What caused FIU’s bridge to fall? More clues come to light.

If tightening support cables caused Florida International University’s signature new pedestrian bridge to collapse, as Sen. Marco Rubio has suggested in a tweet, the rubble spread across the Tamiami Trail could contain obvious clues, bridge experts say.

Adjusting tension cables, which ran like tendons through the 950-ton concrete span and the struts that connected a concrete canopy on the bridge to the walkway, can be a delicate operation. Over-tightening the cables, which are used to reinforce the concrete, can have devastating results, the experts say: It might cause torquing, or twisting, that would cause a specific cracking pattern in the concrete on impact with the road, something investigators would likely detect quickly.

National Transportation Safety Board investigators have confirmed that crews were tightening tensioning cables in a diagonal truss at the north end of the bridge on the day of the collapse. But they stressed they did not know whether that was the point at which the span failed.

Indepent experts have zeroed in on the tensioning work as one of a myriad possible causes for the failure of the unfinished bridge, which crumpled to the pavement on Thursday.

The NTBS investigators, who arrived on Thursday night, began what will likely be an arduous process of reconstructing the accident and assigning a cause. The investigation could take as long as 18 months, the agency said.

Reports that cracking in the bridge had been discovered days before the collapse drew a flurry of attention on Friday, but its significance remains unclear.

The Florida Department of Transportation issued a statement saying the bridge’s engineer had called the agency on Tuesday to report cracking in the concrete at the north end of the span – in the same general area where tensioning work was undertaken on Thursday, two days later. According to the call’s transcript, which was released by FDOT, the FIGG Bridge Group engineer said the cracking did not raise a safety issue.

The design-build team then held a two-hour meeting on Thursday morning, with FDOT participation, during which the cracking was discussed but no safety red flags were raised over it. The meeting broke up at 11 a.m., a couple of hours before the bridge buckled and fell. The precise location of the cracking has not been disclosed.

An NTSB investigator said Friday that a crack in the bridge is not necessarily unsafe. Cracks in new concrete construction are not uncommon, and can be cosmetic or a sign of more serious trouble, experts say.

As crews work to remove the massive slab to retrieve vehicles they suspect hold more victims, investigators will begin combing through debris and document the site. The wreckage itself will likely provide telling details – cracks could reveal the precise location where stresses occurred that caused the bridge to fall.

Interviews with witnesses and video footage could reveal the sequence of events. Investigators may use 3D imaging, preserve larger pieces of the bridge in a local warehouse while the case unfolds and ship other joints or braces back to Washington labs for testing, said NTSB Media Relations Officer Keith Holloway.

“Each accident is different, so it may require different techniques,” he said.

Independent experts focused in on work going on Thursday before the bridge fell. Late Thursday night, Rubio tweeted that he was told by workers at the site that cables connected to the bridge had loosened and engineers were instructed to tighten them.

One subcontractor that provides materials and services for concrete tensioning, Structural Technologies VSL, confirmed on Friday that it had a crew at work on the bridge and that one employee, Navaro Brown, had perished in the accident. Two others were hospitalized.

Company spokesman Michael Biesiada, wouldn’t say exactly what the crew was doing when the bridge collapsed, but he confirmed the workers were “providing installation support for our products.” Documents from FIU and consultant Bolton Perez & Associates, obtained by the Miami Herald, show that Structural Technologies, LLC (VSL) was paid $439,350 for “post-tensioning work.”

Whatever did in fact occur to weaken the bridge, one expert said, it could well be tied to work done Thursday. Engineers say that most bridge and building collapses that occur during construction are caused by work errors, not design flaws, though the latter can’t be ruled out this early.

FIU President Mark Rosenberg told the Miami Herald on Friday there were various tests and construction tasks underway on Thursday. Miami-Dade County Mayor Carlos Gimenez has said the bridge was undergoing stress tests, which typically require placing weights on a structure to test its resiliency,

“I assume they were tests to see the resiliency of the concrete, the stress on the material,” Rosenberg said.

Rosenberg said he has not spoken to anyone from the bridge contractor, Munilla Construction Group, a politically connected firm in Miami. Rubio said he had spoken to partner Pedro Munilla amid the rubble of the bridge collapse on Thursday, but not about the possible cause.

The bridge’s 175-foot main span, built by the side of Southwest Eighth Street over a period of months, was hoisted into place over supports at either end in a matter of several hours last Saturday morning – a relatively novel though increasingly common approach known as accelerated bridge construction.

“It was set on supports on Saturday and it didn’t fall right away, so clearly something happened on Thursday that was different from Sunday, Monday, Tuesday, Wednesday,” said Michael Chajes, a forensic engineering expert at the University of Delaware. “This isn’t the kind of thing that gradually gets weaker over four days. Something weakened it.”

