The Ordinary Engineering Behind the Horrifying Florida Bridge Collapse (wired.com) 276
An anonymous reader quotes a report from WIRED: The people of Sweetwater, Florida were supposed to wait until early 2019 for the Florida International University-Sweetwater University City Bridge to open. Instead, they will wait about that long for an official assessment from the National Transportation Safety Board of why it collapsed just five days after its installation, killing at least six people. In the immediate aftermath of the disaster, many queries have centered on the unconventional technique used to build the bridge, something called Accelerated Bridge Construction, or ABC. But ABC is more complicated than its acronym suggests -- and it's hardly brand new. ABC refers to dozens of construction methods, but at its core, it's about drastically reducing on-site construction time. Mostly, that relies on pre-fabricating things like concrete decks, abutments, walls, barriers, and concrete topped steel girders, and hauling them to the work site. There, cranes or specialized vehicles known as Self-Propelled Modular Transporter install them. A video posted online by Florida International University, which helped fund the bridge connects to its campus, showed an SPMT lifting and then lowering the span into place.
In a now-deleted press release, the university called the "largest pedestrian bridge moved via SPMT in U.S. history," but that doesn't seem to mean much, engineering-wise. SPMTs have been around since the 1970s, and have moved much heavier loads. In 2017, workers used a 600-axle SPMT to salvage the 17,000 ton ferry that sank off the coast of South Korea in 2014. The ABC technique is much more expensive than building things in place, but cities and places like FIU like it for a specific reason: Because most of the work happens far away, traffic goes mostly unperturbed. When years- or months-long construction projects can have serious effects on businesses and homes, governments might make up the money in the long run. Workers installed this collapsed span in just a few hours. These accelerated techniques are also much safer for workers, who do most their work well away from active roads. The report goes on to note that the bridge collapse is still under investigation and the search for a culprit is ongoing. "The answers could run the gamut, from design flaws to fabrication flubs to installation issues," reports WIRED. As of publication, The Washington Post is reporting that an engineer called the state to report cracking two days before its collapse.
In a now-deleted press release, the university called the "largest pedestrian bridge moved via SPMT in U.S. history," but that doesn't seem to mean much, engineering-wise. SPMTs have been around since the 1970s, and have moved much heavier loads. In 2017, workers used a 600-axle SPMT to salvage the 17,000 ton ferry that sank off the coast of South Korea in 2014. The ABC technique is much more expensive than building things in place, but cities and places like FIU like it for a specific reason: Because most of the work happens far away, traffic goes mostly unperturbed. When years- or months-long construction projects can have serious effects on businesses and homes, governments might make up the money in the long run. Workers installed this collapsed span in just a few hours. These accelerated techniques are also much safer for workers, who do most their work well away from active roads. The report goes on to note that the bridge collapse is still under investigation and the search for a culprit is ongoing. "The answers could run the gamut, from design flaws to fabrication flubs to installation issues," reports WIRED. As of publication, The Washington Post is reporting that an engineer called the state to report cracking two days before its collapse.
The usual pattern (Score:5, Informative)
Like every engineering disaster, somebody found the problem, and failed to communicate its severity. In this case, they decided it wasn't a safety issue (cracks in a brand new bridge!) and left a voice mail with somebody else who was out of the office for a few days.
There's no substitute for risk assessments by fully qualified engineers, of course. But those engineers also need communication skills â" including persuasive skills. Engineers who can find somebody in authority and convince them to take action save lives.
Re: The usual pattern (Score:4, Insightful)
The question I havenâ(TM)t seen asked, much less answered, is this: the final bridge was supposed to be supported by cables from a central pier. Why was safe to install the sections without that support?
Re: The usual pattern (Score:5, Informative)
The bridge was basically a truss, self-supported. The stays were to be pipes to add stiffness and controlling harmonics in high winds. As well as for aesthetics.
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It'll probably be here somewhere. Probably with a score of zero or less.
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Apparantly not; I'm guess it was sure-as-shit cheaper, tho'
Re: The usual pattern (Score:2)
Re:The usual pattern (Score:5, Insightful)
Like every engineering disaster
That isn't even remotely true. The vast majority of engineering disasters happen without identification and without warning. An even larger number of infrastructure disasters have nothing to do with engineering as well.
somebody found the problem, and failed to communicate its severity
Nope. Somebody found *a* problem and didn't know of the severity. Cracks don't mean much at all. It may or may not have had anything to do with the collapse.
