By Andres Viglucci, Joey Flechas, Jenny Staletovich And Rene Rodriguez

The unfinished pedestrian overpass that toppled onto the Tamiami Trail on Thursday was being built under a relatively novel approach called accelerated bridge construction -- a fast, tested method that carries some risks if not rigorously carried out.

Until it's fully secured, a quick-build structure is unstable and requires the utmost precision as construction continues. Properly shoring up the bridge can take weeks, a period during which even small mistakes can compound and cause a partial or total collapse, said Amjad Aref, a researcher at University at Buffalo's Institute of Bridge Engineering.

Just before the bridge's concrete main span abruptly gave way on Thursday, crushing four people in cars to death and injuring others, a contractor's crews were conducting stress tests on the incomplete structure, Miami-Dade Mayor Carlos Gimenez said. The 950-ton span, assembled by the side of the road over a period of months, was hoisted into place in a matter of hours on Saturday morning.

That stress testing typically involves placing carefully calibrated weights on the span and measuring how the structure responds to ensure it's within safe parameters, Aref said. Crews may also have been adjusting tension cables that provide structural strength for the span's concrete slabs.

"The loads have to be calculated precisely in the analysis to make sure the partial bridge would be able to carry them safely," Aref said.

That doesn't mean that testing or tension adjustments caused the structure to fail, he said. Other factors, from heavy wind to design flaws to a crane hitting the structure, can also come into play in a failure. It's still too early to even guess at a cause, engineers say.

"It might not be one factor," Aref said. "It could be a combination of things."

In almost all bridge or building collapses, though, construction errors are to blame, not design, said Ralph Verrastro, a Cornell-trained engineer and principal of Naples-based Bridging Solutions, which is not involved in the FIU project.

Determining what exactly went wrong will likely take months. The National Transportation Safety Board has opened an investigation.

Over the coming weeks, forensic engineers will try to unravel what happened in a complicated analysis that involves picking through debris, looking at designs, and piecing together inspections, said Princeton University civil engineering professor Maria Moreyra Garlock. The construction phase, she noted, is often the most dangerous point in the life of the bridge.

Engineers could sample material at the site to test for strength, she said, and look at the sequence of inspections to determine what happened when. Site inspections might also reveal what caused the sudden collapse.

"Maybe there's some sign that a support got unseated," she said.

Thursday's tragic accident is sure to raise questions over the decision by Florida International University to take the quick-build approach, adopted in large part to minimize the need to interrupt traffic on the busy highway. The decision by its contractors to undertake testing while traffic flowed along the busy roadway below will also be scrutinzed. FIU was running the project under an agreement with the state.

Accelerated bridge construction has become more common in the past decade, especially in urban areas with heavy traffic, Verrastro said.

"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."

FIU's engineering school has become a hub for accelerated bridge construction training and research in recent years.

In 2010, after recognizing the need for more engineers trained in the method, FIU started a center focused on the approach. It has drawn 4,000 people to its webinars since launching in 2011, according to a center website, and in 2016 became one of just 20 programs nationwide to receive federal funding amounting to $10 million over five years.

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.

"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."

The FIU center, however, was not formally involved in the pedestrian bridge project, a university spokeswoman said last week when the span was laid lifted into place.

But FIU administered the $12 million bridge project, which was funded by the federal government. Because it has its own building department, the school was also in charge of approving plans, permits and inspections for the bridge. Although the structure spans a state highway, the Florida Department of Transportation was only tangentially involved, the agency said in statement issued Thursday.

FDOT did raise one potential red flag: Under its agreement with the state, FIU was supposed to hire a "pre-qualified" engineering firm to conduct an independent design check -- meaning a firm previously approved by the state. FIU used a large international firm, Louis Berger, that was not pre-approved, according to FDOT. The agency also emphasized that FIU is responsible for overseeing all aspects of design and construction for the project.

