It started as an idea for a footbridge to get college students safely over the busy Tamiami Trail.
But fueled by millions of dollars in available federal stimulus grants, Florida International University's doomed pedestrian bridge morphed into something far more ambitious: A gleaming testament to FIU's lofty institutional aspirations and the linchpin in a grand plan to create a true college town in the neighboring working-class suburb of Sweetwater.
As FIU's ambitions grew, the need to shape a uniquely memorable bridge drove key engineering decisions, resulting in a striking but unorthodox concrete structure. But the design hid a fatal flaw that its designers and reviewers failed to recognize, according to experts who have examined plans and mathematical calculations for the project.
The unconventional placement of diagonal supports in an uneven zig-zag pattern along the bridge produced a complex structural web with a glaring weakness at a key connection point, apparently overlooked by designers at FIGG Bridge Group, say three independent structural engineers who reviewed nearly 2,000 pages of calculations for the bridge at the Miami Herald's request.
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The weakness was likely a key factor in its fatal March 15 collapse, the independent engineers told the Herald.
The engineers believe FIGG significantly misjudged what would happen when a large amount of structural stress was placed on a single diagonal strut — a concrete brace that helped support the bridge — at the north end of the 174-foot span. That resulted in an undernourished strut and anchor that could not adequately bear the weight of the bridge and the substantial forces placed on them when the span was lifted into place over two support piers, the outside experts said. That joint is precisely where the bridge appears to have failed — and where potentially worrisome cracks began to appear even before the bridge was installed over the eight-lane road on March 10.
As a consequence of the apparent design error, the diagonal support at the span's north end was so overloaded that additional stress put on it by construction crews tightening internal support rods on March 15 likely caused it to separate from the walkway deck, instantly sending the entire 950-ton span crashing to the roadway in a chain reaction of structural failure, the engineers said. The accident killed one construction worker and five people sitting in cars at a stoplight below, including an FIU student, 18-year-old Alexa Duran.
"The tensioning work could have pushed it over the edge," said Linwood Howell, an Austin-based engineer who is contracted to inspect bridges for the state of Texas.
FIU and FIGG, which say they have been told by federal authorities not to discuss the collapse of the bridge in detail for the time being, said the National Transportation Safety Board's investigation must run its course before conclusions can be drawn. The engineers consulted by the Herald cautioned that the apparent design error may not be the sole cause of the collapse.
The span was an unusual re-interpretation of a traditional truss bridge, which is supported in part by a series of interconnected upright triangles. But FIU's bridge was designed to mimic the dramatic look of a cable-stayed bridge, where the deck is suspended from cables fanning out from a tall mast. In FIGG's design, though, the "cables" — actually metal pipes — were mostly just for show. The diagonal, v-shaped struts of the truss did the structural work.
To carry through the cable-stayed look, the struts had to line up with the pipes from the mast, resulting in a highly irregular arrangement that the experts say may have made the critical defect harder to detect than it might have been in a conventional truss design.
"It made the trusses more complicated," said David Beck, a New Hampshire engineer who helped the Boston Globe uncover mistakes in Boston's $10.8 billion Big Dig project. "If you were not trying to go ahead and conform to the aesthetics of the 10 cables ... the geometry on the trusses would have been more structurally efficient."
Crucially, had the strut and its connection to the walkway deck been stouter, the engineers say, the unusual bridge likely would have held up just fine. But FIGG's engineers appear not to have sufficiently considered potential vulnerabilities in their novel design, the experts concluded.
The Herald obtained the structural calculations and design plans through a public records request and shared them with the engineers. Howell and Beck, who both have expertise in bridge design and structural engineering, analyzed the plans and calculations independently but came to similar conclusions.
A third structural engineer with similar qualifications, Ralph Verrastro, closely examined the bridge plans but did not review the calculations and said he thus can't say whether there was a design weakness in the bridge. But he said the bridge design led to atypical details and unusual complications in the bridge move that could have played into the collapse.
"They were out on an edge," Verrastro said.
