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Experts: Miami bridge collapse 'terrifying' but 'incredibly rare'

Pedestrian bridge collapses at FIU in Miami. (WSVN via CNN Newsource)

It was designed to stand for 100 years, fending off hurricane-force winds.

It fell after five days, killing at least six people.

Authorities continued digging through what remains of a collapsed pedestrian bridge in Miami Friday as attention turned to why the bridge buckled on Thursday afternoon, raining debris down on a busy eight-lane road below, and whether similar structures elsewhere pose any danger.

Built at a cost of $14.2 million, the bridge was intended to connect the campus of Florida International University (FIU) to a neighborhood where many students live, enabling them to cross safely above a busy road where an FIU student was fatally struck by a vehicle last summer.

A span of the bridge stretching 174 feet and weighing 950 tons was constructed near the road and out of the way of traffic. Last Saturday, that portion was lifted and maneuvered into place over the course of about six hours.

“FIU is about building bridges and student safety. This project accomplishes our mission beautifully,” FIU President Mark B. Rosenberg said in a statement Saturday. “We are filled with pride and satisfaction at seeing this engineering feat come to life and connect our campus to the surrounding community where thousands of our students live.”

Much of the bridge had not yet been constructed. It was scheduled for completion in early 2019 and the full bridge was intended to be 289 feet long. Based on design images, it appears the final structure would have had diagonal cables connecting the main deck to a tall mast.

The first victim of the collapse has been identified as 18-year-old Alexa Duran. According to the Miami Herald, she was driving her Toyota SUV under the bridge when it fell.

The National Transportation Safety Board has opened an investigation of the incident. Workers were reportedly trying to tighten cables when it occurred, and witnesses say it sounded like a bomb going off when it crashed to the ground.

It remains unclear what caused the bridge to fall, but an engineer unaffiliated with the project said the fact that the central span was installed in six hours was likely not the problem.

“That’s actually a fairly common practice in a number of states,” said Michael Culmo, chief technical officer at CME and an expert on what is known as Accelerated Bridge Construction (ABC).

ABC is a technique planners use to reduce the impact of bridge construction on traffic by doing as much of the work as possible off-site and then installing completed pieces of the structure.

Although the FIU bridge did have unique elements—it was the first pedestrian bridge to be made with “self-cleaning concrete,” according to the university—Culmo stressed that “literally a hundred bridges a year” are built this way.

“We’re at a point right now with accelerated construction in the U.S. where it’s become standard practice,” he said.

According to John Myers, a structural engineering professor at Missouri University of Science and Technology who studies the use of advanced construction materials, the technique is also often used in bridge repairs, quickly swapping in pre-cast deck segments to replace parts of a structure overnight.

“We have been doing prefabricated, or if its concrete we call it precast concrete, for many, many decades…,” he said. “Think of it like you have Lego blocks, stacking them, and connecting them together.”

ABC has become a common approach to bridge-building in communities around the country. Culmo worked closely with transportation officials in Utah to implement it in many projects there. He could not provide an exact number of bridges that have been built using ABC nationwide, but he said the Federal Highway Administration stopped counting after it hit a few thousand several years ago.

In Dade County, Georgia, a bridge over Highway 299 was built entirely off-site and then installed in 56 hours last May.

The State Highway 86 Bridge over Mitchell Gulch in Colorado was replaced in 46 hours in 2002, installing a new 40-foot bridge without impacting rush hour traffic at all. Using conventional methods, officials estimated it would have taken months to complete.

In 2005, the New Jersey Department of Transportation replaced three Trenton bridges in three weekends instead of 22 months. The contracts included financial incentives to complete each bridge in under 57 hours, and penalties for each hour over that. None took longer than 56 hours.

According to the Pennsylvania Department of Transportation, about 40 bridges in the state were built using ABC methods. An official noted that they have never used the exact technique utilized in the Miami bridge.

“Before using any accelerated bridge construction method similar to what was used in Florida, we will work to understand what happened in the Florida case,” Rich Kirkpatrick, communications director for the department, told WHP.

Engineers emphasized that the disaster in Miami should not trigger immediate concerns about the thousands of other bridges built similarly.

“Everyone's perception of safety is different but I believe that a fear of engineered structures is irrational,” said David Corr, an associate professor of civil and environmental engineering at Northwestern University. “Collapse incidents like the ones in Miami are certainly terrifying but are also exceptionally rare.”

ABC construction tends to be popular with drivers since it minimizes the amount of time they spend trudging through construction-clogged roads.

“The public satisfaction is incredibly high,” Culmo said.

The Federal Highway Administration has promoted the use of ABC technologies as the nation faces daunting challenges to update or repair nearly 150,000 crumbling bridges.

“Approximately one-fourth of the Nation's 600,000 bridges require rehabilitation, repair, or total replacement,” an FHA fact sheet states. “However, the work that occurs from on-site construction activates can have significant social impacts to mobility and safety. In many cases, the direct and indirect costs of traffic detours that result from the loss of a bridge during construction can exceed the actual cost of the structure itself.”

Florida International University has been so enthusiastic about Accelerated Bridge Construction that it established a center devoted to the study of it in 2010.

Last fall, the center’s director, Atorod Azizinamini said off-site construction can save time and money and prevent disasters like the 2007 Mississippi River bridge collapse in Minnesota.

“With ABC, the total time that it takes to construct a bridge doesn’t change, pretty much, it’s about the same, but the major difference is that you do almost 95 percent, sometimes 99 percent, of the work offsite,” he said in an interview on the university’s website.

There are a number of reasons why ABC has become so common. In addition to limiting the disruption for drivers, it can save cities money that would be lost due to traffic delays with traditional methods.

“The motto we used to use is, ‘Get in, get out and stay out’…,” Culmo said, “because people hate driving through work zones.”

The method is ostensibly safer for both workers and pedestrians. Building a structure on the ground eliminates some of the risks of conventional bridge construction over live traffic.

“Instead of having a two-year work zone, I’m going to have a two-week work zone,” Culmo said.

Working in the controlled environment of a construction yard also helps contractors maintain quality.

“When we go to ABC, there’s two things we don’t do,” he said. “One is we don’t sacrifice quality…. The second thing is we never, ever sacrifice safety for speed.”

It could be months or years before the exact cause of Thursday’s collapse is known.

“The process will generally involve documenting the scene of the collapse, including preserving any pertinent evidence, followed by careful review of design, construction, and inspection documentation, and may include further engineering analysis to determine the cause and origin of the collapse,” Corr said.

According to Myers, it is too soon to speculate, but accounts provided by witnesses could give investigators clues.

“Something clearly triggered a very rapid progressive failure of the structure,” he said.

Beyond studying the debris and taking samples of materials at the accident site for testing, officials will also be scouring nearby cameras for any relevant footage. One video of the collapse that was recorded by cell phone from a monitor inside Florida’s Sunguide traffic center has already emerged on social media.

However, Miami-Dade's Transportation and Public Works Department told the Miami Herald the traffic cameras at the intersection are not set up to preserve footage and the agency apparently has no recording of the collapse.

Whatever happened on Thursday, Corr said, once all the facts are known, engineers will work hard in the future to prevent it from happening again as technologies and techniques for Accelerated Bridge Construction advance.

“If there was something unique about the techniques or materials that led to this collapse,” he said, “the engineering design and construction community will learn from it, make any retrofits to similar existing structures that are necessary, and our future structures will be safer because of the knowledge we acquire from this collapse.”

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