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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.
"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."
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.
Log in or piss off.
USAToday reports that the bridge was a truss design. They quote the design firm saying the central tower and stay-like wires shown in architect's drawings were decorative, not structural.
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.