The Florida International University (FIU)-Sweetwater University City pedestrian bridge was planned to connect the FIU campus to student housing neighborhoods in Sweetwater. It was intended to improve pedestrian safety, as the crosswalks at this wide, busy intersection had been a safety hazard. The project was funded with a $19.4 million Transportation Investment Generating Economic Recovery (TIGER) grant from the United States Department of Transportation (USDOT) in 2013. The bridge cost $9 million to construct, not including the installation cost. It was meant to last more than 100 years and to withstand a Category 5 hurricane. But on March 15, 2018, a 175-foot-long section collapsed onto the multi-lane Tamiami Trail, crushing eight motorists under 950 tons of concrete and metal. Six people died; ten were injured. 

The bridge’s main span was assembled adjacent to the highway using accelerated bridge construction (ABC), for which FIU was heralded.  It was lifted into place on the morning of March 10, 2018, just five days before the collapse.

A post-tensioned concrete structure, the bridge was about ten times heavier than an equivalent steel-designed bridge. A new formulation concrete that was used was intended to stay cleaner than standard concrete formulations. The bridge was styled to look like a cable-stayed bridge, with a pylon tower and high cables for dramatic effect, but functionally and structurally, it was actually a truss bridge, with the spans being fully self-supporting. The bridge spans used a novel concrete truss design invented for this project—a “re-invented I-beam concept.” Concrete truss bridges are rarely constructed, and few exist.  

The concrete walkway deck was to act as the horizontal bottom flange of a wide I-beam, and the concrete roof canopy was to function as the horizontal top flange of the I-beam. The deck was to carry the entire weight of the bridge span, as a tension load. The canopy was to carry the structure’s main compressive loads. The diagonal struts would carry either compression or tension forces, depending upon their angle and position.

The bridge was to cross both a major roadway and a parallel water canal, with two separate spans connected at a faux cable-stayed tower. The main roadway-crossing span was 175 feet long, and the shorter canal span was to be 99 feet long. An elevator and stairs at the south end added 31 feet, and at the north end, 15 feet, for a total bridge length of 320 feet. 

The doomed bridge, the linchpin of a broader “prosperity” project, was meant to help the working-class town of Sweetwater improve its tiny downtown by tying it into the school’s sprawling main campus, while making it safer for students and others to get across the busy, multi-lane roadway. But the university also wanted an iconic design to serve as a new campus gateway.  

For aesthetics, ease of maintenance, and durability, the design of the bridge called for steel-reinforced concrete instead of a more common, simpler-to-erect, steel span. The main companies behind the construction project were MCM, a Miami-based construction management firm, and FIGG Bridge Engineers, a renowned Tallahassee-based engineering firm. Unlike most bridges in Florida, the design for this project was overseen by the university, not the Florida Department of Transportation (FDOT). 

Plans called for the bridge to be completed by construction of a shorter back span connecting over a canal that runs along the Trail to Sweetwater. That back span would have provided additional support and strength to the entire bridge, uniting separate components into one self-supporting structure that required no columns or supports in the roadway, only at the ends. The plan also required one unusual element: steel rods placed inside the last two horizontal support trusses at either end. Those were inserted to support the bridge only while the span was being transported. The rods were tightened before the span was lifted in the air to prevent sagging at each end. 

Once the bridge span was resting on the pylons and supported by its own weight, the rods were “de-stressed” because their support was no longer needed. That element would prove key in design errors and the bridge’s failure.

The span was elevated into place five days before its catastrophic collapse on March 15, 2018. During that time, workers documented growing cracks on the north end of the bridge, at a node where a single plane of load-bearing concrete trusses began. The cracks were 40 times larger than the maximum “acceptable” in a reinforced concrete bridge. Yet, none of the parties involved in the construction recognized the gravity of the problem. At the time the bridge fell, workers were attempting to stop the cracking by tightening the tension rods of one of the trusses.

FIU is known for its expertise in accelerated bridge construction (ABC) and has attracted international scholars as doctoral students. It is home to the federally-funded Accelerated Bridge Construction University Transportation Center. However, with respect to the pedestrian bridge, FIU had no professional engineers on its staff and relied solely on the expertise of its hired contractors. 

The first sign that something was terribly wrong came on February 24, 2018, nearly three weeks before the bridge collapsed. The span was still resting on temporary supports just off the ground. As crews stripped off the framing pieces used to shape and support the concrete structure, a loud, sharp cracking sound rang out, and startled workers ran out from under the span. FIGG, which was ultimately responsible for ensuring the bridge’s structural soundness, still gave the green light to keep working.

The cracks were significant—a symptom of a hidden, catastrophic weakness in a key structural connection at the bridge’s north end. As cracks spread and expanded and a chunk of concrete split off a support column, no one acted to exercise independent judgment or oversight.  

On March 13, 2018, two days before the collapse, the project’s lead engineer discovered cracks at the north end of the span and reported it via voicemail to a FDOT employee as “not being an immediate safety issue.” The FDOT recipient was away and did not hear the message until the day after the collapse.