There's no substitute for risk assessments by fully qualified engineers, of course
You're right. But you fail to realise that this takes time.
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and then identify where the bridge broke, it's a pretty fucking safe bet that the cracks are direct evidence
Now if only engineers could identify where the bridge "broke" (past tense) in advance, then cracks would be direct evidence of a problem. That in itself doesn't mean anything. There's cracks in bridges all the time. Most of them are benign. (I'm a lyrical poet).
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If you read the transcript of the engineer's call, and then identify where the bridge broke, it's a pretty fucking safe bet
Cool bet bro, I'll leave it to people who actually reviewed the engineering design along with the cracks: https://news.fiu.edu/2018/03/f... [fiu.edu]
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Meanwhile, in your lazy world
Actually in my "lazy" world I'm an incident investigator (though not for civil structures). You on the other hand are the source of most of my problems, people who *think* they know what happened because they have just enough knowledge to be dangerous and absolutely nothing to do with the project.
Mind you in my lazy world I do get great job satisfaction from presenting people like you with final reports. The smug expression on the face of an "expert" shatters faster than a bridge which was incorrectly tensi
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I would doubt that the cracks were the source of the problem, and I well understand that there are many benign causes for cement cracking. But I still suspect that it will eventually turn out that the cracks were the external sign of a more serious internal problem. Perhaps in improperly secured support joint.
This doesn't mean I think he should have known that from looking at the cracks. It's the kind of problem even an xray might not reveal. When you've got to guess by looking at external symptoms that
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Cracks are so common you can't do that or you could never build a bridge. And an internal flaw is likely impossible to detect. Sometimes you can detect it with an xray, or sound test, but not always.
OTOH, further down the thread it looks as if someone has come up with a plausible reason having to do with over tensioning a support member.
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The bridge was still being constructed.
Yes. A post-tensioner on member 11 snapped while being either tightened or loosened (no one is sure yet which, but the design called for loosening it after the bridge was put in place). There's a good dashcam video where you can see small explosions along with a projectile out the top of member 11 (where about 4 people were working) about 3 frames before the bridge starts to buckle. Here's a picture of the hydraulic ram which ejected from member 11: https://s.abcnews.com/images/U... [abcnews.com]
Re:The usual pattern (Score:5, Informative)
Like every engineering disaster, somebody found the problem, and failed to communicate its severity. In this case, they decided it wasn't a safety issue (cracks in a brand new bridge!) and left a voice mail with somebody else who was out of the office for a few days.
There's no substitute for risk assessments by fully qualified engineers, of course. But those engineers also need communication skills â" including persuasive skills. Engineers who can find somebody in authority and convince them to take action save lives.
A recent press release [fiu.edu] reports that there was a meeting the morning before the collapse in which engineers and persons in authority concluded that the cracks did not compromise the structural integrity.
So no, I don't think there was a problem with "failure to escalate". Really, the big message here is that one needs to reserve judgement until the facts have a chance to surface. There is an official investigation underway. Amongst other things, It will determine if the analysis of the cracks was accurate and it will also determine if an appropriate escalation process was followed.
While waiting, we should be asking if other "civilians" are at risk due to the lessons we have not yet learned from this collapse. For example, we may temporarily decide to prohibit "civilians" from being underneath active construction sites until we better understand how to protect them.
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"Who's a weak force *now*, bitch?" responded gravity.
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Risk management, my friend, risk management!
(Typing this from bed) :)
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Gee, I wonder why Australian steel managed to get an exemption. Perhaps it's not bullshit, and never has been, i.e. it doesn't present a "threat to US security".
Yes, Oz steel doesn't represent much of the total, but there are countries other than the USofA who can make good steel. Perhaps you should check out products from Germany and Sweden.
