge of beauty will say something powerful about our town's ambition and our place not just in our county, or our region, or our country, but in the world."
I expressed the feeling that perhaps Suffolk were being over-ambitious, and I also noted the way that what had supposedly been a contest to choose a design partner had subtly slipped into being a contest to identify the best design.
Now, I must say I don't really trust the AJ on bridge procurement. They have played a key role in exposing the shambles of London's ill-fated Garden Bridge, but they have also manufactured some completely nonsensical controversy with regard to the Canary Wharf to Rotherhithe crossing.
Nonetheless, their take on the Upper Orwell Crossings does not make for happy reading. I'll pick out only a couple of the key points.
First, the AJ draws attention to the scoring of submissions by the judges, and they include an image of the scoring sheets in their article. What jumps out here is that judges Michael Hopkins and Patty Hopkins each scored the Foster + Partners submission 10/10 not only on every criterion, but for all three of the bridge spans presented (there were three spans required, A, B and C). For those who don't follow architecture closely, Michael and Patty are husband and wife. Michael Hopkins was, of course, also one of Foster's close early associates, both having worked together on the Willis Building in Ipswich, Suffolk.
Here are the evaluation criteria set out in the original invitation to tender document (which is helpfully archived online):
What is notable here, in the quality marking, is that 50/60 of the marks are for the quality of the designs submitted, and only 10/60 for "method of working", despite this supposedly being a contest to select a design partner, not a design.
As did all the contestants, Foster + Partners submitted two designs for each of the two main spans. Looking at the score sheets provided by the AJ in their article, Michael Hopkins did not even bother to mark the second design (it appears that all the other judges did). Patty Hopkins did mark it slightly lower than the first design.
Some of the other judges appeared more critical. In comparison to the two Hopkins judges, Kevin Drain, the lead WSP engineer responsible for the project and who is presumably now working with the architect to take the project forward, awarded Foster's designs 102 out of the maximum possible 120 marks. Jonathan McDowell, the member of the jury with perhaps the greatest experience as a bridge architect, gave only 79 out of 120.
Here is the jury panel as originally announced in the tender invitation:
The AJ remarks on the notable absence of Patty Hopkins from that list.
The designs of the other competitors have not been made public, so it's impossible to comment on their relative quality. I note that the Foster design for the main highway bridge (all the images used in this post) shares some distinctive design elements with their Chateau Margaux Winery, tree-like supports designed for a small building roof and here adapted for a much heavier highway bridge. It will be interesting to see whether the competition-phase design is taken forward.
Second, this extract from the timeline put together by the AJ makes interesting reading:
The original plan was for a winner to be announced in December 2016. By the time of shortlisting, the timetable had slipped, and the announcement was then scheduled for January 2017. The timeline shows that instead, Suffolk sought "financial clarifications" from bidders, delaying the announcement until March.
The AJ reports that the amendments to tender prices may not in fact have been legally appropriate: the "restricted procedure" (shortlisting followed by tender submission) under EU law does not normally permit post-tender negotiations. It is intended to be used by clients where their requirements are clear, such that bidders can price the scope of work fairly.
The AJ has uncovered the range of prices submitted, with the lowest being £537,202, and the next lowest £1.1m. The highest was around £2.63m. This seems an extraordinary range of prices (even in light of the range of bidders involved), and suggests that the scope for the architectural services was not clear at all.
The winning competitor, Foster, is reported to have submitted a price around £1.4m. According to the AJ, the scoring system used to compare prices was that the lowest bidder received 100 points for price; with others receiving 100 minus one point for each percentage point by which their price exceeded the cheapest. Under this system, unless prices are close, most bidders (including Foster) would have received negative scores for the price component of their bid.
However, in February 2017, Foster wrote to Suffolk proposing a reduced fee of £845,000. The following month, they were declared the winner.
The overall impression created is clearly not good. No doubt the process has been entirely above board. The same, of course, was claimed in the Garden Bridge's notoriously skewed procurement which appointed Heatherwick Studio instead of seemingly better-qualified competitors. On that occasion, the winning designer's fee was more than three times as high as either of the other bidders, yet they alone were given the opportunity to reduce their price and ended up being awarded the job. It's hard not to see the distinct similarity in the Upper Orwell Crossing procurement.
