South Florida

NTSB discloses new details about work performed before FIU bridge collapsed

By Sunday workers had cleared most of the debris blocking Southwest Eighth Street after the pedestrian bridge collapsed. Some large pieces have been secured by federal transportation investigators for closer scrutiny.
By Sunday workers had cleared most of the debris blocking Southwest Eighth Street after the pedestrian bridge collapsed. Some large pieces have been secured by federal transportation investigators for closer scrutiny. pportal@miamiherald.com

Before workers began adjusting support cables at the north end of the Florida International University bridge that collapsed and killed six people last week, they performed the same work at the opposite end of the bridge, the National Transportation Safety Board said Wednesday.

Adjusting the cables has come under scrutiny as investigators try to explain why the 950-ton span fell suddenly, just five days after it was lifted into place over Southwest Eighth Street near the sprawling campus.

The new detail was included in the latest update from the NTSB on its ongoing investigation. Spokesman Chris O’Neil said he could not confirm precisely when the work at the south end occurred: whether it was done immediately before workers moved to the north end, or earlier.

fiu bridge collapsed
Engineers not connected to the investigation say adjusting post-tension cables used to strengthen concrete may have played a role in the bridge’s sudden collapse. Sebastián Ballestas sballestas@miamiherald.com

Workers had begun adjusting the tension in two rods threaded through diagonal bridge trusses on the north side, near the C-4 canal, just before the collapse.

“They had done the same work earlier at the south end, moved to the north side and had adjusted one rod,” the update said.

The crew was beginning work on the second rod when the 174-foot-long span abruptly dropped.

The work may have been related to cracks in the span’s concrete, reported just two days before the collapse by the project’s lead technical engineer. While he said repairs would need to be made, he did not consider them to be a safety concern. Why the road remained open during the work is part of the investigation.

Engineers not working on the project have speculated that the cracks may have led to adjusting the post-tensioning cables, which strengthen the concrete against the stress of being pulled, and caused the failure.

FIU bridge roof tensioning boxes
A frame-grab from a video shot by FIU planning coordinator Stuart Grant shows the canopy and dimples where workers could access tensioning cables. Stuart Grant

“They’ll likely find a number of reasons or forces behind the cracking which started all this,” said Richard Hartzell, a California bridge designer. “The [post-tensioning cables] were the ‘straw that broke the camel’s back.’ This is particularly true if the PT bar broke during their adjustment efforts.”

As part of its investigation, the NTSB is focusing on the span’s north end and said Wednesday it has hired a company to remove sections of the span’s floor, the canopy and at least one truss that includes vertical and diagonal members. Investigators also took more core samples from the concrete, adding to earlier samples collected.

The samples, along with rebar and some of the tensioning rods, will be shipped to a U.S. Department of Transportation research center outside Washington for more tests. Investigators have also obtained an “exemplar” of the tensioning rod and hydraulic unit used to adjust the cables, but it’s not clear if they were rescued from debris or are an example obtained from the manufacturer.

The agency also said large sections of the span identified immediately after the collapse for closer scrutiny are being stored at a Florida Department of Transportation facility. But O’Neil said the NTSB will control access to the pieces.

Actions by the FDOT are being examined in the investigation although Gov. Rick Scott said the agency had little oversight. FIU President Mark Rosenberg, however, has said state officials were “involved at every step.”

fiu bridge illustration
The FIU bridge relied on a ‘one-of-a-kind’ construction that allowed the span to be more open. Florida International University

In the coming weeks, four different teams working on the investigation will focus on various aspects, including the bridge work and the decision to keep traffic flowing on busy Southwest Eighth Street, O’Neil said. Once investigators return to Washington, they generally take about two weeks to complete a preliminary report, which will include facts they’ve confirmed but not analysis of what caused the collapse, he said.

“It might talk about what structures are being examined, but really it’s a compendium,” he explained.

Investigators are also interviewing witnesses and have requested that anyone with pictures or video of the bridge submit them. While the NTSB investigation will focus on the cause of the collapse, with the goal of preventing such a catastrophic accident from ever happening again, law enforcement authorities will look for blame.

A Miami-Dade police homicide squad is conducting a criminal investigation to determine whether any negligence occurred. But prosecuting such a case would be difficult, Miami-Dade State Attorney Katherine Fernandez Rundle told the Miami Herald, coming down “to science and the experts.”

Follow Jenny Staletovich on Twitter @jenstaletovich

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