Okay-----serve a second cake decorated like the French flag.
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.
FWIW, just worked for me. :-)
Been checking FEC records for some of the Munilla brothers. RINO/Uniparty players, gave to local Critters, McCain, Rubio, and Bob Menendez. Haven’t found any donations to Trump so far...
Just about every over pass I see on highways and local roads have steel beams. Why the need for a cable stayed bridge for this; what should have been a mundane pedestrian footbridge? Oh, the Feds are on the hook, let’s splurge ?
“If stress testing being conducted, why would the road be open, with vehicles stopped directly under the bridge? Doesn’t make sense.”
You may find a contractual clause penalizing the company if they interrupt traffic. Interruptions to traffic probably subtract from award fee or performance fees. The site manager would say, “we need to close down traffic for two hours to conduct a stress test.” The owner/manager would see a reduction in his take-home of x dollars and say, “Can we do it without interrupting traffic?”
“Well, so long as the test is successful, yes.”
“What? You don’t trust your design? Do it without interrupting.”
Okay-----serve a second cake decorated like the French flag.
You are spot on. Concrete roof indeed. The Chinese would take the company principles outside and put one between the lookers for such things.
I still mourn the dog in the car.
I think this was a pure constructor screwup.
See this video animation of the bridge:
This shows a cable-stayed bridge design. The upper stays were not in place, yet they had no temporary supports at the midpoint of the bridge until that tower on the one side was constructed and the cable stays in place. There was nothing providing necessary up force, and the bridge was simply not able to support itself.
I don’t think this had a thing to do with tweaking the tension on the post-tensioning cables/tendons. Or if it did, that was not the root of the problem.
Thanks, Liz, I’m sure we’ll keep this in mind. s/
Any bets that the replacement bridge will be a much simpler design?
One of Obama's crew?
In the article, all that speedy building and shortcuts and concrete is to keep the mayor’s phone from ringing. Hope the mayor is getting an earful now.
Just about every over pass I see on highways and local roads have steel beams. Why the need for a cable stayed bridge for this; what should have been a mundane pedestrian footbridge? Oh, the Feds are on the hook, let’s splurge ?
...
Yep, and all that money makes it easier for politicians and their friends to steal it. Supposedly this bridge was faster to build, but it wasn’t going to open until 2019.
Pretty bridges are an urban fad these days. Everyone wants to copy the Millennium Bridge in London. (It was pretty, but required a couple of years of re-engineering after the original crew collected their money and called it complete.) https://en.wikipedia.org/wiki/Millennium_Bridge,_London
When politicians start re-directing money and undercutting specifications, you get fiascoes like the Margaret McDermott Bridge in Dallas. (Which they didn’t bother to stress test, hiding the problems until later) https://archpaper.com/2018/01/santiago-calatravas-margaret-mcdermott-bridge-cracking-wind/
“..and self-cleaning gimmick”
I believe this was loudly bragged about. I have to wonder how many other “green building” projects are in the works that have the same feature.
“”””I’ve seen too many times engineers with no mechanical aptitude rely on software.””””””
I deal with them all the time. They have never built anything themselves. They have never touched a welder, wrench, shovel.
They only know what their computer tells them. They are taught by professors who have never touched a tool or been outside.
I had a small open structure go down in a micro burst a few years ago. They all blamed me. I explained that it was built according to the engineered specifications. Every step was inspected by engineers. It was signed off by engineers.
I told them the design was wrong and the anchor bolts were too small. I told them if they changed it a little bit the wind wouldn’t catch it so much. I was told the computer models show the roof with its funky design was capable of withstanding those winds. What did they do? Rebuilt it the exact same way.
Look, if you own a construction company, accidents are part of doing business.
I will not jump to a conclusion on that issue yet.
There’s nothing overly complicated about the bridge design, it has been commonly used for many years, and they do look cool, which is nice.
You just can’t ignore basics about how they work during the construction.
As long as the project met its minority ‘goals and quotas’, it’s difficult to see what the problem is here.
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