What Happened to the FIU Pedestrian Bridge?

12 min readMar 5, 2024

On March 15, 2018, a tragic accident occurred near the campus of Florida International University (FIU) in Miami, Florida. A 174-foot-long, 950-ton pedestrian bridge that was under construction collapsed onto a busy highway, killing six people and injuring ten others. The bridge was designed to connect the university with the nearby city of Sweetwater, where many students lived and worked. It was also part of a larger project to improve the safety and mobility of pedestrians and cyclists in the area.

But what went wrong? How did a bridge that was supposed to be innovative and resilient end up crashing down in a matter of seconds? And what lessons can we learn from this engineering disaster?

Background of the Project

The FIU pedestrian bridge was one of the components of the University City Prosperity Project, which received an $11.4 million grant from the U.S. Department of Transportation’s Transportation Investment Generating Economic Recovery (TIGER) program in 201³⁴. The project aimed to create a multi-modal, pedestrian-oriented corridor that would enhance the quality of life and economic development of the university and its surrounding communities.

The bridge was intended to span over eight lanes of traffic on Southwest 8th Street, a major arterial road that separated the FIU campus from Sweetwater. The bridge would provide a safe and convenient crossing for students, faculty, staff, and residents, as well as a symbolic link between the two sides. The bridge would also feature a plaza, seating areas, bike racks, lighting, and landscaping.

The bridge was designed by FIGG Bridge Engineers, Inc., a firm that specializes in cable-stayed bridges. The construction was led by Munilla Construction Management (MCM), a family-owned company based in Miami. The steel fabrication and erection were done by Structural Technologies VSL (STVSL), a subsidiary of Structural Group.

The bridge used an innovative method called Accelerated Bridge Construction (ABC), which involves prefabricating large sections of the bridge off-site and then installing them on-site with minimal disruption to traffic. ABC is supposed to reduce construction time, cost, and risk, as well as improve worker safety and quality. The FIU bridge was one of the first in the country to use ABC for a cable-stayed bridge.

Bridge Layout

The FIU bridge consisted of two main sections: the main span and the canopy span. The main span was 174 feet long and weighed 950 tons. It was composed of a concrete deck supported by two concrete trusses that formed an inverted V-shape. The canopy span was 109 feet long and weighed 320 tons. It was composed of a concrete deck supported by four concrete columns that extended above the deck to form a canopy.

The main span was built on temporary supports on the north side of Southwest 8th Street, while the canopy span was built on permanent supports on the south side. On March 10, 2018, four days before the collapse, the main span was lifted from its supports and rotated 90 degrees across the highway using self-propelled modular transporters (SPMTs). The operation took about six hours and required closing the road to traffic.

The main span was then lowered onto its permanent supports: two concrete piers on each side of the road and one concrete pier in the median. The canopy span remained on its permanent supports on the south side. The next steps were to connect the main span and the canopy span with steel pipes and cables, install post-tensioning bars inside the trusses to provide additional strength and stability, pour concrete over the steel pipes and cables to form a continuous deck, and complete the aesthetic and functional features of the bridge.

Bridge Design and Construction

The FIU bridge used a novel design that combined elements of cable-stayed bridges and truss bridges. Cable-stayed bridges are bridges that have one or more towers that support the deck with cables. Truss bridges are bridges that have triangular structures that distribute the load along the bridge.

The FIU bridge had two concrete trusses that formed an inverted V-shape under the deck. These trusses were connected to four concrete columns that extended above the deck to form a canopy. The columns were also connected to each other with steel pipes and cables that formed an X-shape over the deck.

The design aimed to create a slender and elegant appearance for the bridge, as well as to reduce its weight and cost. However, it also introduced some challenges and complexities for its analysis, fabrication, erection, and testing.

One of these challenges was how to ensure that the trusses could resist shear forces, which are forces that act parallel to the surface of an object. Shear forces can cause an object to slide or tear apart. In the FIU bridge, the trusses were subjected to shear forces from the weight of the deck, the traffic on the road, and the wind.

To prevent the trusses from sliding or cracking, the design included several features:

The trusses had diagonal members that formed triangles to provide rigidity and stability.
The trusses had vertical post-tensioning bars that ran through the nodes, which were the points where the members met. These bars were tightened to create a compressive force that held the members together.
The trusses had horizontal post-tensioning bars that ran along the bottom chord, which was the lower edge of the truss. These bars were also tightened to create a compressive force that balanced the tensile force from the weight of the deck.
The trusses had steel reinforcing rods that were embedded in the concrete and connected to the deck. These rods provided additional strength and stiffness to the truss-deck interface.