The bridge was devised to provide FIU students and others a safe way to cross multi-lane Southwest Eighth Street, also known as the Trail, to the small town of Sweetwater, where the school estimates some 4,000 students live. At least one student was hit and killed by a car at that busy crossing, at 109th Avenue, which leads to new apartments built by private developers designed to cater to the university.

FIU selected the contracting team in a competitive process. It consists of MCM Construction, a family owned contractor based in Miami, and Figg Bridge Group, a design and engineering firm based in Tallahassee.

MCM is one of the most influential contractors in Miami-Dade, and a top contributor to county races. Gimenez said he spoke to co-principal Pedro Munilla by telephone from Hong Kong, where the county mayor is leading a county trade mission.

"Obviously, they're devastated," Gimenez said, adding that Munilla did not share an explanation as to what might have happened.

MCM has done substantial bridge and highway work for FDOT, including on the massive Palmetto Expressway and Dolphin Expressway intersection reconstruction. That project made use of accelerated bridge construction, U.S. Rep. Carlos Curbelo said Thursday.

"This is a new technology," Curbelo said. "My understanding is even on major highways like the 836-826 exchange, this type of bridge technology is in place and there's never been any issue with it, so this is just very surprising and we need to figure out what happened."

Figg has designed bridges all over the world, including the iconic Sunshine Skyway Bridge across Tampa Bay. The firm was chosen to build the replacement to the Interstate 35 bridge in Minneapolis, Minnesota, after an aging and structurally deficient bridge collapsed in 2007. The $233.8 million, 10-lane replacement bridge was heralded for its state-of-the art safety systems, including sensors designed to monitor bridge stress and corrosion.

In 2012, the Virginia Department of Labor cited Figg Bridge for four violations after a 90-ton piece of concrete fell from a bridge under construction near Norfolk, Virginia. The builder did not get the manufacturer's written consent before it modified a girder that ultimately failed, causing the concrete to crash to the ground, according to the Virginia Pilot Ledger.

Figg designed the FIU bridge as a signature structure. Yet to be installed was a distinctive support mast from which cables would be suspended to hold up the span in an upside-down V shape.

According to documents on the university's website, a concrete rather than steel structure was chosen for the span to minimize vibrations and provide a comfortable environment for pedestrians. The broad span would have open sides and was also wide enough to accommodate benches. It would also provide Wi-Fi service to users. One reason for the amenities: pedestrians tend to shun confined bridges.

The 175-foot span that collapsed was not the full bridge. Still to be built were stairs and elevators and connectors to take the north end of the bridge across a canal that runs along the Trail into Sweetwater.

The span had undergone testing and tension cables adjusted as it was assembled from precast pieces along the side of the road.

It's possible the cables were over-tightened, causing the bridge to elevate slightly in what's called a camber, said Verrastro, the Naples engineer. Adjusting the cables to address camber would be appropriate, but that would not impact the structural strength.

"So maybe it had too much or not enough. But that's just a guess. It wouldn't be because of structural strength," he said. "If they were adjusting the structural cables, it was to try to put more or less camber."

However, adjusting the camber, or tuning the bridge, can be tricky. Robert Bea, a University of Californai Berkeley engineer and catastrophic risk expert, has studied hundreds of structural failings including the BP's Deepwater Horizon, and said workers adjusting the camber on a bridge in Australia in the 1970s led to a similar collapse.

"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 and engineers need to be exact about how much weight is used during load-bearing tests, Buffalo's Aref said.

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 steel trusses, it used 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." A steel bridge weighs about one-tenth of a concrete one, he said.

Verrastro, an expert in accelerated construction who had spoken at FIU's bridge engineering program, suspects that using concrete was part of the bridge's aesthetic, rather than structural, design. The Figg firm 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.

But Bea cautioned that innovative methods sometimes produce new ways for structures to fail.

"Innovations always bring potential 'failure modes' that have not been previously experienced," he said.

Douglas Hanks Jr. contributed to this report.

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