FIU Bridge Collapse: Design Flaws Cited By NTSB

WASHINGTON, DC — The National Transportation Safety Board on Thursday said the deadly collapse of the FIU pedestrian bridge was consistent with design errors and not the steel and concrete used in the massive structure. The 174-foot-long main span killed six people, including a bridge worker when it collapsed on March 15. Eight others were injured in the tragedy.

"Although the evaluation is ongoing, the assessment has determined that errors were made in design of the northernmost nodal region of the 174-foot-long span, where two truss members were connected to the bridge deck," said the agency in an investigative update released Thursday.

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"These design errors resulted in (1) overestimation of the capacity
(resistance) of a critical section through the node comprised of diagonal member 11 and vertical member 12; and (2) apparent underestimation of the demand (load) on that same critical section," investigators concluded.

The NTSB said that the cracking "observed in the node prior to the
collapse" is consistent with such errors.

Investigators said earlier that large cracks had been documented in Florida International University's failed pedestrian bridge days before the 950-ton main span came crashing down. The main span was lifted from its temporary supports, rotated 90 degrees across an eight-lane road and lowered into its permanent position in a matter of hours over busy SW 8th Street days before the collapse.

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Earlier photos released by the agency showed cracks before the bridge was moved into place. The structure was intended to improve safety for the thousands of students who cross into the campus from the city of Sweetwater each day.

Kenneth Jessell, senior vice president for finance and administration and Chief Financial Officer of FIU, said the university has assisted in the investigation.

“At FIU, we continue to fully cooperate with the NTSB so that it may finish its investigation and complete a final report and safety recommendations. We hope the results of the investigation will help bring closure to the families and loved ones of the victims.”

NTSB investigators have already interviewed employees involved in the design, inspection and construction of the bridge.

Test results taken by the Turner-Fairbank Highway Research Center found that the concrete and steel samples met minimum requirements specified in the project’s build plans, according to the NTSB.

Here is a summary of the findings related to steel and concrete samples:

• Test results showed that all the concrete core specimens obtained from both the bridge deck and the canopy met the compression requirements per the project plans.

• The design plans specified that the concrete used for the project was required to meet Florida Department of Transportation specifications. All specimens from both the bridge deck and the canopy were within the specified range for total air content.

• Two types of specimens were tested during evaluation of the post-tensioning rods: machined round bar specimens and full-size bar specimens. For all tension tests, the specimens met the specified minimum yield strength, tensile strength, and percent elongation at fracture.

• The steel reinforcing bars collected from the collapsed structure included several different sizes, identified as #5, #7, #8, and #11. Collapse-induced deformation of the size #7 bar precluded their testing. Tension test results for the remaining size #5, #8, and #11 bars found that all met the minimum yield strength, tensile strength, and percent elongation at fracture for their respective sizes.

Photograph of 174-foot-long main span on March 10, 2018, prior to placement on south pier and north pylon pier, looking west. Labels showing diagonal member 11 and vertical member 12 added by NTSB. Photograph courtesy National Transportation Safety Board.