Posted on 03/16/2018 3:59:12 AM PDT by csvset
Aerial footage shows the aftermath of the FIU pedestrian bridge collapsing on Southwest Eighth Street in Miami on March 15, 2018. Pedro Portal Miami Herald In the hours after a 950-ton pedestrian bridge over Tamiami Trail collapsed Thursday afternoon, killing at least four people, civil engineers began to speculate about potential causes.
Was it a design error? Did something go wrong during construction?
The answer may be buried deep in the calculations made by workers who were conducting a stress test on the unfinished and vulnerable bridge. Any such test, experts told the Miami Herald, requires extreme care and precision to avoid overwhelming the structure. Too much weight on the bridge or over-tightened cables could cause problems.
The firms behind the project are Miami-based MCM and Figg Bridge Group, a well-known Tallahassee design company. Miami-Dade Mayor Carlos Gimenez said that crews were conducting a stress test on the bridge Thursday, and Miami-Dade Fire Rescue confirmed two workers were on the bridge when it collapsed.
The bridge was designed to enable students at Florida International University to safely cross the busy six-lane roadway between campus and a popular residential area. It was built using a method known as “accelerated bridge construction” — an innovative way to build bridges more speedily than with traditional building methods. While support columns were constructed on both sides of Tamiami, the 175-foot span was built on the side of the road. In a matter of hours Saturday morning, the span was installed onto the columns.
The accelerated bridge construction (ABC) approach has become more common in the past 10 years, particularly in urban areas with heavy traffic, said Ralph Verrastro, a Cornell-trained engineer and principle of Naples-based Bridging Solutions.
“That’s the driver and why ABC is so popular, because it allows you to keep the road open,” he said. “It’s more expensive to do, but it gains the advantage of keeping traffic moving and that’s what makes the phone ring at the mayor’s office.”
Aerial footage shows the aftermath of the FIU pedestrian bridge collapsing on Southwest Eighth Street in Miami on March 15, 2018. Pedro Portal Miami Herald
As was the case with the FIU bridge, the structure typically is assembled from pieces placed alongside the road before being moved into place. Cables running through the bridge slab that are tightened to strengthen the pre-fabricated portions are adjusted and stress tests completed before the pieces are moved over roads, for obvious safety reasons.
If workers were adjusting cables once the bridge was in place, the cables should not have connected to the bridge’s structural integrity, Verrastro said. “Once you’re done tensioning those cables, you’re done,” he said.
It’s possible the cables were over-tightened, causing the bridge to elevate slightly in what’s called a camber. Adjusting the cables to address camber would be appropriate, but that would not impact the structural strength.
“If they were adjusting the structural cables, it was to try to put more or less camber,” he said.
Still, adjusting the camber — called tuning the bridge — can be tricky. Robert Bea, a University of California Berkeley engineer and catastrophic risk expert, has studied hundreds of structural failings including the BP Deepwater Horizon. According to Bea, when workers adjusted the camber on a bridge in Australia in the 1970s, it also collapsed.
“The steel buckled while they were attempting to tune this camber, so it’s very plausible,” he said.
Another vulnerability: the span’s weight capacity. At this stage in the accelerated timeline, bridges typically need additional temporary support; engineers must not exceed weight limits during load-bearing tests.
“The loads have to be calculated precisely in the analysis to make sure the partial bridge would be able to carry them safely,” said Amjad Aref, a researcher at University at Buffalo’s Institute of Bridge Engineering.
Because precision is key, multiple factors may have contributed to the bridge failure. The investigation, Aref said, will need to examine the construction sequence, testing, environmental conditions such as wind and other possible factors.
“It might not be one factor,” he said. “It could be a combination of things.”
The bridge also had some unusual design features.
The bridge’s superstructure was something Verrastro said he’s not seen in 42 years of designing bridges. Rather than using steel trusses, it employed heavier concrete trusses. The bridge also had a concrete roof, adding even more weight.
“This was a very long span and then they used very heavy material,” he said. “The majority of pedestrian bridges are steel.” Steel bridges are about one-tenth the weight of concrete, he said.
FIU installed a new pedestrian bridge over the perilous Tamiami Trail in a single morning, part of a project to provide students a safe crossing and directly connect its main campus to Sweetwater. Pedro PortalThe Miami Herald
Verrastro, an expert in accelerated construction who has spoken at FIU’s bridge engineering program, suspects that using concrete was part of the bridge’s aesthetic, rather than structural, design. The FIGG Bridge Group that designed the bridge is known for its signature bridges, he said.
“They typically get involved in ones that look fancy, but they’re competent,” he said.
Using the accelerated process doesn’t necessarily change the design, just the construction, he said. However, it does require trained contractors who specialize in the method.
In almost all bridge or building collapses, he added, construction is at fault, not design. The flattened bridge will likely remain in place, he said, while a forensic engineer conducts an investigation.
While the accelerated bridge construction process is not well known outside the engineering world, FIU has become a hub for fostering the new approach.
FIU started a center to “advance the frontier” in the field in 2010 after identifying a need for more engineers trained in the method. Since launching in 2011, the center has drawn 4,000 people to its webinars, according to the website. In 2016, it became one of just 20 accelerated building programs nationwide to receive federal funding that amounts to $10 million over five years.
The center was not formally involved in constructing the pedestrian bridge.