I don't have any reason to think that the judging was unfair: the two Hopkins partners were perfectly entitled to show enthusiasm for whatever they thought was best. They were not the entire judging panel.
However, the root of the mess clearly lies in the way the original tender evaluation scoring was set up, which would be shame had it not been utterly predictable. Set up to give the impression that price was secondary to quality, the scoring regime instead made certain that price was all, and eliminated the possibility of making a sensible judgement primarily on quality.
As always, the project will be one to watch as it moves forward. I very much doubt that the designs shown at competition stage will be what is built, and in the absence of any meaningful cost or buildability evaluation, it's entirely possible they are beyond what Suffolk County Council can actually afford.
I recently had some time on a trip to Leeds to see a few bridges, both in the city and nearby.
The first bridge I visited was the Stanley Ferry Aqueduct, which is claimed to be the largest cast iron aqueduct in the world, and possibly the first iron aqueduct to have been supported from suspension hangers. It is both a Scheduled Monument and also Grade I Listed. This photo makes it look much smaller than it really is:
Built in 1839, the aqueduct carries the Aire and Calder Navigation, a canal, across the River Calder. The structure was designed by engineer George Leather (probably with his son John Wignall Leather), and comprises a cast iron trough suspended via wrought iron rods from two cast iron arches.
Leather had originally developed a multi-span design, which met with the approval of Thomas Telford, but it was eventually decided to build a single span structure, to reduce any hindrance to water flow in the river. As built, the arches span 47.2m across the river, although the canal trough is longer at around 50m. The trough is 7.3m wide and 2.6m deep, containing some 955 tonnes of water. This phenomenal load is carried on a series of cast iron cross-frames, supported from the hanger rods.
None of this can be seen directly, as it is all hidden behind decorative colonnaded fascia panels. The bridge abutments are also hidden, disguised behind faux-Greek pavilions (you can only see the tops of these in some of my photos). The support arrangement is show in this diagram (taken from Broad's paper, see link below):
The two arches are also cast iron, each cast in seven segments, with each segment having four Vierendeel-type openings. The arch ribs taper from 1.83m deep at the crown to 3.02m deep at the supports. In contrast to the rest of the bridge, the form of the arch is surprisingly modern, compare for example the Taunton River Tone bridge.
The bridge was extensively refurbished circa 1986. Impact to the sides of the trough from large canal barges was repaired using the Metalock process. The hangers were in some cases highly corroded, and found to be carrying uneven loads (ranging from 5 to 25 tonnes), so several hangers were replaced, and all were re-tensioned. The bridge was completely repainted.
It's a magnificent structure, but not an easy one to see well. It can be viewed from a road bridge to the west, or from a field and canal towpath to the east. Closer viewpoints on its west side are within private property, and I could not get access.
If you want to see how the bridge looks from close-up, there are some good photos at the Stanley History website linked below.
Immediately to the east of the bridge, a second aqueduct was built in 1981. This is a huge, deep prestressed concrete trough structure. It resembles a dam more closely than a bridge.
East of that there is a "trash screen footbridge", which provides access along the canalside but also helps to trap the great piles of debris which wash down the Calder. All three bridges have surprisingly little clearance to the river below. When the Calder floods, the aqueduct is nearly entirely submerged.
It's a shame Stanley Ferry Aqueduct isn't more accessible for visitors: it is a one-of-a-kind structure. The Canal and River Trust held an open day at their adjacent workshops back in 2016, from where great views are possible, and perhaps they will do so again. Further information:
The US National Transportation Safety Board (NTSB) have been crawling all over the bridge wreckage (see videos below). It has been reported that they have asked project participants not to share anything with the media. Their preliminary report should be published this month.
A couple of stories note that the project was running over budget and behind programme, caused in part by designchanges instigated by Florida Department of Transportation (FDOT). Is this relevant? It is hard to tell. Design-and-build projects often proceed to a difficult programme, never with a positive impact on quality and safety, but most are, of course, built safely.
Anonymous Canadian YouTuber AvE is said to have found the "smoking gun", and offers up a useful analysis of some of the evidence publicly available (I've embedded the video further down this post). His explanation suggests that stressing rods in truss member 11 (see diagram above, taken from the preliminary design drawings) were over-tightened, causing the rods to fail suddenly - but this was a compression member at the time of collapse, and the rods in it should not have been relevant to its load capacity.