Another challenge was how to fabricate and erect the bridge using ABC methods. The bridge was divided into several segments that were built off-site and then transported and assembled on-site. This required careful coordination and quality control among the different parties involved in the project.

The fabrication of the bridge segments was done by STVSL at its facility in Tampa, Florida. The segments were made of steel-reinforced concrete that was cast in steel forms. The segments were then cured, inspected, and tested for strength and durability.

The erection of the bridge segments was done by MCM at its site near FIU. The segments were delivered by trucks and then lifted and placed on temporary or permanent supports using cranes or SPMTs. The segments were then aligned, connected, and post-tensioned according to the design specifications.

Reports of Cracking

On March 13, 2018, two days before the collapse, an engineer from FIGG sent an email to officials from FIU and FDOT (Florida Department of Transportation) reporting that there was “some cracking” observed at node 11 of the north end of the main span. Node 11 was one of the critical nodes where several members of the truss met.

The email included a photo of a large crack at node 11 that extended from the bottom chord to the diagonal member. The engineer stated that this crack did not compromise the structural integrity of the bridge and that repairs were needed to address “mostly a serviceability issue”. He also stated that he would conduct further analysis and provide an update.

On March 15, 2018, at 9:00 a.m., a meeting was held at FIU to discuss the cracking issue. The meeting was attended by representatives from FIGG, MCM, FIU, FDOT, Bolton Perez & Associates (BPA), which was a consulting firm hired by FDOT to oversee the project, and Louis Berger Group (LBG), which was an independent peer reviewer hired by FIU to verify the design calculations.

At the meeting, FIGG presented its analysis and proposed solution for node 11. FIGG claimed that node 11 was overstressed due to a design error in calculating its capacity. FIGG also claimed that node 11 was not a safety issue but a durability issue that could be fixed by applying additional post-tensioning force to member 11/12, which was one of the diagonal members connected to node 11.

FIGG’s proposed solution involved tightening four post-tensioning bars inside member 11/12 from both ends using hydraulic jacks. FIGG estimated that this would require applying a force of about 280 kips (thousand pounds-force) per bar, for a total of about 1,120 kips. FIGG also recommended applying epoxy injection to seal the cracks at node 11.

The meeting participants agreed with FIGG’s proposed solution and decided to proceed with it later that day. They also decided not to close Southwest 8th Street during the operation, as they did not consider it a safety risk.

Collapse

At around 1:30 p.m., workers from MCM and STVSL began to perform post-tensioning on member 11/12. They used two hydraulic jacks: one on each end of member 11/12. They also used two pressure gauges: one on each jack. They followed a procedure provided by FIGG that instructed them to apply pressure gradually until reaching a target value of about 280 kips per bar.

However, as they applied pressure, they encountered some problems:

The pressure gauges did not match each other or with FIGG’s calculations. One gauge showed higher values than expected, while another gauge showed lower values than expected.
The hydraulic jacks did not move smoothly or evenly. One jack moved faster than another jack, causing uneven distribution of force along member 11/12.
The cracks at node 11 did not close or reduce as expected. Instead, they widened and lengthened as more pressure was applied.
The workers heard loud noises coming from the bridge, such as pops, bangs, and cracks. They also saw concrete spalling and falling from the trusses.

At around 1:47 p.m., after about 15 minutes of post-tensioning, the bridge suddenly collapsed. The main span fell onto the highway, crushing eight vehicles and their occupants. The canopy span remained standing but tilted to the south.

The collapse was captured by several video cameras and witnesses. One of the videos was recorded by a construction pickup truck that was approaching the bridge from the east¹. The video showed the initial failure at node 11, followed by the rapid collapse of the entire main span.

NTSB Findings

The NTSB issued its final report on October 22, 201⁹⁴. The report identified several probable causes for the collapse of the bridge, as well as several contributing factors. Some of the main findings were:

