The Atlanta Journal-Constitution
» More on the bridge collapse in Florida,
Accelerated process carries risks if not fully secured.
MIAMI — The unfinished pedestrian overpass that toppled onto the Tamiami Trail at Florida International University in Miami 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 at least six 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 960-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 cal- culated precisely in the analysis to make sure the par- tial bridge would be able to carry them safely,” Aref said.
That doesn’t mean that testing or tension adjust- ments 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 fac- tor,” 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 Trans-
portation 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 Univer- sity civil engineering profes- sor Maria Moreyra Garlock. The construction phase, she noted, is often the most dan- gerous 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 whathappened 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 traf- fic on the busy highway. The decision by its contractors to undertake testing while traffic flowed along the busy roadway below will also be scrutinized. 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.