The National Transportation Safety Board has determined that the probable cause of the deadly March 15, 2018, Florida International University pedestrian bridge collapse was related to load and capacity calculation errors made by FIGG Bridge Engineers Inc.

The board also found that consulting engineer Louis Berger Group failed to catch FIGG’s design calculation errors that contributed to the collapse.

Louis Berger, the board said, did not detect the inadequate design of the main span truss member 11/12 nodal region and connection to the bridge deck, which was the area that exhibited significant cracking prior to the collapse. Demand on the node was almost double FIGG’s calculations, and mistakes by the bridge design team resulted in an overestimation of the bridge’s capacity to resist shear in that region.

The NTSB also noted that Louis Berger was not qualified as an independent peer review engineering firm by the Florida DOT and did not have a sufficient budget to evaluate the nodes or the stages of bridge construction.

The NTSB offered 30 findings, including that the retensioning of member 11, which was taking place when the bridge collapsed, constituted a design change and should have been reviewed by an independent engineer and that the cracks should have indicated to the project team that the bridge was failing.

The board also offered its safety recommendations for use on future projects to FIGG, the FDOT, the Federal Highway Administration (FHWA) and the American Association of State Highway and Transportation Officials.

The victims’ families and survivors of the 2018 accident, which killed six people and injured several others, have settled on monetary damages with most of the project team. As of August, Louis Berger was the only holdout. The accident also drove Magnum Construction Management, known at the time of the incident as Munilla Construction Management, to file for bankruptcy protection.

After the NTSB meeting Tuesday, MCM, in an emailed statement to Construction Dive, did not address the ruling specifically. “We are a family-owned and operated company with a 35-year history. This is the first timed in our over three decades of operation that we have ever experienced anything like this tragic accident. 

“We will continue to work closely with all parties to resolve ongoing legal and financial issues in an expeditious manner.”

Louis Berger did not respond to a Construction Dive request for comments.

FIGG, on the other hand, disputed the NTSB’s determination that bad design was the reason the bridge fell that day. The company hired forensic structural engineering firm Wiss, Janney, Elstner Associates (WJE), which FIGG said identified the cause of the collapse as failure to build the construction joint at member 11 according to FDOT standard construction specifications.

“In its review of the FIU pedestrian bridge accident, WJE conducted detailed research, in-depth analysis and physical testing. When the Federal Highway Administration (FHWA) Turner-Fairbank Highway Research Center issued its October 19, 2018 analysis concluding that the concrete cold joints were not intentionally roughened, WJE went a step beyond analysis and performed testing of full-scale replicas of the critical connection with both roughened and un-roughened surfaces.

“WJE's tests revealed, contrary to the findings of the NTSB, that the failure to roughen the concrete beneath bridge member 11 was the fundamental cause of the collapse.”

The NTSB addressed FIGG’s assertions about the cold joints and found that roughening the joints would not have necessarily prevented the collapse. The agency also found that the bridge engineer’s construction plans “inconsistently identified when intentionally roughened surfaces were needed to fulfill assumptions of the bridge design.”

In June, OSHA’s Office of Engineering Services issued a report that also laid most of the blame at FIGG’s door for both its design and failure to recognize the severity and danger of the cracks, which the NTSB said were 40 times the size one would normally see in concrete construction. The agency took most of the project team to task for not acting to shore up the bridge and stop the flow of traffic below.

The bridge was built near the point of installation and moved into place according to accelerated bridge construction (ABC) techniques. During the meeting, NTSB engineers said this method of construction did not contribute to the collapse.

Among the NTSB’s other findings:

• There were no deficiencies in the materials or the ladder jack used in post-tensioning operations on the day of the collapse.
• FIGG’s bridge design was nonredundant and provided only a singular load path.
• The FDOT should have verified Louis Berger’s qualifications as an independent peer review firm as part of the agency’s oversight.

During the NTSB’s presentation, Chairman Robert Sumwalt said that the finger-pointing since the accident was “unprecedented” but correct because everyone involved in the project carried some responsibility.

“Oversight of the project, like the bridge itself, collapsed,” Sumwalt said.