BOSTON (WHDH) - As the 950-ton concrete bridge section was swung into place over a highway last weekend, Florida International University officials were beaming with pride.
The pedestrian bridge on the edge of the Miami-area campus was a signature achievement of the school’s Accelerated Bridge Construction University Transportation Center, a research group set up with federal funding a few years ago to show how spans could be built faster and cheaper in the U.S.
“FIU is about building bridges and student safety. This project accomplishes our mission beautifully,” FIU President Mark B. Rosenberg boasted that day. “We are filled with pride and satisfaction at seeing this engineering feat come to life and connect our campus to the surrounding community.”
Five days later, the bridge collapsed onto the busy six-lane highway, crushing cars and killing at least six people in a tragedy now under federal investigation.
While it is not yet clear what caused the failure of the unfinished span Thursday, the disaster has cast a spotlight on a rapid construction technique widely used around the U.S.
Accelerated bridge construction, or ABC, involves assembling large sections of a span offsite, and then moving the massive pieces into place all at once.
The technique eliminates the lengthy road closings and other traffic disruptions that can result when a bridge is built out over a highway piece by piece. It is also considered by some engineers to be safer for hardhat workers and motorists because much of the construction isn’t done in the middle of traffic.
The general approach has been around since the mid-19th century — and has been used safely and successfully for a long time — but interest in ABC has increased in recent years as states have looked for quicker, less expensive ways to replace thousands of aging bridges.
In the case of the Florida tragedy, engineering experts said the question is where was the fatal mistake: in the design of the bridge, in the way its construction was carried out, or in the materials used?
Civil engineering experts who viewed photos of the planned structure and the collapse have raised questions about how FIU and its contractors approached the project.
One local expert says the tragic collapse should never should have happened.
“I’m very surprised,” Northeastern Professor Ming Wang said. “It was very simple stuff to do.
Wang, who teaches civil engineering, says he believes the cables supporting the bridge began to sag and that crews weren’t careful enough while adding more tension to them.
“The operation actually is not that easy,” Wang said. “It’s complicated.”
Wang says it’s a must to shut down the street overnight, and temporarily safeguard the structure while working on it.
“The only way to prevent this is to put a middle support in the middle to prevent it from failure,” Wang said.
The collapse has many questioning whether “accelerated bridge construction” is safe. The technique was used last fall when the state replaced two bridges over Route 3 in Braintree, and last summer for a new Commonwealth Ave. bridge over the Mass Turnpike.
So what is accelerated bridge construction? It’s basically when pieces of the bridge are pre-fabricated, then put in place with a crane.
Wang says those bridges are very different.
“They are very simple supported beams,” Wang said. “I don’t think we have that kind of problem at all.”
FIGG Bridge Engineers, which worked on the design of the collapsed bridge in Miami, also helped design Boston’s iconic Zakim Bridge.
Wang says they are very different designs, and he believes the Zakim Bridge is safe.
“Zakim Bridge is over designed. It’s very strong and rigid,” Wang said.
The proportion of the length of the bridge to its depth is vital. Humanity can also affect the strength of the concrete. Wang says investigators in Florida will be looking it those factors.
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