A fourth engineer, also a bridge expert who examined the FIGG calculations in detail, arrived at conclusions similar to those of Beck and Howell. The engineer asked not to be named.
The Herald did not compensate any of the engineers for their time or work, but flew Beck to Miami for a day to consult with reporters.
Only the NTSB can give an official account of why the bridge fell. But the federal agency's inquiry could take between one and two years from the date of the accident. Any conclusions drawn before then by outside experts do not serve the public's interest, the NTSB says.
"There is one agency tasked with conducting a comprehensive, independent and objective investigation into this bridge collapse and that’s the NTSB," Chris O'Neil, the agency's spokesman, said in an interview. "As qualified as other experts may be, if they are drawing conclusions about this accident, it is speculation at best. Unless you’re part of the investigative team, you may not be privy to all the information."
That information is no longer accessible to the public, however. The NTSB has severely restricted access to records related to the accident, leading the Miami Herald to file a lawsuit demanding access to documents that were previously available for public review under Florida public records laws.
All the engineers consulted by the Herald said their initial findings could change with further information gathered by the NTSB.
Even so — given the available evidence — the design for the bridge does not appear "structurally logical," Beck said.
While the singular design was unorthodox, it did fulfill FIU's aesthetic vision.
When two competing teams presented their plans for the bridge in 2015, only one struck Tom Gustafson, an FIU administrator and former speaker of the Florida House, as matching the grandeur sought by the university.
The proposal, by Munilla Construction Management, a politically connected South Florida construction company that has won local, state and federal contracts, and FIGG, a renowned bridge-design firm from Tallahassee, called for a sturdy concrete structure bedecked with a pylon tower for 10 imitation cables that could be dramatically lit at night. It also included amenities such as WiFi, planters, benches — even vendors selling FIU gear.
The rival plan, said Gustafson, a member of the selection committee, was simply conventional and banal.
"[The other design] is not a place I’m going to want to be. It’s not a place," Gustafson said, according to an audio recording of the meeting. "It’s a 12-foot-wide sidewalk across a busy highway and I don’t think that’s what we wanted."
He did not respond to a request for comment.
A simpler bridge — like the $6 million steel walkway constructed over South Dixie Highway at the University Metrorail Station last year— would have been easier to design and build. The FIU bridge, made of more visually pleasing and longer-lasting concrete, cost $14.3 million and weighed nearly 10 times as much as the South Dixie walkway, built by Miami-Dade County and the Florida Department of Transportation at the request of the University of Miami after eight students were killed or injured while trying to cross the road. FIU's consulting planners had at first envisioned something more similar to the South Dixie project.
Kenneth Jessell, chief financial officer for FIU, disputed the suggestion that the bridge's complexity played a role in its failure.
"Simplicity in design is not synonymous with safety, just as innovative design cannot be equated with lack of safety," he said in a statement.
Ron Sachs, a spokesman for FIGG, provided the following statement: "It is a breach of widely held professional standards of ethics for any engineer to judge or speculate on any aspect of a construction accident unless they have complete knowledge of all the facts, which include construction, materials, design, and other factors, and are highly experienced in bridge design."
Arthur Schwartz, deputy director and general counsel for the National Society of Professional Engineers, said independent engineers often use their expertise to shed light on tragedies such as Hurricane Harvey and the I-35 bridge collapse in Minnesota.
"I’m not aware of anything in our [ethics] code that says it’s unethical to render an opinion," as long as the engineer has studied the facts and circumstances of the incident and is properly trained, Schwartz said.
MCM, which built the bridge, said it could not respond because of the NTSB prohibition on doing so.
The FIU project had its genesis in a simple need to provide a safe way for students and others to get across the Tamiami Trail from the school's main campus north to Sweetwater's compact main street, Southwest 109th Avenue.
Founded as a commuter school on a former airfield, FIU embarked on a massive building spree in the 1980s as it sought to become a top public university. It added medical and law schools and a football stadium and expanded science and engineering programs until its main campus was bursting at the seams, plagued by traffic logjams and a parking shortage. One solution administrators settled on: Get more students to live on campus or within walking distance.
