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.”
Read more at our news partner, the Miami Herald.