The design of the bridge was nonredundant, meaning that it had only one load path and no alternative way to support the load in case of failure.
The design of the bridge had errors in calculating the load and capacity of the nodal region 11/12, which was a critical connection point between the truss and the deck. The design underestimated the load and overestimated the capacity of this region, resulting in a structural deficiency.
The design of the bridge did not account for the effects of post-tensioning on the nodal region 11/12, which increased the stress and strain on this region and caused cracking.
The cracking observed at node 11 before the collapse was an indicator of structural failure, not a serviceability issue. The cracking compromised the integrity and stability of the bridge and reduced its load-bearing capacity.
The post-tensioning operation performed on member 11/12 on the day of the collapse was a flawed attempt to close the cracks and restore the capacity of node 11. The operation increased the stress and strain on node 11 and triggered its failure.
The peer review performed by LBG was inadequate and failed to detect the errors in FIGG’s design calculations. LBG did not review all the relevant documents, did not use independent software, and did not verify all the load factors and safety margins.
The communication and coordination among the parties involved in the project were ineffective and insufficient. There was no clear documentation or agreement on roles and responsibilities, decision making, quality control, inspection, testing, and reporting.
The decision to keep Southwest 8th Street open during the post-tensioning operation was a result of a failure to recognize and act upon the risk posed by the cracking. None of the parties involved took appropriate action to close the road or stop the work when the cracking reached unacceptable levels.

OSHA Findings

OSHA issued its final report on June 11, 2019. The report cited several violations by MCM, STVSL, FIGG, BPA, FIU, and FDOT, and proposed penalties totaling $86,658. Some of the main findings were:

MCM and STVSL exposed employees to crushing hazards by allowing them to work under a bridge that had multiple cracks without adequate precautions or engineering controls.
MCM failed to provide adequate training for employees on how to recognize and avoid hazards associated with post-tensioning operations.
MCM failed to ensure that employees wore hard hats when working under or near overhead loads or structures.
STVSL failed to ensure that employees used fall protection systems when working at heights above six feet.
FIGG failed to perform adequate engineering analysis and oversight for its design of node 11/12 and its post-tensioning operation on member 11/12.
BPA failed to perform adequate construction engineering and inspection services for node 11/12 and its post-tensioning operation on member 11/12.
FIU failed to exercise proper authority over its contractors and subcontractors regarding safety issues related to node 11/12 and its post-tensioning operation on member 11/12.
FDOT failed to exercise proper authority over its grant recipient (FIU) and its contractors and subcontractors regarding safety issues related to node 11/12 and its post-tensioning operation on member 11/12.

Lessons Learned

The collapse of the FIU bridge was a preventable tragedy that resulted from multiple errors, oversights, and failures by various parties involved in the project. The collapse also revealed some systemic issues and gaps in the design-build process, especially for innovative projects that use ABC methods.

Some of the lessons learned from this incident are:

Designers should ensure that their designs are redundant, robust, reliable, and resilient. They should also perform rigorous analysis and verification of their designs using appropriate software tools and standards.
Designers should account for all possible loading scenarios and conditions during construction and service. They should also consider how different construction methods and sequences may affect their designs.
Designers should communicate clearly and effectively with contractors, engineers, inspectors, owners, and other stakeholders about their designs. They should also provide adequate documentation and guidance for their designs.
Contractors should ensure that they follow the design specifications and instructions accurately and consistently. They should also perform adequate quality control, inspection, and testing of their work.
Engineers and inspectors should ensure that they review and verify the design calculations and assumptions thoroughly and independently. They should also use appropriate software tools and standards to check the design validity and reliability.
Owners and oversight agencies should ensure that they exercise proper authority and responsibility over their contractors and subcontractors regarding safety issues. They should also establish clear and effective communication and coordination mechanisms among all parties involved in the project.
All parties involved in the project should ensure that they monitor and report any anomalies or defects observed during construction and service. They should also evaluate and address any potential risks or hazards posed by these anomalies or defects promptly and adequately.
All parties involved in the project should ensure that they prioritize public safety above all other considerations. They should also take appropriate action to close or restrict access to any areas that may pose a danger to the public.

Conclusion

The FIU pedestrian bridge was a tragic example of how a combination of errors, oversights, and failures can lead to a catastrophic outcome. The bridge collapse claimed six lives, injured ten others, and caused significant damage and disruption. The bridge collapse also raised questions and concerns about the design-build process, especially for innovative projects that use ABC methods.

The investigations into the bridge collapse revealed several causes and factors that contributed to the incident, as well as several lessons learned that can help prevent similar incidents in the future. The investigations also resulted in several recommendations that aim to improve the safety and quality of bridge design, construction, inspection, and oversight.

The FIU pedestrian bridge collapse was a wake-up call for the bridge engineering community and the public. It showed that even with advanced technology and methods, bridges are not immune to failure. It also showed that bridges require constant vigilance and care from all parties involved in their life cycle.

The FIU pedestrian bridge collapse was a reminder of the importance of bridge safety and the responsibility of bridge engineers. It was also a reminder of the value of human life and the impact of bridge engineering on society.

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