Emergency personnel respond after a brand-new pedestrian bridge collapsed onto a highway at Florida International University in Miami on March 15. The pedestrian bridge collapsed onto the highway crushing multiple vehicles and killing several people. (Pedro Portal/Miami Herald via AP photo)
The deadly collapse of the pedestrian bridge at Miami's Florida International University (FIU) on March 14 is likely to have been caused by a combination of factors, according to bridge and structural engineers.
One thing is certain, however: the answers to why the 950-ton structure pancaked onto a heavily-traveled street on the edge of the campus at FIU may not be fully determined for some time.
Six people were killed and nine more injured when the FIU-Sweetwater UniversityCity Bridge suddenly fell onto SW 8th Street (also known locally as Tamiami Trail) around 1:45 in the afternoon. The bridge was put in place to connect the FIU campus in Miami to the city of Sweetwater and offer a safer alternative for pedestrians to cross the busy, eight-lane road.
The main span of the bridge was erected only the Saturday before the accident.
The bridge was designed to link FIU's new dorms with off-campus housing and mitigate pedestrian-vehicle accidents, such as one last August where an FIU student was killed trying to cross the busy street.
When finished, the bridge would have been 289 ft. long, 109 ft. tall at its center suspension tower and 32 feet wide. It was scheduled to be completed and open in 2019.
It was also designed to withstand a Category 5 hurricane and the deck was being post-tensioned in the longitudinal and transverse directions to give it a durability that, it was assumed, would keep it standing a century or more.
Tragically, it lasted a mere four days.
Investigators Have Their Hands Full
In the wake of the collapse and horrible loss of life, a myriad of questions and controversies have arisen that will surely keep the probe in the public eye for the foreseeable future.
Now, state and federal investigators are among the examiners combing through the bridge debris gathering evidence and seeking to find answers as to why the $14.2 million bridge pancaked onto several cars and trucks underneath it.
Possible reasons for the failure include possible cracks discovered by engineers in one of the bridge's supports just hours before the disaster, as well as a lack of temporary support structures for the heavy concrete bridge while still under construction. In addition, reports have been confirmed that workers had tightened the footbridge's support cables both before and at the time of the accident while the roadway was opened — a fact that has raised eyebrows among bridge experts.
The public and local officials in Miami, along with many engineers, are now questioning why traffic was allowed to continue traveling under the bridge while work was ongoing.
Contractors Under the Microscope
In the hours after the accident, questions arose, too, as to the culpability, as well as the safety records, of the two Florida-based contractors behind the construction of the bridge: Munilla Construction Management (MCM) and FIGG Bridge Group. Both are well-known in the building industry for their successful public-works projects and have built many high-profile building complexes and bridges.
They have also each been cited for a number of safety violations by various state and federal agencies.
Miami-based MCM, which has worked on major projects at Miami International Airport and at PortMiami, has received several fines from the federal government and fought negligence suits over safety issues at its job sites — something most big firms typically face if they have been in business long enough.
One of those suits was filed earlier in March by a workman who fell from a temporary footbridge at a MCM project at the Fort Lauderdale-Hollywood International Airport. The plaintiff alleged shoddy construction of the bridge.
Figg, with headquarters in Tallahassee, designed the iconic Sunshine Skyway Bridge over the south end of Tampa Bay, as well as the replacement to the I-35 bridge in Minneapolis that collapsed into the Mississippi River in 2007.
But, among other infractions, it was cited in Virginia in 2012 after a 90-ton piece of concrete fell from a bridge being built in Norfolk. It was reported by The Virginian-Pilot that FIGG did not receive written permission to modify a girder that ended up causing the fall.
Inevitably, the Miami bridge collapse has brought them each unwanted publicity and scrutiny. Investigators will undoubtedly closely examine each firm's entire bridge-building process at the FIU site.
The contractors have each declined to return calls to CEG for comments about the accident. They have, however, released statements through social media expressing their sorrow for the loss of life and have vowed to provide complete cooperation with the investigation
Who Knew What and When?
Bizarrely, it was disclosed by the Florida Department of Transportation (FDOT) on Friday evening, March 16, that two days before the accident, a FIGG engineer had left a voicemail for an FDOT employee saying that a crack was found in the north portion of the bridge's concrete span. The voicemail was not discovered until the day after the disaster.