Engineers have pointed to the lack of structural redundancy in the design, with its single truss carrying all the load. Catastrophic failure of any individual truss member would therefore inevitably result in collapse of the bridge. However, there's nothing wrong with "fracture-critical" design so long as members (and their connections) are designed to be invulnerable to fracture.
The most comprehensive discussion of the failure that I've seen can be found on the eng-tips forum, currently extending over five separate discussion threads: 1, 2, 3, 4, 5. Be prepared to give up several hours if you want to dive into those in any serious depth.
I think the cause of the collapse will be found to be multi-dimensional. There must be an immediate physical cause of failure: the structure was not adequate for the loads applied to it (at the time of collapse, the only loads of significance were self-weight and prestress). That may relate to defects in construction and/or design, and it may relate to failures of process (doing the wrong thing). That in turn may have been caused by human failures: miscommunication, or plain irresponsibility. Behind this, there will be a wider context of budget, programme, regulatory, political, commercial factors and the like. Some of this is captured in Alfred Pugsley's enduring phrase, the "engineering climatology", the cultural environment within which engineers operate.
I have some questions I would want answered before hazarding a clear speculation as to what happened, and why.
1. Who was responsible for what?
FDOT have used every opportunity to disassociate themselves from the bridge failure, issuing press releases to make clear that their role was only budgetary (channelling funding), or administrative (monitoring progress in use of funding). FDOT are clear that FIU, the contractor MCM and the designer Figg were entirely responsible for the safety of the structure and its construction.
But this is not clear at all.
FDOT have acknowledged that part of their role was to "authorize utilization of aerial space above the state road". They also attended meetings with the design-build team, including one just a few hours prior to the collapse to review cracks found in the concrete. Their representative at that meeting was an engineer, not an accountant.
It seems to me that the designer, Figg, was responsible for the safety of their design, and any amendments made to the design that they had knowledge of. The contractor, MCM, was responsible for following the design and any standard specifications. Both clearly have a duty to the public to ensure the works are safe.
However, FDOT also have a duty to the public. If they had any reason to suspect the works were not safe, presumably they would not have authorised use of the space above the road.
In the United Kingdom, they would have considered the competence (and available insurance) of the project participants. They would also have reviewed the technical proposals for the design to ensure they were appropriate and in line with good practice, and they would have accepted a certificate from the designer confirming the design had been prepared in accordance with what had been agreed. They may also accept a certificate from the contractor confirming the structure had been built in accordance with the approved design. In the UK, they would have required the appointment of an independent design checker, with further check certification.
A particularly prudent public authority might also consider that before reopening a road to traffic passing below a partially complete structure, they might seek specific assurances regarding the safety of the structure in its interim state, to confirm that the design covered the state the structure was being left in temporarily, and to confirm that the construction completed to that point was compliant.
I don't think FDOT can have expected anything to go wrong. The question, however, is whether their technical assurance procedures were sufficient for them to reasonably judge that it was safe to open the road below an incomplete bridge. A "hands-off" approach is clearly a nonsense, otherwise they would be obliged to let all kinds of dangerous work take place without regard to highway safety. The highway authority should, in my view, only be relying on the word of the design-build team if they have a process in place to ensure that word is trustworthy.
2. Why prestressed concrete?
This really does need explaining. Concrete truss bridges are pretty rare, and those that do exist are generally historic.
The reasons for this are not primarily safety-related. A steel truss will be lighter than a concrete truss, making foundations and temporary works less expensive. Parts can be largely prefabricated and assembled, rather than requiring complex cast in-situ works. Temporary construction arrangements are made easier due to the material's better ability to deal with reversal of load.
In some countries, steel will be preferred because there are fewer hidden critical details, a nervousness born out of a past history of failures in post-tensioned bridges when hidden prestressing tendons corrode. That is presumably less of an issue in a warm-weather climate such as Florida.
The positive side-benefit of selecting steel is that it is not normally prone to sudden, brittle failure. It will tolerate overstress by undergoing plastic deformation; yielding and sagging, and giving forewarning before failure.
The same is not true of prestressed concrete, and especially where it is subject to high shear stresses. Failure of a prestressing tendon can be sudden and explosive. Both compressive and shear failure of concrete can be sudden, with little prior warning, especially if there is a lack of conventional reinforcement.
