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.
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.
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 traffic on the busy highway.
The decision by its contractors to undertake testing while traffic flowed along the busy roadway below will also be scrutinzed. 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.
FIU’s engineering school has become a hub for accelerated bridge construction training and research in recent years.
The bridge was devised to provide FIU students and others a safe way to cross multi-lane Southwest Eighth Street, also known as the Trail, to the small town of Sweetwater, where the school estimates some 4,000 students live.
At least one student was hit and killed by a car at that busy crossing, at 109th Avenue, which leads to new apartments built by private developers designed to cater to the university.
FIU selected the contracting team in a competitive process. It consists of MCM Construction, a family owned contractor based in Miami, and Figg Bridge Group, a design and engineering firm based in Tallahassee.
MCM is one of the most influential contractors in Miami-Dade, and a top contributor to county races. Gimenez said he spoke to co-principal Pedro Munilla by telephone from Hong Kong, where the county mayor is leading a county trade mission.
Please like, share and tweet this article.
Pass it on: Popular Science