Why did a newly built bridge collapse? The keys to the Miami accident investigation
Rescuers were still going through the rubble Saturday to extract victims two days after a pedestrian bridge under construction collapsed in Miami killing six people in their cars.
Authorities have already begun investigating what caused the collapse of the 950-ton bridge that linked Florida International University (FIU) with the city of Sweetwater, in western Miami.
Investigators will try to determine both the cause of the accident and those responsible, a process that Michael Culmo, a bridge expert, estimates could last up to a year.
Who’s responsibility is it?
There are three principle entities involved: FIU is the supervisor of the $14.2 million project and two companies were contracted to carry it out: the designer, FIGG Bridge Group, and the construction firm Munilla Construction Management (MCM).
Two other companies commissioned to install the bridge last Saturday, Barnhart Crane & Rigging and a subcontractor, BDI, ruled out any involvement in the design or construction.
The university claims that it followed all the proper steps, while both MCM and FIGG say they are cooperating with the investigation.
What is known about the companies involved?
The construction of bridges involves very specific skills and technology. FIGG and MCM have extensive experience in this type of work, but the two companies have faced at least two claims over accidents in structures of this type (both non-fatal).
MCM is a family business based in Miami founded by a Cuban-American, Jorge Munilla, who sits on FIU's board of directors and has bought enormous influence in local politics thanks to the company's generous contributions to political campaigns at the local, state and federal levels.
The company has participated in numerous projects in the United States and Latin America, including the rehabilitation of the emblematic Bridge of the Americas on the Panama Canal. According to The Miami Herald newspaper, the family that created the company previously worked on the construction of Havana's waterfront, before the 1959 Cuban revolution.
MCM, which has about 500 employees, is currently building a military school at the U.S. naval base at Guantanamo in Cuba.
Based in Tallahassee, FIGG has worked on more than 230 bridges in the United States and is an expert in suspension bridges such as the Leonard P. Zakim Bunker Hill in Boston and the Sunshine Skyway in Tampa Bay. In addition, it has designed about 35 miles of similar structures that "have endured multiple hurricanes," the company told Univision News in a statement.
What exactly is an "instant bridge"?
The bridge was built using a relatively new 'Accelerated Bridge Construction' (ABC) pre-fabricated method, which is taught at the Civil and Environmental Engineering department. According to the university, the modular construction method reduces the potential risks for workers, travelers and pedestrians and has become key to resolving the nation's deteriorating infrastructure.
According to Culmo, an accelerated bridge expert at CME Associates in Connecticut, the technology has been in use for at least 15 years and is common in countries such as the Netherlands and . He said well over 5,000 accelerated structures have already been built, with dozens being completed each year in the United States.
However, he described the Miami pedestrian walkway as "unique and very sophisticated" because of its heavy, two-tier concrete design.
What could have caused the collapse?
Determining this is the job of the NTSB (National Safety Transportation Board) and results are unlikely to be known for at least a year. But there are some immediate clues. The bridge suddenly bent and collapsed about 20 feet from the northern end, according to a bystander's video. The lead engineer reproted a crack in that same area a day before the collapse, though he did not think it posed a major safety risk.
The NTSB investigators will look at the design specifications, materials and worksite tests. For example, every batch of concrete is tested during construction.
Experts consulted by Univision News say that there may be two main causes: a design or material failure (concrete, cables, and bolts) or a sum of the above factors. "Way too early to tell," says Culmo. "It's forensic investigating. Sometimes there is an obvious answer. But most collapses result from multiple problems,” he added.
The NTSB is asking the public to share any videos of the accident that could help determine the cause of the collapse.
What is a stress test, and could it be the cause of the accident?
Local officials pointed out that the day of the accident was carrying out a "stress test" of suspension cables, but what were they referring to?
A stress test usually occurs at the end of construction before it is opened to the public in order to make sure the structure is safe. In this case, the bridge was still under constructrion and was not due to open to pedestrians until 2019. Suspension cables and a 110-foot pylon near the center of the bridge had not been installed yet. So it’s more likely that workers were testing steel cables, or ‘tendons’ in ducts running through the 30-foot-wide, 174-foot long concrete span that fell.
These can be tightened using a jack to create greater tension, in order to strengthen the bridge.
Experts consulted by Univision News said it depended on what type of test was being carried out that day. The test may have nothing to do with the collapse, although the timing raises suspicion.
Should the road have been closed for the test?
The concept of accelerated bridge construction is designed to limit the number of days highways have to be closed to traffic. This can limit road closures to one day only instead of 7-8 days. “Accelerated bridge construction has tremendous benefits for the public and saves lives every day,” said Culmo, noting the frequent workplace accidents that can occur during construction.
SW 8th St, also known as the Tamiami Trail is a busy highway running west to east across the entire width of Miami Dade County from the coast to the Everglades, connecting downtown Miami to the Florida Turnpike. While it would be unusual to conduct a full stress test with cars ing, “it’s not unusual” to conduct a less rigorous stressing of the steel tendons with traffic running underneath, said Culmo.
The Florida Department of Transportation (FDOT) said Friday that it never received a request to close the road on Thursday and was unaware of any scheduled stress testing of the bridge.
“Per standard safety procedure, FDOT would issue a permit for partial or full road closure if deemed necessary and requested by the FIU design build team or FIU contracted construction inspector for structural testing,” the state said.
Can those responsible be subject to criminal prosecution?
That is unlikely. The bottom line to determining whether or not criminal charges can be filed is whether there was negligence or culpable negligence. “If there is proof that there was culpable negligence, then manslaughter charges could be filed,” according to former state and federal prosecutor, David Weinstein, with the law firm Hinshaw & Culbertson.
Culpable negligence is reckless disregard for human life or a grossly careless disregard for the safety and welfare of the public. Simple negligence is the failure to use reasonable care and would not result in criminal charges.
Even if culpable negligence exists, it will not be an easy case for the state to prove. “Proof beyond reasonable doubt is a high burden and the interplay of manufacturing defects, design flaws, weather and other conditions will play into the equation,” said Weinstein.
On the civil side, FIU is a public institution so it enjoys limited immunity. There is a cap on the claims of either $200,00 or $300,00 depending on the number of claims. Any claims in excess of those amounts have to be approved and paid out by the legislature.