So the university embraced an increasingly common alternative to dorms, recruiting private developers to build high-rise student housing across the Trail and a narrow canal in Sweetwater, a mostly immigrant city in need of an economic boost. FIU's plan soon expanded in scope to include improvements to Sweetwater's main drag, so its modest downtown could serve as a gathering place for college students. A broad new plaza at the center of campus would connect to Sweetwater through a new pedestrian and bike pathway and a bridge.
The so-called "FIU University-City Prosperity Project" eventually ballooned into a $120 million vision that encompassed far more than just town-and-gown improvements, extending to campus trolleys and a transit station for Miami-Dade transit's planned rapid-bus system in a new FIU parking garage alongside the Trail.
To help finance the Trail bridge and the campus and Sweetwater pedestrian projects, FIU turned to TIGER grants, which funded shovel-ready transportation plans as part of the Obama administration's stimulus package to help the country recover from the economic crash. FIU was awarded $21 million in federal funds for the overall project through FDOT.
Because the bridge was the most visible piece of the University City grand plan, FIU did not seek an ordinary span, but felt it needed some design razzle-dazzle, university documents and brochures show.
Initially, FIU sought a cable-stayed bridge — that is, a bridge supported by cables hung from a tall pylon — and drew up renderings, according to its TIGER grant applications and other documents.
But FIU and its planning consultants, T.Y. Lin International, a global design and engineering firm, then settled on a different alternative: a time-tested truss structure, a common design consisting of a deck with open, V-shaped vertical supports on each side tied together at the top in a structural web.
Such structures are generally sturdy and reliable because they're "redundant" — meaning that failure of a single strut on one side would not bring down the entire bridge because the corresponding piece on the opposite side would still provide support. In creative designers' hands, truss bridges can also lend themselves to some flashy variations.
Something else was also likely driving FIU's change of heart about the type of bridge it wanted: traffic.
As the bridge plan was being formalized, FIU had also begun promoting the use of an increasingly popular approach to bridge construction. In so-called Accelerated Bridge Construction, or ABC, bridge spans are prefabricated, then moved and hoisted into place in a matter of hours to avoid the long road closures so dreaded by motorists, highway authorities and elected officials.
FIU had launched and heavily promoted a center for ABC at its engineering school, and explicitly saw use of the technique for its pedestrian bridge as a demonstration of its efficiency.
Such quick-bridge construction effectively ruled out a true cable-stayed bridge. Cable-supported bridges are built in sections and in place, which requires extended road closures. Truss designs, in contrast, are ideal for the accelerated approach, engineers say.
Bid documents drawn up for the project by T.Y. Lin made it abundantly clear that FIU wanted no ordinary truss bridge. The document outlined a broad scope for the design of the pedestrian bridge, which would serve as a "landmark" for the school and a "gateway" for west Miami-Dade.
"This structure should function as more than just a path for circulation; it should be a place to be and a place to be experienced, and the FIU campus and its students must be proud of it," the document's introduction reads. "It should be a destination in its own right where community members might linger, gather, and create an urban social space — a linear park."
Bid specifications emphasized the message that a conventional approach would not win the contract: "The selection criteria will be weighed heavily towards an innovative design that represents the intentions of this project, creating a distinctive landmark for the region."
The specs called for a broad walkway at least 20 feet wide, but preferably 30 feet, and explicitly invited inventive "hybrid" variations on the truss design. But it also sought to establish some constraints.
The consultants sought a "primarily" steel structure with a concrete walkway. They also directly discouraged use of "non-redundant, fracture critical" designs — meaning designs in which failure of one structural element could lead to a catastrophic collapse. But they left the door open to other designs, saying those would be subject to approval by FIU.
What FIU got in the winning proposal by contractor MCM and FIGG was indeed unconventional. The MCM team easily outscored the other finalist, Facchina, with a design that offered everything FIU wanted, including fast-bridge construction and a structure that looked like — but was not in reality — a cable-stayed bridge.