And if that was not enough, an ugly war of words (in the form of press releases) broke out between FDOT and FIU over the following weekend as to whether FDOT was informed about the crack in the structure during a meeting at the work site held less than four hours before the accident.
FIU contends that the state transportation agency was informed of the crack's existence by the FIGG engineer, but FDOT later said in a statement that “at no point” was a “life-safety issue” ever communicated to its representative in the meeting at the MCM construction trailer.
Since the collapse, FDOT has steadfastly maintained that its role in the building of the FIU-Sweetwater bridge was purely administrative and there was little input from its consulting engineer, who attended the March 15 meeting.
Florida Gov. Rick Scott also seemed to further distance the agency (run by one of his appointees) from the accident by pointedly telling reporters just hours after the collapse “It was not a FDOT project. It was an FIU project.”
The university countered by saying that the FDOT engineer was, indeed, informed about the crack because that was the entire purpose of calling the March 15 meeting.
Footbridge Erected Rapidly
The main 174-ft. long span of the FIU-Sweetwater structure was made from prefabricated concrete by MCM on temporary shoring along the south side of SW 8th St., before being moved, rotated and lifted into place on concrete supports on each side of the road in just six hours on March 10.
To many, that may seem like an unusually short period of time to erect a 950-ton structure, but in this case, the general contractors did so using a tried-and-true method called Accelerated Bridge Construction (ABC).
Ideally, ABC's advantage is it is a technique that allows bridges to be built or replaced with minimum disruption to traffic.
Typically, like the ill-fated FIU pedestrian bridge, an ABC-built bridge can be moved and placed onto its supports so that the highway or railroad passing underneath need only be closed for a weekend. In contrast, conventional techniques usually replace only half of a bridge at a time, with all traffic redirected under the other half —often for a year or more —while construction progresses.
In the case of the FIU-Sweetwater ABC-built bridge, self-propelled modular transporters (SPMTs) with pull-up gantries lifted the new structure, then rotated it 90 degrees before sliding it onto the permanent supports (SPMTs are essentially huge platform vehicles with several heavy-duty wheels — perfect for transporting objects such as weighty bridge sections).
Indeed, the FIU-Sweetwater structure was the largest pedestrian bridge in United States history to be moved by a SPMT.
ABC Has Long Been in Use
With the accident, the ABC technique is also facing a lot of scrutiny.
But one prominent engineer said the bridge-building method has been used in many places around the country for 30 years. Ralph Verrastro, owner and president of Bridging Solutions LLC in Naples, Fla., is a technical expert in the use of ABC and has been involved in designing, building and erecting hundreds of bridges over his 40 years in the industry.
“Whether they are concrete or steel, these bridge components are prefabricated in a factory or in an off-site location and then brought to the site, where they are installed in segments,” said the Cornell-trained engineer. “Even though the technique has been around for decades, the one element of the process that has really taken off in recent years is the use of SMPTs to transport these components before pushing or sliding them onto supports using hydraulics. I think this big bridge at FIU was brought up to a point and then pushed into place on rollers.”
Multiple Causes Suspected
Currently, forensic engineers are on-site inspecting portions of the collapsed bridge. Professionals from the federal National Transportation Safety Board (NTSB) and the Occupational Safety and Health Administration (OSHA), among other state and local agencies and insurance investigators, are searching for the causes of the accident.
Although he has not been to the accident scene, Verrastro feels certain that, like most structural disasters, more than one cause will be found to have led to the catastrophe.
Verrastro has read several accounts of the disaster. He also has viewed dash-cam video from a vehicle that captured the moment of the footbridge collapse, and his expert eye, coupled with studying blueprints of the structure, have led him to form a few firm hypotheses about the accident.
“I am sure from watching that video that that type of collapse was a result of a failure of one of the truss members, the diagonal member [or component], and it was near the north support,” he said. “From looking at a copy of one of the bridge's drawings, I also saw that some of those diagonals have two post-tensioning strands within them and some have four. If it was one of the members and it only had two strands, a loosening of one of the strands, for whatever reason, would mean that it would only have half the capacity to carry the load.
“Then, if by chance, the second one loosened just a little bit it could have reached a point where the force was too much, and the failure occurred. That is the only way a bridge like that would fail catastrophically: a failure in shear, one that occurred near the one support.”
Varrastro cautioned that if there had been a moment failure due to bending it would have occurred at the center of the bridge, where, on the FIU pedestrian bridge, the center tower and support had not yet been built.
