The National Transportation Safety Board (NTSB) said that multiple factors came into play in the March 2018 collapse of a pedestrian bridge that was under construction at Florida International University near Miami.

The NTSB said that key factors that led to the accident included bridge design errors, inadequate peer review of the bridge design, poor engineering judgment and response to the cracking that occurred in the region of the eventual failure and lack of redundancy in the bridge design.

The investigation focused on the performance of what the NTSB described as the northernmost nodal region (identified as the 11/12 node) of the main span. This nodal region was the triggering event for the bridge collapse, the NTSB said.

In particular, the NTSB said that FIGG Bridge Engineers made "significant errors in determining loads for the bridge. In particular, the NTSB said that FIGG chose the wrong interface shear demand value that led to a "severe underestimation" of demand. In addition, the firm chose the wrong load factor in calculating the permanent compression loading, which led to a "significant overestimation" of capacity.

In a statement, Robert Sumwalt, NTSB chairman, said that FIGG Bridge Engineers "severely underestimated the demand on the bridge, significantly overestimated the bridge’s capacity, and incorrectly determined the bridge to be a redundant structure," among other calculation errors.

He said also that public safety oversight was lacking and that the project "should have ensured that a qualified, independent firm provide effective peer review of bridge plans, as required." He said that engineering consulting firm Louis Berger "was not qualified and produced an incomplete review." When cracks in the bridgework reached unacceptable levels, the oversight structure should have resulted in a suspension of work and in road closures. "It did not," he said. "Oversight of the project, like the bridge itself, collapsed."

NTSB said that contributing to the severity of the collapse outcome was the failure of MCM; FIGG; Bolton, Perez and Associates Consulting Engineers; FIU; and the Florida Department of Transportation to cease bridge work when the structure cracking reached unacceptable levels and to take appropriate action to close SW 8th Street as necessary to protect public safety.

One bridge worker and five vehicle occupants died as a result, and 10 other people were injured.

Accident sequence

The NTSB said that on Thursday, March 15, 2018, at about 1:46 p.m., the partially constructed pedestrian bridge crossing an eight-lane roadway experienced a catastrophic structural failure in the nodal connection between truss members 11 and 12 and the bridge deck.

On the day of the collapse, a construction crew was working on retensioning the post-tensioning rods within member 11, connecting the bridge canopy and the deck at the north end. About 1:46 p.m., a video camera on a construction pickup truck traveling east, approaching the bridge, recorded the collapse sequence. The video showed the blowout of the concrete north of truss member 12, and the truss losing geometric stability.

OSHA action

In June, the Occupational Safety and Health Administration’s (OSHA’s) Office of Engineering Services (OES) said that FIGG Bridge Engineers, the Engineer of Record on a pedestrian bridge that collapsed in March 2018, failed to recognize that the bridge was in danger of collapsing when it inspected the structure hours before the failure.

OES also said in its 115-page report that the bridge had structural design deficiencies that contributed to its collapse. The cracks occurred due to “deficient structural design,” it said.

(Read the report.)

In the hours after the collapse, OSHA sent officials including a forensic structural engineer to determine the cause and whether industry or OSHA standards were violated. OSHA said it worked with the NTSB and local officials, inspecting the remnants of the fallen bridge, and interviewing contractors to determine construction activities preceding the collapse.

OSHA published its report independent of ongoing investigative work into the accident by the NTSB.

In a statement, FIU President Mark Rosenberg thanked the NTSB for its work and said that as a party to the investigation, the university was limited in what information it could share. He added that the university observed the NTSB's rules with regard to commenting publicly on matters associated with the bridge and that the university cooperated "fully and promptly" throughout the investigation and provided "all information requested by the NTSB during the course of the investigation."

Rosenberg said that the university will build a replacement pedestrian bridge and that additional information would be released later.

Design errors

In November 2018, the NTSB said that errors were made in the design of the pedestrian bridge and that cracking observed before the bridge's collapse was consistent with those errors. The NTSB said that the errors were made in the design of the northernmost nodal region of the span where two truss members were connected to the bridge deck.

Samples of rebar recovered from the collapsed pedestrian bridge await transport to the Federal Highway Administration’s Turner-Fairbanks Highway Research Center. There the samples underwent materials testing as part of the NTSB’s ongoing investigation of the March 15, 2018, fatal, bridge collapse. NTSB photo by Adrienne LammThe design errors resulted in an overestimation of the capacity (resistance) of a critical section through the node and an apparent underestimation of the demand (load) on that section.

The Turner-Fairbanks Highway Research Center, part of the Federal Highway Administration, conducted tests and examinations of concrete and steel samples taken from the bridge following its collapse. The concrete and steel specimens reportedly met the project’s build plans specified minimum requirements.

The NTSB in November said that findings from the materials tests included:

• Concrete core specimens from the bridge deck and bridge canopy met the compression requirements in the project plans.
• The design plans specified that concrete used for the project had to be in accordance with the Florida Department of Transportation specifications. All specimens from the bridge deck and bridge canopy were within the specified range for total air content.
• Tension test results of size #5, #8 and #11 steel reinforcing bars revealed that all met minimum yield and tensile strengths and percent elongation at fracture, for their respective sizes. Size #7 bars could not be tested due to collapse-induced deformation.

Recommendations

The NTSB in its October 2019 final report said that as soon as the bridge had to support its own weight, cracks appeared at the under-designed nodes, particularly node 11/12. Over a period of 19 days, the cracks grew until the bridge collapsed. The construction and inspection firms working on the bridge were aware of the cracks and reported the cracks to the design firm, asking for guidance, NTSB said. The engineer of record at the design firm repeatedly indicated that the cracks were of no safety concern.

On the day of the collapse, the firms met to discuss a plan by the engineer of record to remediate the cracks. The bridge collapsed as the firms were implementing the remediation plan.

In addition, NTSB said the repair work was done without closing the road below the bridge to traffic. The NTSB recommended changes to Florida bridge construction oversight procedures to emphasize the need for bridge closure to protect public safety when structural cracking (beyond what sound engineering judgment considers acceptable) occurs and to increase state oversight of complex bridge construction.

The NTSB also said that the design of the pedestrian bridge did not include redundancy in the bridge load path. As a result, when the 11/12 nodal region failed, the bridge collapsed. The design firm "incorrectly believed" that the bridge had a redundant design, NTSB said.

For typical bridge designs, it said that a bridge designer would use a safety factor greater than one to ensure that the bridge was overdesigned to prevent a collapse. The NTSB, recognizing that no design guidance exists discussing redundancy in concrete truss bridges, recommended that bridge design guides include a discussion of redundancy in concrete bridge designs.