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I think it's more likely that the cracking had the same root cause. I got the impression from various news articles that the cables lost tension for some reason. Perhaps the cables inside the bridge itself snapped or some welds broke, allowing the whole b
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Was the suspension complete? (Score:4, Insightful)
The engineering drawings I have seen show a tall tower with suspension supports coming down to support the span. Was the tower complete and the suspension in place when it broke or was that for completion later? Also the suspension when to both sides of the tower so it was balanced. Are there any pictures of the bridge while it was "good" to compare with the engineering drawing?
See here for a 3d view of the design. As far as I could tell the tower, the section over the water and the suspension lines were not in place when it collapsed.
https://youtu.be/Q2A1wS09p0k?t=5
Re:Was the suspension complete? (Score:4, Funny)
Re:Was the suspension complete? (Score:5, Interesting)
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A true cable-stayed bridge would be built out starting from a center pier. This was a truss bridge, self-supported. The stays are pipes to stiffen structure and harmonics in high winds, as well for aesthetics. See my other post for more details.
Re:Was the suspension complete? (Score:5, Funny)
Anonymous Coward says "see my other post for more details."
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It is straightforward to identify which posts are by the same AC. See my other post for details.
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It is straightforward to identify which posts are by the same AC. See my other post for details.
I showed you this wasn't the case in my last AC post. Please see it for details.
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Re:Was the suspension complete? (Score:4, Funny)
Anonymous Coward says "see my other post for more details."
It's a little known fact that all AC posts are written by the same guy.
He's actually quite brilliant but his writings are of wildly inconsistent quality since he hasn't slept in almost 20 years.
Re:Was the suspension complete? (Score:4, Insightful)
I don't know if there are any pictures, but the situation has been confused by media reports that workers were "tightening cables" at the time it collapsed. However it turns out the pylon for the suspension cables had not been erected yet; the workers were tightening cables that ran through the structure.
It's impossible to say whether the cable-tightening, or the cracks reportedly found in the span, had anything to do with the collapse. It's possible that neither did; it's possible that both did. Engineering disasters tend to be complex and they take time to nail down.
Cable tightening.. Post Tension Slab (Score:5, Informative)
They were tightening the cables that run through the structure - it's called post tension concrete - Concrete is strong in compression but weak in tension, so the idea is to preload the structure with large cables under tension - which squeezes the concrete.
This is extremely common with slab on grade houses these days.
If you consider a beam supported between two supports, there's a bending moment - the top is in compression, the bottom is in tension. If you externally apply a force to squeeze the ends together then you can make it so that the bottom is in compression and the top is in even more compression, both of which are easily handled by the concrete.
In this bridge that is done by cranking cables through the bridge. In something like, say, the Roman Arch, it's done by having massive side supports. In Gothic cathedrals, it's flying buttresses.
This isn't exactly new engineering - although the details change over time.
Re:Cable tightening.. Post Tension Slab (Score:4, Insightful)
I assume that's what they were up to, but of course a bridge by its nature is more extreme in one direction. It may have had nothing to do with the collapse, it may have caused the collapse.
People in these situations want fast answers, so they jump to conclusions based on reports of cracks, or cables being tightened. But good answers take time, and it's a fair bet by the time we get good answers the public's curiosity will have waned. Even if the things drawing our attention now are involved, they're embedded in a much more complex scenario than most people have the patience for.
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And until they tightened those cables, the truss was weak. P(ost)TC that doesn't get T'd until after the truss is installed is a disaster waiting to happen.
P(re)TC is the norm.
They screwed up in how they chose to make/install this truss. Pretty obvious.
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Well, because the road was a major artery and shutting it down for weeks would cause huge disruptions -- much less shutting it down for months.
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Well, sure if you *know* that's going to happen. That's the trick to planning anything, isn't it? Knowing what's going to happen in the future.
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Re: Was the suspension complete? (Score:2)
https://m.youtube.com/watch?v=... [youtube.com]
Nope (Score:2)
They slide this 950 ton slab onto the pilings and congratulated themselves. Great job of building a suspension bridge without you know the suspension part that holds it up.
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Sure looks like one to me http://www.mcm-us.com/projects... [mcm-us.com] you cunt.
Truss Bridge Self Supported. Not Cable Stayed. (Score:5, Informative)
To start, it was a truss bridge, self-supported. The stays shown in final drawings are pipes for stiffening and harmonics in high winds.