The center’s director, Atorod Azizinamini, recognized by the White House in 2016 as one of the world’s leading bridge engineers, said the method is safer and more efficient than conventional construction methods.
“We are able to replace or retrofit bridges without affecting traffic, while providing safety for motorists and workers who are on site,” he said in a 2016 press release about the program. “The result is more durable bridges.”
But Bea was more skeptical of too much innovation.
“Innovations always bring potential ‘failure modes’ that have not been previously experienced,” he said.
VIew from a parking garage as crews continue working on rescuing victims of the FIU bridge collapse on March 16, 2018. Monique O. MadonMiami Herald
Herald staff writers Andres Viglucci and Douglas Hanks contributed to this article.
Yes, that was the story, thanks.
You’re good.
.Not the best video, some of the comments are interesting regarding the construction process.
And I DO BLAME THE CONSTRUCTION COMPANY. The bridge fell on their watch. They were doing stress test and, now adjustments, per Little Rubio, yet they did not close the road! They murdered people for their incompetence and they should be accountable.
Cars weren't stopped, they were crushed under the collapse. Death toll now 6 and bodies still underneath today. https://www.msn.com/en-us/news/us/at-least-6-killed-in-florida-bridge-collapse/ar-BBKhNbI?OCID=ansmsnnews11
Aesthetically pleasing. I’m sure they had a committee review several alternatives but this one was selected.
As a former County Engineer I was involved in a project that was much needed and had been under development for several years prior to my employment. The preferred design that had been selected had an estimate of $13 million but every attempt at securing grant funding to construct it, including TIGER grants had come up empty.
Basically I reviewed the other alternatives and chose the 2nd place alternate which only had a price tag of $3.5 million. I did this without a committee and then sold the plan to the Board of County Commissioners with the only drawback that we would have to throw away the previously spent $1 million on engineering. They said go for it and it was completed a a year later.
What was the difference? The expensive design called for the existing bridge to be removed and a new bridge be constructed. The one I selected utilized the existing bridge.
The one thing to take away from all of this is that never use a committee to select the design. They always take the one regardless of the practical factors.
When will we find out the construction company hired “day laborers” (i.e. Illegal immigrants), who did less than adequate work?
JoMa
Wow. I’d never heard of that incident, was probably deployed overseas at the time.
Mexican concrete.
Thanks, but I’m not good. I’m pi$$ed.
The Autoban in Germany has some pretty high bridges.
I dont think I would be able to drive across that thing without a hand full of valium. Especially after seeing their handiwork in Miami. I like big bridges built with Yuge American steel and gigantic rivets and physical supports I can see and have become accustomed to. (Channeling my inner “Bones” on the hazards of “transporter use”.)
This beautiful magic bridge crap just doesnt do it for me.
The bridge in Miami was dinky compared to this structure! And there is the same amount of gravity pulling down on both of them.
Do you think maybe the same engineers from the same company that built both the huge bridge and the tiny one in Miami are the same ones or even consulted with the engineers that built the smaller one THAT COLLAPSED under its own weight?
Update at the link-—
UPDATE, March 16, 2018, 11 a.m.: To clarify, Leonor Flores did not work on the FIU-Sweetwater UniversityCity Bridge project in any capacity.”
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No, the Miami bridge builders are small potatoes compared to the companies that build those massive expansive bridges.......
I’ve been in construction for over 40 years. I took a good look at the situation and had a pretty good idea right away what happened.
Have you seen the surveillance video of the actual collapse? Here it is: https://www.cnn.com/videos/us/2018/03/16/video-of-florida-bridge-collapse-sot.cnn/video/playlists/florida-bridge-collapse/
One would suppose the structure could have been substantially post-tensioned while yet on the ground once the fabrication process was completed.
OTOH, what are the odds this was a Hyatt walkway type of failure?
I’ve been in construction basically all my life. My son is an engineer. We were watching the news reports together and i noticed the bolts on top of the center truss and said there is a structural element missing and where is the falsework at center span? This was all before seeing the rendering of the bridge or seeing the photos of the span placement over the weekend.
I agree about post-tensioning before the set. They could have cambered the deck in place and pulled tension on the tendons at that time. That said, a falsework support was still needed IMHO. Setting the span and then pulling tension without the center falsework probably contributed to the collapse again IMHO.
The bridge was only supported by the ends placing it in a catenary position (a downward parabolic position), now you pull tension and what happens? You pull the camber the wrong way. They may have been pulling above spec to try and camber the structure the proper way and instead caused the failure.
Just speculation mind you but the whole thing is a cluster and people are in deep doo doo.
No I haven’t, thank you.
I don’t think this was a Hyatt style deal. There was no live load and no dynamic live load. If you recall the Hyatt incident was precipitated by a high live dynamic load ( people at a party filling the spans and dancing).
The mechanism of failure was structure fatigue of the horizontal members at the vertical tie-ins. The vertical support members were to be continuous so each individual span was supported independently, instead the erectors hung the verticals from the flanges of the channel above, meaning the flange on the top span carried the load of all the lower spans and the flange failed. That was a failure induced by the constructors.
IMHO the Miami failure was an engineering failure. The engineering firm must have specified the methodology here and that was lacking safety margins.
The Hyatt changes to the vertical were approved by the engineers when they reviewed the shop drawings. The engineer was eventually found guilty if I remember my ethics courses correctly.
You may be correct, it’s been a long time.
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