MCM and FIGG proposed a highly unusual, hybrid version of a truss design. Instead of two sets of parallel, regularly spaced vertical truss pieces running along both edges of the bridge as in a straightforward truss, the bridge would have a single set of irregularly shaped struts running down the center of the bridge, tying together the deck and a shading canopy.
That approach had the advantage of giving the bridge an expansive, open look and feel. It would be built entirely of reinforced concrete, for a cleaner appearance and greater durability and ease of maintenance than plain steel.
But the design also appears to contravene T.Y. Lin's recommendations.
The bridge FIU ended up with was steel-reinforced concrete, and not primarily steel. Concrete can be harder to work with and more unforgiving, as well as substantially heavier than steel. The independent engineers and other outside experts say concrete truss bridges are exceedingly rare, and they can find no other design similar to the FIU bridge anywhere.
The single row of support struts, meanwhile, is precisely the kind of "non-redundant" design T.Y. Lin's specs sought to avoid. On the FIU bridge, the failure of a single strut could — and in fact, did — cause the entire span to fall because there was no backup support for it, the outside engineers say.
"It's only as strong as its weakest link," said Howell, the Texas-based bridge engineering expert who reviewed FIGG's calculations for the Herald.
Making things even more complicated was a decision to mimic the look, but not the function, of a cable-stayed bridge, Beck and Howell said. FIGG, which has said publicly the bridge is a truss design and not cable stayed, added a central pylon with mostly decorative steel pipes, instead of cables, connected to the bridge's canopy. FIGG said the pipes would help dampen vibrations from wind and people walking across.
That decision, the engineers said, determined the irregular size, spacing and angling of the diagonal truss support pieces, which had to line up with the pipes to complete the illusion of a cable-stayed bridge and give the span a dramatic profile visible from a distance, especially when lit up at night. The pylon was designed to be 109 feet tall to mark the location at 109th Avenue, a height that helped set the angles or the pipes and trusses, Beck said.
"The whole design is driven by the aesthetics," Howell said. "Bottom line is, FIGG is trying to be innovative to get the job. That's what they had to do. That's how they won the proposal."
The engineers stress it's not uncommon for projects to put engineering at the service of aesthetics. But that means designers and engineers must then take extra care to make sure they understand the structure's function and look for potential flaws.
In the case of the FIU bridge, though, back-checking their conclusions appears to be where the school's contractors ran into trouble, the engineers say.
The faux cable-stayed bridge design created a highly irregular pattern for the diagonal struts. The irregular pattern, in turn, complicated the calculations for determining the stresses at different points and resulted in each of the 12 pieces being of different length and thickness, the three engineers who undertook a review of FIGG's calculations say.
The designers utilized two commonly used computer models to analyze stresses on their bridge design to determine the strength required for each structural piece. Beck and Howell, who say they did their own simpler calculations to back-check the numbers used by FIGG, concluded the figures in the firm's calculations significantly under-represent the forces working on the No. 11 truss piece, the last diagonal strut on the bridge's north end.
The fourth engineer, who has asked not to be named, agreed that the point where the No. 11 strut met the deck was too weak to hold up once the span was installed over the roadway.
That's critical, Beck and Howell said, because the No. 11 strut bore the greatest stress of any of the diagonal pieces in the design in comparison to its strength.
"That configuration put heavy stress on one member, but for whatever reason, [the designers] did not realize it was underdesigned for the stress," Beck said.
"It's sitting there stressed to the limit," Howell said.
That resulted in a support piece that was likely not thick enough, and even more significantly, lacked sufficient steel rebar reinforcements at the point where it connected to the deck, all three engineers who reviewed the calculations said. That meant it could not adequately withstand the load of the bridge weight it was meant to carry, nor resist "shearing" stress at the connection to the deck that tended to pull the base of the diagonal strut from its anchor, they said.
By comparison, the diagonal piece at the opposite end of the span, No. 2, was a foot thicker and carried less structural stress, Beck and Howell pointed out.
