Investigators from the National Transportation Safety Board (NTSB) probing the March 15 collapse of a pedestrian bridge at Florida International University say that workers were adjusting the tension on two tensioning rods located in a diagonal member at the north end of the span when the bridge collapsed.

The bridge collapsed March 15, killing 6. Workers had done this same work at the south end, moved to the north side, and had adjusted one rod, NTSB says. They were working on the second rod when the span failed and collapsed. The roadway was not closed while this work was being performed.

The 950-ton bridge segment fell onto a busy highway, killing six people. The 174-foot-long segment had been lifted into place on March 10.

NTSB investigators and contractors remove and catalogue core samples from the pedestrian bridge deck. Credit: NTSBReports shortly after the accident said that engineers had reported cracks in concrete on one side of the bridge two days before the collapse. Cracks are not uncommon in concrete and may not have been related to the accident.

Components from the bridge that NTSB believed needed additional examination and testing were removed from the site soon after the accident. These include sections of the floor, the canopy, a vertical member and a diagonal member, all from the structure's north end and were in the area of where the failure occurred, NTSB says.

(Click to enlarge.) The bridge segment was lifted into place March 10.NTSB also obtained core samples. These samples, along with recovered rebar and tensioning rods, were shipped to the Turner Fairbank Highway Research Center, in McLean, Virginia, for testing and evaluation.

The investigative team obtained an exemplar tensioning rod and hydraulic unit used by the construction crew to make tension adjustments to the tensioning rods. These items also were shipped to Virginia for evaluation and testing.

Segments of the bridge are being stored at a Florida Department of Transportation facility, but no plans exist to reconstruct the bridge as part of the NTSB investigation. It says the nature of the structure and the way it failed make reconstruction impractical.

NTSB says the next update on the investigation will likely be the release of its preliminary report. Preliminary reports do not contain analysis and do not address probable cause.