“If there was a failure in bending, the bridge never would have collapsed catastrophically, but would have just sagged and redistributed the load,” he said, adding that everyone from the public to the project engineers would have noticed the sag before the disaster.
He also noted that besides being a shear failure, it was also “non-redundant,” meaning once the one member gave way, nothing could stop the catastrophe. If the contractors had reached the point where they were able to install the cables from the tower of the bridge, the redundancy needed to keep the span standing would have averted the accident, he said.
Are Cracks, Post-Tensioning Linked?
Independent engineers have speculated that the cracks may have led to the adjustment of the post-tensioning cables, which strengthen the concrete against the stress of being pulled, and caused the bridge's failure.
At a news conference on March 15, NTSB investigator Robert Acetate was more cautious in his assessment by telling reporters that the investigation had not yet been able to determine if the crack detected by the FIGG engineer was a major factor in the collapse.
“I would have to say a crack in the bridge doesn't necessarily mean it's unsafe,” Acetate said. “I know crews were out there applying tension to a member, but I don't know if that was related to the cracks they discovered.”
Verrastro was puzzled as to why post-tensioning was being done to the bridge without the street being closed to traffic.
“That is not a normal procedure and that is what is surprising to me,” he said. “If they thought a member had lost all capacity, they would have closed the bridge. The engineers made the judgment that it was safe to do that, obviously. I am sure they didn't realize how much capacity they had lost prior to the collapse. No one would take the chance to do something so negligent — certainly no engineer would ever do that.”
Could any post-tensioning done after the bridge was moved into place March 10 have caused the cracks in the north support discovered just before the accident?
After a moment of thought, Verbasco answered “It shouldn't have,” adding that he wouldn't think there was a relationship between the two “because apparently there were engineers from FIGG and the FDOT that looked at it a few hours before the failure and I assume they were looking at something different from the tensioning.
“As I have said, it is likely not one thing, but, rather, multiple factors. Most all big accidents, including plane crashes, are the result of several problems.”
Concrete for Aesthetics, Low Maintenance
Then there is the matter of the massive amount of heavy concrete used to construct the footbridge. Included in the 950 tons that fell onto Tamiami Trail was both a walkway surface and a canopy made of concrete. Light-weight steel was not employed in the structure.
Varrastro said that much concrete in a bridge is out of the ordinary in most places, but is not as surprising in tropical South Florida, where he now works after many years based in the Northeast.
“I think it was done on purpose so that they would not have any maintenance concerns, as they would with steel,” he said. “Being an open structure, the problem with using steel, especially down here with the corrosion, is you have to paint it and clean it periodically. The FDOT, for instance, is all about that. They prefer concrete and will use steel only when they absolutely must.”
But, Verrastro, added, using concrete on the bridge was also an aesthetic choice. FIU and the city of Sweetwater were envisioning the bridge to serve as a beautiful, elegant welcome for students and visitors to the area, located on the western edge of Miami.
“It did not need to be a cable-stayed bridge, rather it could have been a slab or girder bridge. Whoever made that decision — I assume the owner — was wanting that look for the bridge. For a 174-foot span, that easily could have been a steel truss and a clear span with no tower or no cables. Plus, a steel bridge weighs only a fraction of a concrete bridge.”
Closer Forensic Work Ahead
Although investigators only needed a week to complete their initial work at the collapse site, the real task of discovering the reasons for the failure of the FIU-Sweetwater pedestrian bridge will be done in various labs and offices in Miami, Tallahassee and Washington.
A week after the accident, the NTSB narrowed its examination to the span's north end and had brought in a firm to remove sections of the span's floor, canopy and at least one truss that includes vertical and diagonal members. The federal agency also took another in a series of core samples from the concrete.
Tests on those samples, along with rebar and some tensioning rods, will be taken to a federal research center outside Washington for more tests. An exemplar of the tensioning rod and hydraulic unit used to adjust the cables was also taken to D.C., as well.
In addition, the NTSB shipped off large sections of the span to an FDOT facility for closer story, a department official said, the NTSB will control access to the pieces.
A quartet of investigative teams will be focusing on various aspects of the accident, including the bridge work and the decision to keep traffic flowing on busy Tamiami Trail. A preliminary report should be released in a few weeks, according to NTSB officials, which will include facts they've confirmed but not an analysis of what caused the collapse.
But, the evidence collected will hopefully help engineers to find ways to prevent a similar tragedy from occurring again. CEG