Slashdot Mirror
The Ordinary Engineering Behind the Horrifying Florida Bridge Collapse (wired.com)
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.
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.
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.
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.
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!
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
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.
The possible explanation to the failure (over tightening of one or more post tension rods) as detailed in this AvE video, seem to make sense. https://www.youtube.com/watch?...
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.
AvE over on youtube did a very good commentary on how and why the bridge collapsed. Video 1 (overview): https://www.youtube.com/watch?... Video 2 (technically in depth): https://www.youtube.com/watch?...