MIAMI _ 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 zigzag 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.
The weakness was likely a key factor in its fatal March 15 collapse, the independent engineers told the Miami 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.
"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 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 Wi-Fi, 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 by the University of Miami 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 UM's. FIU's consulting planners had at first envisioned something more similar to the UM 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.
Survivors of the collapse and families of the dead have filed lawsuits alleging wrongful death and negligence. Their attorneys have also criticized the NTSB for denying them access to documents.
