MIAMI _ Design flaws led to cracking in the Florida International University pedestrian bridge that collapsed during construction earlier this year, federal investigators conclude in a brief update issued Thursday.
The two-page report by the National Transportation Safety Board stops short of blaming the design errors for the bridge's collapse, which killed six people. The board's full findings on the cause of the bridge's catastrophic failure on March 15 are expected to come in a full report expected next year.
But the investigative update bolsters conclusions reached by independent bridge engineering experts consulted by the Miami Herald and others posting in online professional forums. Three experts consulted by the Herald separately concluded that design flaws at the north end of the unfinished bridge's 174-foot span over Southwest Eighth Street were likely a leading contributor to the collapse.
The NTSB brief echoes what the experts told the Herald after examining publicly available engineering calculations and plans for the bridge: Design errors meant that a key structural connection in the span, a point at which a diagonal strut identified as Number 11 met the deck of the bridge span and a vertical column, was too weak to support the large forces it was supposed to withstand.
Experts from the Federal Highway Administration who examined the plans and calculations for the NTSB concluded that the FIU team's engineers, who worked for Tallahassee-based FIGG Bridge Group, underestimated the structural load on that section while overestimating the strength of the connection.
Cracks that appeared at a critical structural connection point after the unorthodox bridge span was lifted into place, dismissed by FIU bridge design engineer W. Denney Pate as posing no safety threat, were "consistent with the identified errors," the NTSB report says without further elaboration.
The experts consulted by the Herald said the cracks were concerning enough that they should have prompted a stop to work on the bridge and a redirecting of traffic below the span for a thorough evaluation, something that available records suggest did not happen.
Instead, after a morning meeting called to discuss the cracking, a crew was sent out to tighten steel support rods that ran inside the Number 11 strut _ possibly an effort to close the cracks, the independent experts say. Traffic continued to flow below the span, which collapsed without warning while that "post-tensioning" work was going on, killing one crew member and crushing motorists stopped at a red light beneath.
Because the NTSB has barred release of critical records surrounding the accident, including any record of the meeting, it's unclear just why Pate and FIGG concluded the cracks were not a hazard or precisely why the crews were told to tighten the rods. That's critical information for determining why the bridge fell, said David Beck, a New Hampshire engineer who has assisted the Herald's reporting without charge.
"I am disappointed they haven't addressed the issue of the post-tensioning of member number 11," Beck said.