The structure was non-redundant. A failure of any truss was near guaranteed to lead to collapse. It's theorized by some that truss member #11 at the junction of the pier was initial failure point.
In preliminary drawings, #11 is shown with no post-tensioning bars, but the actual construction shows it with two. While those bars in #11 may have been necessary due to the move, since the ends of the bridge were cantilevered (which is different than shown in the preliminary drawings), they likely weren't needed after placement; not needed to be post-tensioned, since #11 would be in high compression.
It appears workers were post-tensioning #11 using a crane and other equipment attached to one of the post-tension rods. It appears tensioner (blue) and part of the bar is sticking out several feet in photos of the collapse. According to some, this likely lead to the collapse.
Re:Truss Bridge Self Supported. Not Cable Stayed. (Score:5, Interesting)
Mod AC up!
I'm a Professional Engineer, though not licensed in Florida nor am I an expert in concrete bridges. Based upon the pictures of the debris I've seen, the bridge was made using prestressed concrete. Prestressed concrete is an amazing material - the steel reinforcement inside the concrete is used to compress the concrete, which causes the concrete member to act like high strength steel when it is loaded with tension forces.
AC's comment above explains what happened:
In preliminary drawings, #11 is shown with no post-tensioning bars, but the actual construction shows it with two. While those bars in #11 may have been necessary due to the move, since the ends of the bridge were cantilevered (which is different than shown in the preliminary drawings), they likely weren't needed after placement; not needed to be post-tensioned, since #11 would be in high compression.
It appears workers were post-tensioning #11 using a crane and other equipment attached to one of the post-tension rods. It appears tensioner (blue) and part of the bar is sticking out several feet in photos of the collapse. According to some, this likely lead to the collapse.
In its final placement as a bridge, truss member #11 would be in compression, so there was no reason to prestress it. Per AC, the preliminary design drawings did not show the two post-tensioning rods in this member.
My guess, based upon AC's post, is that when the builders decided to assemble the bridge using the accelerated technique, the designers realized that there would be tension forces on truss member #11 during the move.... so the design drawings were changed to add the two post tensioning rods to truss member #11.
Once the bridge was in place, the construction workers evidently began tightening the two post tensioning members even more. Member #11 was already being loaded with compression forces, from the dead weight loading of the bridge.... and tightening the post tensioners would have placed more compressive loading onto member #11. Once the combined compressive loadings (from the dead weight and the tensioning) exceeded the compressive strength of the concrete, the concrete would fail (it makes a loud popping sound when it fails in lab tests) and the single, non-redundant truss would fail.
If my speculation is correct, it will be interesting to see whether the Figg, the engineering firm that designed the bridge, or the construction contractor gets blamed.
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Other stories say they were loading testing the bridge at the time. Pictures in the UK Daily Mail show that there was green crane that seems to have snapped the cable holding whatever it was carrying. That cable ended up wrapped around the street light away from the mashed up side of the bridge.
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Pictures in the UK Daily Mail show that there was green crane that seems to have snapped the cable holding whatever it was carrying.
Thanks for pointing me to the Daily Mail story. Their first video of the collapsing bridge shows workers on top of the bridge directly above the part of the truss that seems to fail and fall first.... their second video, from the perspective of a driver approaching the bridge, more clearly shows that the side of the bridge near the crane fell first. Their article then says:
At a news conference Friday night, officials from the National Transportation Safety Board said they have just begun their investigation, and cannot yet say whether any cracking contributed to the collapse.
They also said workers were trying to strengthen a diagonal member on the pedestrian bridge at Florida International University when it collapsed.
Robert Accetta, the investigator-in-charge for the NTSB, said crews were applying post-tensioning force, but investigators aren't sure if that's what caused the bridge to fall.
I assume the "strengthen a diagonal member" description is referring to post tensioning a prestressed concrete member.
The story and
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i think the that's just Journalist Telephone. Some engineer probably said they were "tensioning a prestressed concrete member", meaning they were inducing stress to the truss by tightening the cables that run through the concrete, and the Journalist wrote "stress test" and that got into print.
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Yes and no, there is no 'gauge' you can check although you can check the post tensioning being done twice. It's typically what you pay architects and engineers for, not only to design but also check and double check the work during and after construction.