Because the calculations don't fully show how FIGG derived its numbers, Beck and Howell said it's hard to say how the mistake may have been made. But they suggested that over-reliance on the computer models, a common pitfall in the profession, may have played into it.
It's difficult to tell who was responsible for the bridge designs and any possible errors, the three engineers who examined the calculations said. The plans are stamped by FIGG's chief engineer, W. Denney Pate, a lauded designer of bridges around the country and the engineer of record for the bridge.
But those three independent engineers say it's unclear how involved Pate was in design or oversight of the FIU bridge. Notes in the plans bear the notation "M.F.," the initials of a senior bridge engineer at FIGG. The engineers consulted by the Herald say FIGG would probably also have had a team of junior engineers working on it.
It's also unknown how thorough a check was conducted by Louis Berger, a large engineering firm hired to review the design. Both FIU and FDOT, in response to a public records request from the Herald, say they can't find relevant documents from Berger, other than a brief letter certifying the design plans. They were unable to locate even the contract laying out how much Berger was paid. The contract would probably also specify how thorough Berger's review of the design should be and how many hours its engineers should spend on the task.
"Louis Berger continues to review this matter and, at this time, cannot provide additional information as it is part of a government investigation and litigation," a spokeswoman for the firm said in an email.
Such peer reviews can be superficial. But the three outside engineers who studied the plan in detail said someone should have caught an error that Beck and Howell characterized as clear. FDOT and U.S. Department of Transportation engineers also reviewed the plans, emails released in response to a request by the Herald show.
"I would not accept these calculations. It really comes down to some kind of design error in this page," Howell said, referring to a place in the calculations focusing on the area of the No. 11 strut. "There is no way this connection is OK. I'm thinking this was not really checked. You want to do simple checks on your sophisticated program to make sure it's reasonable."
What appears certain, the three engineers who reviewed the calculations say, is that more concrete and more rebar pieces at the deck connection, and perhaps a thicker depth on the No. 11 truss piece, would have prevented catastrophe.
"They could properly design that pattern," Howell said, referring to the irregular structural design of the bridge. "There is nothing particularly difficult about it."
"If they had made member 11 three feet deep instead of two feet deep, we wouldn't be sitting here today," said Beck, as he conducted a review of the plans and calculations for reporters. "It wouldn't have failed, based on what I have seen so far."
All four engineers say other factors could have played a role in the bridge's failure, including last-minute design changes or defects in materials and workmanship.
The bridge was made of a novel titanium dioxide concrete mix, designed to be self-cleaning. Beck said the concrete at the point where the bridge failed shattered into unusually small pieces, suggesting it was highly brittle, though he can't say whether that had anything to with the collapse. Research into so-called Ti02 concrete shows if too much titanium dioxide is added into the mix, the concrete loses strength. FIU said its bridge was the first in the world made entirely of a Ti02 mix.
Other decisions by FIGG and MCM may have played into the collapse, the three engineers who looked at the calculations said. The installed span was only the main portion of the bridge, which would not have opened until next year. A shorter span connecting the north end across the canal to Sweetwater was to be built in place once the main span was up.
Because the back span would be structurally tied into the main span, the bridge would have been significantly stronger had the shorter portion been in place, they said. That could have ameliorated or even prevented the failure of the No. 11 piece, they say.
What the engineers say they still don't know is precisely how the No. 11 strut failed. That determination may have to await a report by the NTSB. The agency has barred the release of public records related to the bridge dated after Feb. 19.
The secrecy has precluded public scrutiny of some critical decisions by FIU's project team, including how they decided to handle cracks that had appeared on the bridge's north end before it collapsed, or what they thought had caused the cracking.
The cracks at the base of the No. 11 piece, which have drawn substantial public attention, could have been a sign that something within the structure was amiss — another critical element that the bridge designers and engineers on FIU's contracting team may have failed to fully account for, the three independent engineers who looked at the calculations say.