At 9 am on March 15, 2018, a university employee heard a loud “whip cracking” sound while stopped at a red traffic light under the bridge span. At that very same time, the design-build team was in a meeting at the construction site to discuss the cracks. The FIGG lead engineer concluded that the structural integrity of the bridge was not compromised by the crack and that there were no safety concerns. 

At 1:47 pm on March 15, the north end of the installed bridge span sagged deeply as the first diagonal fractured, folded, and immediately dropped the heavy full span onto the roadway below. As the post-tensioning operation was being carried out on bridge diagonal member support #11, the bottom joint “explosively” failed, and the bridge collapsed. 

Had cracks previously deemed as “insignificant” have been the cause of the collapse of this “iconically-designed” bridge that was meant to withstand a Category 5 hurricane?  Or had something else gone wrong? 

The Investigation and Causal Factors of the Collapse

Investigations by NTSB, OSHA, FHA, and FDOT identified 13 causal factors of the disaster:

1. Faulty bridge design by FIGG. Design flaws left the bridge critically weakened at the crucial point at which it collapsed. Specifically, the problem was the design of the main span truss member 11/12 nodal region and connection to the bridge deck.

2. Structural Cracks. As soon as the bridge had to support its own weight, cracks appeared at the under-designed nodes, particularly node 11/12. Over the next 19 days, the cracks grew until the bridge collapsed. FIGG’s engineer repeatedly insisted that the cracks were not safety concerns.

3. A poorly-conceived attempt to close the cracks. FIGG ordered crews to tighten internal steel support rods in a key diagonal strut at the north end in an attempt to close the cracks. The force caused the damaged, badly-overtaxed connection to abruptly fail.

4. Load capacity calculation errors made by FIGG. FIGG Bridge Engineers severely underestimated the demand on the bridge and significantly overestimated its load capacity. The overestimation was the result of the designer having used incorrect loads and load factors in his calculations.

5. Contract bridge design experts had violated basic FDOT construction requirements. The concrete truss at the center of the investigation had not been built to specifications required by the FDOT.

6. Public safety oversight. All parties failed to close SW 8th Street to protect public safety.

7. Failure to fully account for the shifting forces on the bridge in each stage of construction. FIGG’s plans and calculations lacked sufficient detail to assess the extent to which this variable figured into the collapse.

8. Failure of project leaders to raise “red flags.” Even as cracks grew to proportions that are considered abnormal, no engineer or project leader discussed suspending bridge work or raised any red flags.

9. Organizational dysfunction. “A paralyzing culture of groupthink” prevented the project team from recognizing that the bridge had become a 950-ton deathtrap.

10. Inadequate peer review performed by Louis Berger. The review conducted by Berger 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.

11. Lack of Redundancy. The design of the bridge did not include redundancy—it was not designed so that failure of a single structural element would not bring it down. As a result, when the 11/12 nodal region failed, the bridge collapsed.

12. FIGG’s lack of “due diligence” in selecting Louis Berger to conduct a peer review. The firm was not rated as “qualified” under state rules to review a project like the FIU bridge and had not been so since at least 2013. That didn’t stop a Berger employee from telling FIGG that it was qualified.

13. Failure of FDOT to provide enough oversight of FIU during the bridge construction. The FDOT had delegated responsibility for managing the project to FIU, even though FIU had no professional engineers on its staff and relied solely on the expertise of its hired contractors.

FIGG disputed that its design had been flawed, arguing that the cracks had shown no signs of expanding and had not presented safety concerns. But by the time of the collapse on March 15, the cracks had grown so much that a subcontractor called them “scary,” as the size and depths of the splitting in the concrete were far larger than what is considered normal and acceptable—and by a jaw-dropping margin. 

A veteran engineer who independently analyzed FIGG’s bridge plans and reviewed the NTSB’s report concluded that none of the designers had a full understanding of the structure and mechanics of the bridge’s main span. No one had studied the cracks in a serious way or had the knowledge to understand the danger they presented. 

The After-Effects of the Disaster

On March 19, 2018, the first civil lawsuit was filed against FIGG and MCM.  Two days later, U.S. Transportation Secretary Elaine Chao asked the department’s inspector general to probe whether the federally-funded pedestrian bridge had been constructed in compliance with all USDOT rules. 

The NTSB issued several recommendations to ensure that additional guidance will allow designers to better determine loads; that plans will undergo peer review by a qualified independent firm; that proper action will be taken any time structural cracks are noticed; that uncommon designs will receive appropriate attention from the FDOT to ensure compliance; and that discussions of redundancy will be included in manuals and guidelines, especially in relation to uncommon designs. 

In the meantime, MCM changed its name to Magnum Construction Management and filed for Chapter 11 bankruptcy protection. 

In an October 2019 letter to the community, FIU revealed that it still planned to build a pedestrian bridge over Eighth Street, and that it would include a memorial to the victims of the collapse.