FIU Bridge: NTSB Identifies the Causes of the Collapse

On March 15, 2018, a partially constructed pedestrian bridge crossing an eight-lane roadway in Miami, FL experienced a catastrophic structural failure. The 174-foot-long bridge span fell about 18.5 feet onto SW 8th Street, which consists of four through travel lanes and one left-turn lane in the eastbound direction, and three through travel lanes in the westbound direction. Two of the westbound lanes were closed to traffic at the time of the collapse; however, one westbound lane and all five eastbound lanes were open.

The pedestrian bridge was under construction as part of the Florida International University (FIU) University-City Prosperity Project. Eight vehicles below the bridge were fully or partially crushed, seven of which were occupied. One bridge worker and five vehicle occupants died. Five bridge workers and five other people were injured. The Accident Report NTSB/HAR-19/02 PB2019-101363 of the National Transportation Safety Board (NTSB) identifies the causes of the collapse of the FIU Bridge.

The NTSB determined that the probable cause of the bridge collapse was the load and capacity calculation errors made by FIGG Bridge Engineers, Inc., (FIGG) in its design of the main span truss. Contributing to the collapse was the inadequate peer review performed by Louis Berger, which failed to detect the calculation errors in the bridge design. Further contributing to the collapse was the failure of the FIGG engineer of record to identify the significance of the structural cracking observed in this node before the collapse and to obtain an independent peer review of the remedial plan to address the cracking.

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.

The investigation of the collapse focused on the performance of the northernmost nodal region of the main span. The failure of this nodal region was the triggering event for the bridge collapse. Factors in the collapse 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 eventual failure, and lack of redundancy in the bridge design. Specifically, the investigation focused on the following safety issue areas:

  • Bridge design and construction plan errors and unique bridge characteristics and mechanisms of failure. The uniqueness of designing a concrete truss bridge led to the circumstances that accounted for the collapse of the pedestrian bridge. The bridge design team made two errors that resulted in the under-design of the nodal area that failed, resulting in the collapse. First, the design team underestimated the demand (loads imposed on structural members) that would be acting on the nodal area. The investigation compared post-collapse calculations for the demands on the node with the design calculations, and this comparison found that the demand for the node was nearly twice what the design team had calculated. Second, the design team also overestimated the capacity of the node to resist longitudinal shear where the nodal region was connected to the bridge deck. This overestimation was the result of the designer using incorrect loads and load factors in its calculations. These two design errors resulted in a node that lacked the capacity to resist the shear force pushing the node to the end of the bridge.
  • Independent peer review of complex bridge design. Errors in design may occur, but systems should be in place to catch those errors when they do occur. In this case, a firm was hired to independently review the bridge design for errors. However, the review conducted by this firm did not evaluate the nodes of the bridge truss where they connected with the bridge deck and canopy, nor did it consider the multiple stages the bridge construction involved. Although the design reviewer recognized that he should have examined the nodes and stages, he indicated that there was not enough budget or time to evaluate those factors. Contributing to this review failure was the reviewing firm’s lack of qualification to do the work.
  • Shortcomings in oversight of evaluation of and response to significant observed bridge structure distress prior to collapse. As soon as the bridge had to support its own weight, cracks appeared at the under-designed nodes. Over the next 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. 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, the repair work was conducted without closing the road below the bridge to traffic.
  • Lack of redundancy guidelines in specifications for pedestrian and concrete truss bridges. The design of the pedestrian bridge did not include redundancy in the bridge load path. As a result, when the nodal region failed, the bridge collapsed. The design firm incorrectly believed that the bridge had a redundant design. For typical bridge designs, a bridge designer would use a safety factor greater than one to ensure that the bridge was over-designed to prevent a collapse.