Photographs of the FIU bridge do not reveal large quantities of conventional reinforcement, indeed they seem to show the opposite. The bridge may therefore have been highly dependent on the integrity of the prestressing rods for its load capacity. The interaction of forces at the truss nodes will have been especially complex, given the proximity of the prestress anchorages to these nodes.
With all this in mind, the choice of prestressed concrete seems likely to have contributed to the suddenness of the bridge collapse. So: why was prestressed concrete chosen?
3. What was the nature and location of the reported crack?
It's known that there was a crack at the north end of the bridge, the end which failed. The project's design engineer had phoned FDOT in the days before collapse to report the crack. FDOT had joined the project team for a site meeting to discuss the crack on the morning just a couple of hours before the bridge collapsed.
After the meeting, work was undertaken on the bridge to adjust prestressing rods. It's not entirely clear whether this work was intended to address the cracking, although a link is clearly possible.
It's not clear at this stage whether the crack is actually relevant. It is evidence of a problem, but not necessarily the same problem as was being dealt with at the time of collapse, and not necessarily the same problem which caused failure.
4. Why was work being undertaken on the stressing system immediately prior to collapse, what was this work, and who instructed it?
According to the NTSB:
The investigative team has confirmed that workers were adjusting tension on the two tensioning rods located in the diagonal member at the north end of the span when the bridge collapsed. They had done this same work earlier at the south end, moved to the north side, and had adjusted one rod. They were working on the second rod when the span failed and collapsed. The roadway was not closed while this work was being performed.
This refers to member 11. Attentive readers will note from the truss diagram above that member 11 was shown (in the preliminary design) with no prestressing. In the permanent load case, it does not require prestressing, as it is under compression under all permanent and imposed loads. However, the design was evidently changed to suit the construction arrangement, which required the span to sit temporarily on a self-propelled modular transporter during installation, supported at the truss node below members 9 and 10. The end part of the truss cantilevered beyond this during transportation, which will have induced tension in member 11.
The prestressing bars in member 11 were therefore required only as a temporary measure during transportation. You would expect them to have been de-stressed (and possibly removed) once the bridge was sat on its permanent supports.
Indeed, that's precisely what a construction representative appeared to say would happen in the "smoking gun" video (starting at 8 minutes in):
There is an obvious discrepancy here. It makes sense that rods in member 11 would be de-tensioned before traffic was allowed back under the bridge, simply because it was convenient to do so while the highway remained a construction site. It does not make sense that any further adjustments were required afterwards; that implies that the bars had not been de-stressed at the intended time.
As well as knowing what was done, a key question is who instructed it, who agreed to it, and why they considered it to be a safe operation to perform above live traffic. There can have been no consideration that the de-stressing work could endanger the bridge.
5. Why was the end truss diagonal (member 11) insufficiently robust to accommodate whatever change in load effect occurred during the re-stressing operation?
Indeed simple calculations should show that the adjustment of stress in member 11 should have been minimal: the compression due to the bridge's self-weight should have been far greater than any stress induced by the prestressing rods. Follow the earlier link to the eng-tips forum for calculations which set this out.
Even in a temporary condition, where reduced factors of safety are sometimes accepted, the concrete truss member and its end nodes should have been robust enough to accommodate any small variations in load caused during construction operations. This should be true even for unexpected changes in load.
The prestress in the stressing bars was being adjusted by means of a hydraulic jack. According to the NTSB statement, one of the two bars had been adjusted, and the second was being worked on when the bridge failed. This will have created an eccentric load effect in member 11, but I doubt that on its own is sufficient to cause failure, and it can be checked beforehand.
There are other issues with hydraulic jacking: in order to loosen the nuts securing the stressing rod, a greater prestress has to be applied initially to allow the nut to be freed. There are risks of hydraulic failure in the jack. The possibility of some sort of failure in the jack, the rod, or the rod anchors, could result in a dynamic shock load being applied to the concrete, but it should have been designed to be robust enough to accommodate any foreseeable range of loading, especially considering that member 11 would be required to carry significantly greater loads once the bridge opened to the public.
I've read a lot of speculation about whether member 11 failed at its upper or lower end, or along its length. There isn't yet sufficient evidence available to do more than speculate. However, the general question remains: why was this part of the bridge not sufficiently robust? This is not a question about the load, or about material defects, it's a question about general good practice in design and detailing, especially for one critical member and two critical nodes on which the entire capacity of the bridge depended.
