Design flaws led to cracking in a critical portion of 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 explicitly blaming the design errors for the bridge’s collapse, which killed six people, including an FIU student. The board’s findings on the cause of the bridge’s catastrophic failure on March 15 are expected to come in a full report next year.
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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 engineers on the project team, who worked for Tallahassee-based FIGG Bridge Group, underestimated the structural load on that section while overestimating the strength of the connection.
The report does not name FIGG, which formed a joint venture with politically connected Munilla Construction Management to design and build the pedestrian bridge for FIU. MCM declined to comment on the NTSB report. FIGG issued a statement saying “the investigative update is just that, an update,” adding that the report “underscores that no probable cause conclusion should be drawn from the update.”
In a statement, FIU Senior Vice President Kenneth Jessell said the school was “glad” to see the NTSB investigation progressing and noted the university is “fully cooperating” with the agency.
“We hope the results of the investigation will help bring closure to the families and loved ones of the victims,” Jessell said.
Cracks that appeared at a critical structural connection point after the unorthodox bridge span was lifted into place, cracks dismissed by FIGG bridge design engineer W. Denney Pate as posing no safety threat, were “consistent with the identified errors,” the NTSB report says without further elaboration.
A previous NTSB update in May detailed the cracks and said the agency was looking into their significance, but also did not elaborate. The NTSB had sought to prevent evidence of those cracks from becoming public before it issued that report, citing “investigative need.” But FIU accidentally released photos of the cracks and an accompanying memo in response to a Herald public records request.
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 of FIU’s team 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.
Linwood Howell, a Texas-based bridge engineer who also examined plans and calculations for the Herald, said he expects NTSB’s final report will dig into the handling of the cracks and how the team went about reviewing the design to catch flaws.
“When the cracks appeared, people were concerned and troubled,” Howell said in an email. “But the cracks were dismissed in what seems to have been an ad-hoc process of consensus within an informal committee of non-qualified persons. The NTSB will certainly have something to say about that whole process. The NTSB will also have much to say about the review procedures used during the design phase.”
The Herald sued to obtain the withheld records, contending they should be public under Florida’s broad Sunshine law. A state court ruled in the Herald’s favor, but that decision was blocked by a federal judge.
The new NTSB update also seems to rule out flaws in concrete and steel fabrication as a cause. The highway administration’s Turner-Fairbank Highway Research Center performed tests on concrete samples and steel reinforcement bars from the span and found they met specifications, the NTSB said.
Attorneys for some of the victims of the collapse said the NTSB update corroborates their own conclusions that design errors led to the bridge failure.
“Based upon our investigation and analysis, we were aware that there are fundamental design errors in the plans that directly caused the collapse,” said Alan Goldfarb, an attorney for the family of Alexa Duran, the FIU student killed in the collapse. “All parties in charge of design and working on this bridge completely ignored or missed these design flaws.”
Stuart Grossman, who represents Richard Humble, injured in the car where Duran was crushed, as well as Kevin Hanson, a construction crew member who was in a coma after the collapse, applauded the agency’s report.
“We have long contended that this bridge was defective from its very design beginnings and that this put in motion the ultimate collapse of the bridge,” Grossman said. “We appreciate the NTSB continuing to examine the design further along with the review processes and construction that led to this disaster.”
The span that collapsed was the main portion of a bridge designed to take pedestrians from FIU’s main campus to the town of Sweetwater over eight lanes of traffic. FIGG designed an unusual variation of a traditional truss bridge to provide FIU with a signature structure the university sought as part of a broader project to help turn Sweetwater’s modest main street into a college town center. But independent experts say the bridge design made it vulnerable to failure if a single structural element failed.