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.”
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.
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.
Herald staff writers Andres Viglucci and Douglas Hanks contributed to this article.