Cracks in concrete are common and often minor. But given the unconventional design of FIU's bridge, that should at the very least have prompted work to stop and an in-depth diagnosis to be conducted, including a fresh look at the structural calculations that should have uncovered any hidden deficiencies, those engineers said.
But records released by FDOT before the NTSB issued its ban suggest FIGG engineers saw little reason to undertake a full reassessment, something that would have taken days if not weeks, the independent engineers note. FIGG's principal engineer, Pate, left a voicemail message for an FDOT official two days before the collapse to report that cracking had appeared, but added it did not appear to pose a safety hazard.
If the FIU team relied just on Pate's word, and not a thorough analysis, that was a serious error, Howell said.
"You don't accept a verbal reassurance," he said. "You want a report."
The three engineers who examined the plan calculations — as well as Verrastro, an ABC expert — conjecture that the move of the main span into position over the Trail may have been a contributing factor in the collapse. Because the bridge was lifted into place by two special transporters set toward the center, the ends of the span — designed to rest on pylons — would tend to sag under their own weight while being moved, the engineers say.
To counteract that sag, engineers added steel support rods into the two end diagonal pieces, No., 2 and No. 11. Documents show that, as planned, those rods were tensioned before the move to provide added support to the bridge ends while up in the air. They were then de-tensioned once the span was resting on the pylons, because the added support was not needed at that point.
Beck and Howell said they believe the underdesigned No. 11 node was possibly damaged by twisting and bending forces during the move, making it even more vulnerable to failure. Engineering plans for the move assumed that stresses on No. 11 had been properly calculated, and sensors placed along the span during the move would have warned FIU's team if those exceeded expected parameters.
Thus, the method of accelerated bridge construction, initially scrutinized as a possible contributor, was by itself likely not a probable cause of the accident, the engineers concur.
"The problem was not ABC, it was the execution," Beck said.
The NTSB recently confirmed for the first time that workers were tightening the No. 2 and No. 11 support rods when the bridge fell. The agency's brief report did not say why they were doing so. But the outside engineers say it may have been an attempt to close up cracks at the No. 11 connecting point.
The fact that workers were atop the bridge canopy in a possible effort to repair a defect while traffic continued to flow below strongly suggests that FIGG engineers remained unaware of the critical structural flaw at the No. 11 support, the engineers said.
At least one said that if so, it constitutes an avoidable mistake on a bridge that should never have been so complicated.
“This is not a big project," Beck said. "It’s a darn pedestrian bridge.”
David Beck is a New Hampshire-based structural engineer and construction manager with more than 40 years of national and international experience. He has planned, designed and managed major civil construction projects including bridges, power plants, offshore oil platforms, wastewater treatment facilities, oceanic outfalls, water and rail tunnels, and large urban traffic projects.
Linwood Howell is a senior engineer specializing in bridge engineering at XR Structural in Austin, Texas. The firm has been approved by the Texas Department of Transportation to inspect bridges in the state for over 30 years. Howell has inspected more than 20,000 bridges, designed more than 30 and has provided engineering services for bridge construction to dozens of contractors. Howell has expertise with truss bridges, having inspected and performed load ratings on nearly all the approximately 100 truss bridges on public roads in Texas.
Ralph Verrastro graduated with a BS in civil engineering from Cornell University in 1976. His career includes bridge design experience throughout the United States and he is a registered professional engineer in 37 states. Verrastro specializes in the design, inspection, evaluation, technical supervision, and quality assurance/quality control for new and rehabilitation bridge projects. He is a technical expert in the use of fast track repair/replacement methods using prefabricated bridge components also known as Accelerated Bridge Construction. He has extensive experience in the evaluation and repair of historic metal truss bridges and concrete arch bridges. He has served as the specialty structural engineer for over 500 bridge structures throughout the United States. In 2018, he was named "Engineer of the Year" by the Florida Engineering Society.
A fourth engineer who reviewed plans and calculations asked not to be named.
This story was corrected to reflect that the new pedestrian bridge over South Dixie Highway at the University Metrorail Station was built by Miami-Dade County and Florida Department of Transportation.