Construction technique reviewed after bridge collapse
MIAMI — 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 quickbuild 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 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 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 March 10.
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.