A pedestrian bridge collapsed days after it was built over Southwest Eighth Street, allowing people to bypass the busy street to reach Florida International University. WTVJ NBC6 via AP

Florida International University’s new pedestrian bridge was state-of-the-art. On Thursday, the new span failed, killing six.

Updated: March 16, 2018

On Saturday, officials at Florida International University in Miami-Dade County celebrated an “engineering feat come to life”—a 174-foot pedestrian suspension bridge designed to link the vast FIU campus with the neighboring city of Sweetwater, where many students lived in off-campus housing. Spanning eight lanes of traffic, FIU-Sweetwater UniversityCity Bridge was supposed to give students a safe way to cross a busy roadway—an FIU student was killed by a car while crossing Southwest Eighth Street last year—while also providing a new campus amenity. Plans called for the wifi-equipped structure to boast a shaded deck, benches and seating, and plazas at either end.

“Not only is it a bridge, it’s a place,” Robert Herrada, Sweetwater’s director of operations, told the Miami Herald in August. “This is place-making.”

It was also a showpiece for new and innovative construction methods—the first bridge in the world to be constructed entirely of self-cleaning cement, as well as the largest bridge ever to be moved by Self-Propelled Modular Transportation—or atop platform vehicles—according to an FIU press release. The $14.2 million project, initially funded in 2013 and slated for completion in January of 2019, was being built using “accelerated bridge construction,” or ABC, a technique where prefabricated pieces can be installed in a day, rather than months or years. Think modular construction, but for bridges—“Did Someone Order an Instant Bridge?” one New York Times headline read.

“This project is an outstanding example of the ABC method,” said the chair of the university’s acclaimed civil and engineering department, Atorod Azizinamini, in a statement on March 10.

But just five days after the bridge was installed, tragedy struck. On Thursday, the main span collapsed onto Southwest Eighth Street’s mid-afternoon traffic, leaving at least six people dead, at least ten people injured, and several cars crushed flat under the 950-ton weight of the bridge’s deck. More victims may still be buried in the rubble, the Washington Post reported on Friday afternoon. In a statement, university officials said they were “shocked and saddened about the tragic events unfolding at the FIU-Sweetwater pedestrian bridge.” The street will be closed indefinitely, according to the Herald.

The bridge’s failure comes at a time of growing popularity for both pedestrian bridges and the usage of the ABC method, which the Federal Highway Administration has called a “paradigm shift” in how we conceptualize and construct bridges. Besides lifting pedestrians out of harm’s way, the span was built to withstand a Category 5 hurricane, according to a fact sheet, and supposed to last over a century. Its collapse represents one of the worst U.S. bridge accidents since the Interstate 35W bridge collapsed in downtown Minneapolis in 2007, killing 13. And it may call the safety of streamlined bridge construction methods into question.

So, what happened?

Workers were conducting a “stress test” immediately before the collapse, which may have been a factor, according to the Tampa Bay Times. Bridge engineering and safety experts have also observed from images of the aftermath that the bridge appeared to have no central tower in place. Such columns are typical of cable-stayed suspension bridges, and architectural renderings of the FIU bridge showed it would have one. “When the bridge collapsed, the main tower had not yet been installed, and it was unclear what builders were using as temporary supports,” the Times reported.

The National Transportation Safety Board has launched an inquiry into the cause of the collapse. Chairman Robert Sumwalt stated on Thursday that the missing central column would be “part of our investigation.”

There’s rarely one single factor explaining most bridge collapses, said Sam Schwartz, the former chief engineer of New York City’s Department of Transportation. Instead, a butterfly effect of deficient parts or designs tends to lead to systematic failure. “There’s often a lot of safety built into bridges, and that’s why they don’t fail in large numbers,” he said. “When they do fail—with the Minneapolis bridge collapse, as the most famous recent case—there were many things done wrong, and [that has] a cumulative effect.”

With the Interstate 35W Bridge, which crossed over the Mississippi, the National Transportation Safety Board concluded that it was, in fact, a design flaw that ended up bringing the bridge down—not age or sustained neglect, as many critics had thought. The gusset plates that held the bridge’s steel beams were simply not thick enough. In addition, repair work being done on the bridge helped precipitate its collapse, as the weight of the construction equipment applied more pressure onto the plates, leading to eventual failure.

What makes the collapse of the FIU Sweetwater UniversityCity Bridge different—and, perhaps, more worrisome—is that it has only been standing a few days, without any additional weight present to even stress the structure. (The I-35W bridge had opened in 1967, and at the time of catastrophe, carried 140,000 vehicles daily.) Furthermore, Miami’s “instant bridge” was supposed to be Exhibit A for a technique specifically designed to greatly reduce the risk of this very thing happening.

While it is too soon to know the cause of the Miami collapse, bridge experts have warned in the past that speed can come at the cost of quality assurance when performing ABC. “The quality/speed of construction trade-off can be problematic,” Ted Zoli, the national bridge chief engineer at HNTB Corporation, stated at a 2013 FIU webinar on ABC implementation.“[T]he best strategy is to ensure the critical path of the project does not go through some difficult to inspect and to construct component that invites poor execution.”

Schwartz believes that the ABC method itself is not at fault in Miami; the technique, he says, is “not as new as people make it out to be,” and prefabricated bridge panels have been in use for at least 30 years in New York and elsewhere. Transit engineers and architects favor the technique because materials can be designed and built faster, safer, and more effectively. “Often the quality of the work is better, because it can be done under very controlled circumstances, as opposed to out in the field,” Schwartz said.

In the case of the FIU bridge, another key advantage was the lack of disruption to traffic below during construction. According to the press release, the eight lanes “saw little disruption to traffic” over the weekend, and actual assembly only took several hours. This is, in fact, one of the promises of ABC—that it does little to impact the surrounding environment during construction, as opposed to other construction techniques. On the other hand, because it requires a concentrated effort by construction crews, the technique can be pricier than others. “We pay a premium each time we use ABC, so in the end that means less money to address other bridges using conventional building methods,” Greg Penny, a Pennsylvania DOT spokesperson, said via email.

Before critics jump to any conclusions, Schwartz says, details surrounding the bridge’s construction process must first come to light, and many questions will need answers. “Did they install it improperly? Was it a failure in a joint?” he said. “Was it a failure in the midspan, because the bridge couldn’t support its own weight? Was it a design issue? Was there any damage while moving the bridge?”

Meanwhile, CBS has reported that two of the construction firms involved have a history of safety complaints.

The question other observers may have is how this happened at Florida International University, and what the incident might mean for the school, which has notably partnered with the U.S. Department of Transportation to study innovations in bridge-building techniques and civil engineering, with a track record of industry-transforming research. CityLab has reached out to DOT and FIU experts and will update when more information becomes available.

CityLab will have more on this story as updates become available.

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