A lot of construction companies these days however rather have it done quickly and rather not deal with 'overhead'.
I was involved in a construction project also with prefab pieces where the architect had specified a different size for some of the levels and
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That is why I mentioned displacement although considering the cost of the bridge, a permanently mounted load cell is not *that* expensive. Paint a visible marker on the tension rod and measure displacement compared to the concrete with a tape measure. The accuracy requirements are not that great in this application.
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people killed by collapsing bridges, 2017: 0
people killed by gun violence, 2017: 15,549
great analogy, you fucking moron.
Re:Truss Bridge Self Supported. Not Cable Stayed. (Score:4, Informative)
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"the designers realized that there would be tension forces on truss member #11 during the move"
Any "designer" (which includes the engineers) that would fail to realize that bridge components would have forces imposed on them during transport doesn't deserve to work in engineering or construction. Then again, maybe all the experienced and competent (and expensive) people have been funemployed in favor of cheaper, less-experienced software button-pushers who just think they are competent.
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But they had CAD, 3D models and renderings and no doubt expensive, sophisticated stress analysis software. How could this have possibly happened?
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MCM (Score:2)
It will be interesting as one of the prime's MCM, has had issues in the past. MCM gives generously to Christie and Rubio.
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MCM is best buddies with Paul Manafort.
https://therealdeal.com/2017/08/31/meet-paul-manaforts-real-estate-fixer/ [therealdeal.com]
Also, AvE has detailed the hive mind verdict: The smoking gun. [youtube.com]
The problem here was the bridge itself (Score:5, Insightful)
In my town we have a footbridge that was installed this way, several years ago. It was factory-built in Phoenix, hauled 100 miles up I-17 using one of the smaller roadable version of the SPMT, and installed overnight to cross a creek. There hasn't been a problem since.
Offsite construction should be safer than site-built, so in this disaster let's focus on the design itself, rather than rushing to judgement on the offsite construction.
Re:The problem here was the bridge itself (Score:4, Interesting)
Yeah, we have a couple where I work. Footbridges over a four lane highway. They seem solid... the newest one's been hit by dump and garbage trucks a couple times without any issue. But they're designed to be self-supporting rather than depending on external cable towers.
My gut feeling is that if they'd just put a central support beam on that median in the center of the bridge it would've made quite a difference.
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so in this disaster let's focus on the design itself, rather than rushing to judgement
That is the most ironic statement I've seen in a while here.
How about we don't pass judgement at all, and focus on the incident and all parts of it. A large majority of infrastructure failures have nothing to do with their design.
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In my town we have a footbridge that was installed this way, several years ago. It was factory-built in Phoenix, hauled 100 miles up I-17 using one of the smaller roadable version of the SPMT, and installed overnight to cross a creek. There hasn't been a problem since.
I'm sure your bridge was very economic, quite functional and an asset to your community.... but it is not an artistic "signature bridge" a University would demand for its main entrance.
Here's the collapsed bridge designer's website: Figg Engineers, Creating Bridges as Art [http]
That makes it more tragic (Score:2)
If 6 people died while we were inventing a new technique that improves construction for a decade, it's a sad growing pain. If 6 people died because of a fuckup in traditional design, it's horrific.
Since the 1970's? And the Rest (Score:2)
engineering-wise. SPMTs have been around since the 1970s, and have moved much heavier loads
Depends what you call a SPTM. Bridges have been prefabricated and moved into place since pre-historic times, starting with carved tree trunks. It does not matter how the bridge gets put there, what matters is its strength and that of its supports. Here is a much more ambitious construction, overseen by Robert Stephenson and Brunel no less, over dangerous water too, 150 years ago :- Britannia Bridge, Menai Strait [astronet.ru]
It's probably not the ABC strategy per se (Score:2)
The complexity and risk of the intermediate stages of a project are the most impressive things to me about civil engineering. It's one thing to design a dam, it's another to build that dam in the middle of an actual river; you need a dam to build a dam. It's one thing to draw a bridge that's perfectly stable once built, it's another to ensure the partially completed structure is in perfect equilibrium at every point from the time the moment the ground is cleared until the last cable is tightened.
