Post-tensioning works were being carried out when the Florida International University (FIU) bridge in Miami, Florida collapsed last Thursday, investigators have confirmed.
Investigators from the National Transportation Safety Board (NTSB) have confirmed that engineers were adjusting the tension on two tensioning bars in the 53m long post-tensioned bridge’s concrete truss when it collapsed.
NTSB officials confirmed that the two tensioning bars being worked on were in a diagonal member at the north end of the span – the location where the collapse was triggered. Tension had already been applied to one bar and engineers were working on the second bar at the north end when the span failed and collapsed.
NTSB said the same work had already been carried out earlier at the south end.
The work on the bridge was being carried out over a live, eight lane highway when it collapsed, killing six people.
Investigators said they were now focusing on measuring and documenting the critical structures at the north end of the bridge.
Significant developments in the investigation include the securing of a contract to remove components from the bridge to undergo examination and testing, including sections of the floor, the canopy, a vertical member and a diagonal member; all from the north end of the structure.
Concrete core samples from the failed area have also been extracted and sent to a facility for testing and evaluation along with recovered rebar and tensioning bars.
NTSB said while segments of the bridge were being transported to and stored at a Florida Department of Transportation facility, there was no plan to reconstruct the bridge as part of the collapse investigation.
“The nature of the structure and the way it failed make reconstruction impractical,” said the NTSB.
It emerged last week that engineers had observed, discussed at length and ultimately dismissed the significance of a cracking in the pedestrian bridge just three hours before it collapsed. Investigators have not yet released details of the location of the cracking.
From photographic and video evidence, engineers New Civil Engineer has spoken to pointed to an explosive failure of the bottom end joint at the northern end of the span.