Whatever the case, the collapse is certain to lead to multimillion dollar lawsuits and finger-pointing even as the official investigation proceeds.

“The reason everyone is so shocked is because this should never have happened,” said John Uustal, a Fort Lauderdale trial lawyer who has won major verdicts against General Motors and tobacco companies. “Now, it’s like a murder mystery, and we’ve got to figure out who did it. At best, someone made a horrible, negligent mistake. But in my experience it tends more often to have its roots in a greedy corporate decision to save money or cut costs.”

Uustal warned that the official probe could drag on into 2019 because of federal budget cuts.

“Because of the publicity they might move more quickly, but lately all sorts of government investigations – whether it’s fire, highways, boating, air traffic – are taking longer because these agencies have been losing funding for 20 years.”

The innovative bridge, set to open in early 2019, was lauded by both the university and the FIGG Bridge Group, the award-winning firm that designed it, as a grand new gesture in both style and construction. University officials said it was intended to tell “the story of the technology of its time” while showcasing the school’s accelerated bridge construction center, the instant-bridge technology now widely used in urban areas to avoid closing roads and keep traffic flowing.

That fast-track construction is now raising questions. The span, made of concrete poured into place as it was constructed by the side of the road, is typically tested as work progresses. It can also vibrate as it’s moved into place, but that would likely be accounted for during design, experts said.

Rosenberg said on Friday that crews had continually monitored stress on the span as it was moved into position across Eighth Street and lifted onto the support columns.

“They were continually monitoring the stress on the concrete with the movement,” he said.

Bridges are typically “tuned” once they are put in place, to adjust the arc, or camber, in the concrete that occurs when tension cables are pulled tight.

The FIU bridge also has an unusual design that might have played a part in its failure. It had a heavy concrete canopy, or roof, connected to the walkway slab by v-shaped trusses – the structural element that the NTSB said crews were working to tighten on Thursday before the bridge collapsed.

An NTSB official said the agency had not encountered a bridge design like it previously.

It’s possible that the roof, bearing down on the span, was at risk of twisting if tension wires were released, one expert said.

“If they were releasing tension in the wires to fix the camber,” bridge designer Richard Hartzell said in an email, “it could suddenly increase this twisting (torsional buckling) leading to a quick and sudden collapse.”

The span was incomplete when it fell. A pylon holding up support pipes, not yet installed, was designed to tower over the bridge, but those structural pieces would attach to the canopy, not the walkway itself. A detailed document on FIU’s website says the bridge was designed to be strong enough to hold up without the pylon assembly. The pylon assembly is meant instead to dampen vibrations on the bridge and provide a dynamic aesthetic element.

There could be other causes as well, from contractors failing to follow design instructions to problems in the pre-stressed concrete, which was poured and cured as the span was constructed alongside the Tamiami Trail.

“They’re not going to find out a lot of things until they start poking around,” said Martin Gordon, president of the National Academy of Forensic Engineers and a professor at the Rochester Institute of Technology. “But the first thing is to preserve and record exactly as much as possible. Sometimes that gives clues as to what portion of the bridge failed.”

Because concrete is so vulnerable to stress – it’s stronger when compressed than when pulled – cracks may provide important clues as to whether contractors followed design speculations. Engineers would have specifically designed for the angle of stress. So if a piece were moved or twisted, or if it’s picked up in the wrong spot, it may weaken.

“Anywhere they pick it up, they’re going to change the state of stress,” Chajes said. “So we would calculate each one of these movements to know what we’re doing.”

Using heavy concrete rather than steel has also raised questions.

“Normally a bridge would use steel trusses because of the pull-push pressures,” Gordon said. “With trusses, some members are compressed and some pulled.”

Investigators will likely collect debris as possible to identify defective material, Chajes said, and then try to distinguish the damage caused by failure verses the fall.

“It’s almost like in a fire trying to figure out what was the initiating cause,” he said.

The exact sequence of events could also quickly lead them to where the stress occurred, which could help reveal what went wrong.

“What did you notice starting to happen as it failed? Was there twisting involved? You really want to know because the stresses throughout the structure are of a different nature,” Chajes said. “If you knew exactly what was happening on the site for the five minutes before it failed, that’s going to pretty quickly point you to what the causes were.”

And while the accelerated construction may not wind up being a cause, the collapse may change safety measures.

“What I don’t understand, and I don’t understand it as a lawyer or as a regular human being, is how do you do this in the middle of the day with people driving underneath,” said construction lawyer David Haber. “That’s not just a bad idea, that’s potentially criminally negligent. It’s insane.”

A final report on the accident may not come for more than a year, Holloway said. But the agency will likely release updates, as it has with other cases like the El Faro investigation, as facts are confirmed.

“We’ll look at what’s unique about this particular bridge,” Holloway said. “If we do see something during our investigation that needs to be addressed more urgently, we can issue more urgent recommendations.”