If you
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Making a half-completed project stable enoug
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BEFORE cars started driving on it.
I hope not. It was a pedestrian bridge. And as such, the 'massive load' it was designed to bear was it's own weight. Pedestrians would have been insignificant load compared to the bridge's own 950 ton weight.
Comment removed (Score:5, Informative)
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I say ... (Score:2)
Collapsed FIU Bridge Was Funded by Federal Grant (Score:5, Informative)
The Reason Foundation has published an article about the bridge was funded by a federal program that has come under repeated fire for awarding money based on politics rather than merit:
Collapsed FIU Bridge Was Funded by Federal Grant Program Criticized for Shoddy, Politicized Review Process - The TIGER grant program has come under fire for putting politics ahead of technical concerns. [reason.com]
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A good technical look at the cause of failure (Score:4, Informative)
Wrong transportation strategy (Score:2)
But they could not close the road because there are basically no other modes of transportation. No bicycle routes worth mentioning, no pedestrian alleys, no trams.
Close the road for two days and the town collapses.
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On March 15th, the road got closed with a wall of concrete rubble. That was more than 2 days ago. Did the town collapse?
Were they sure? (Score:2)
What happened? (Score:2)
https://youtu.be/KtiTm2dKLgU [youtu.be]
Re:I blame Trump (Score:4, Interesting)
"Hello, I'm an American. As such I have some sort of pathological need to reference my personal political beliefs in every single interaction I have with anyone and anything. No matter how unrelated the discussion at hand is."
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Re: I blame Trump (Score:2, Insightful)
Now imagine what would happen if enough people say "Fuck this, I am not voting for any shitty candidate."
One of the two losers still wins. There is no quorum requirement in American elections.
Re:It was half a bridge, or even less (Score:5, Informative)
It was not a cable-stayed bridge. It's a truss. See my other posts for details. Hopefully, someone can mod them up for visibility. Thanks in advance!
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You know, you'd be more visible if you didn't post AC.
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People filter zero-score (where AC posts start) due to shit like GNAA.
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I read at -1, showing all posts. I'd still like to suck on some of that dick, though. I hope I'm not ineligible.
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I've been wondering about that ever since I saw the first images of its proposed shape. "Where's the pylon? Where are the supports?"
Obviously it wasn't strong enough to span the distance without them. Doh. But not a word from the media yet about that missing pylon or supports.
Re:It was half a bridge, or even less (Score:4, Insightful)
Obviously it wasn't strong enough to span the distance without them
That isn't obvious at all. That's why we do incident investigations. Just because it's final design was to have cables doesn't mean that during construction this wasn't taken into account. There is a lot of reasons why this incident could have happened.
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man you are a fucking moron.
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It was in the process of being constructed, they hadn't got to that bit yet. It was engineered from day 1 to be put up without the supporting pylon. However some things were changed from the initial design.
AvE does his best to analyse what went wrong.
https://www.youtube.com/watch?v=ioC61QW7SHQ&t=0s
https://www.youtube.com/watch?v=KtiTm2dKLgU&t=0s
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Re: It was half a bridge, or even less (Score:2)
...which means, obviously, why did they need to do any cable tightening to begin with? If it was to ensure the structure that supports the bridge was capable of supporting 1,000 tons, then that suggests it was never safe to begin with
No. Concrete under compression behaves much differently than concrete under tension. A concrete span of that length couldn't possibly survive on it's own; gravity would cause the centre to sag more than the ends, which would put a tensile force on it and literally pull the concrete apart. To avoid this you add cables which compress the concrete, eliminating the tension. With the tension gone your span goes from fail-under-its-own-weight mode to being able to support several times it's own weight.
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I would argue that there should be enough cable redundancy so that one or two failures wouldn't bring down the whole bridge, which IMO makes it at least partially a design issue, too, but....
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Is there any huge advantage to making something out of concrete/rebar when it's designed to handle only foot traffic?
I think this could be a case of style over substance.
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How about a box girder?
https://en.wikipedia.org/wiki/... [wikipedia.org]
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Like supporting it's own weight, perhaps?
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This bridge had to withstand hurricane force winds, thunderstorms and salt water erosion, plus not get in the way of traffic
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