State and local officials said Friday that the cables supporting the deck of the pedestrian bridge at Florida International University near Miami were being tightened after a stress test when the bridge collapsed over traffic on March 15, killing at least six people.
Miami-Dade Mayor Carlos Gimenez said crews had conducted the stress test on the span earlier in the day, and Sen. Marco Rubio tweeted that engineering firm Figg Engineering had ordered the tightening of cables that loosened, the Associated Press reports. “They were being tightened when it collapsed,” Rubio said.
A major search-and-rescue effort is underway beneath the shattered pieces of the partially completed pedestrian bridge.
The Florida Highway Patrol confirmed six fatalities resulted from the sudden failure of the 32-ft wide, 950-ton main span, which had been set in place on March 10 across the eight-lane Tamiami Trail using accelerated bridge construction (ABC) techniques. Miami Dade County Fire Rescue confirmed at least eight vehicles that had stopped for a red light were trapped beneath the rubble.
Police officials expect the final count of fatalities to go higher.
An eyewitness interviewed on local television stated that the failure began from the north end of the bridge and quickly continued toward the center. At least 10 victims had been transported to area hospitals. Among them were two construction workers who were reportedly on the bridge before it gave way.
A 15-member multi-disciplinary National Transportation Safety Board (NTSB) investigative team was scheduled to be on site Friday night to join state and local investigators to pinpoint the cause.
NTSB Chairman Robert L. Sumwalt, who is leading the team, said the agency can issue urgent interim recommendations should it uncover any deficiencies with ABC methods. “To my knowledge,” he said, “there have been no problems” with other ABC structures.
Although officials insist that no conclusions can be drawn pending a full investigation, Amjad Aref, a professor in the University at Buffalo department of civil, structural and environmental engineering department, told USA Today that the span should have been stabilized with temporary supports until permanent column and foundation connections were completed.
“When collapses like this happen very quickly, it’s called instability,” Aref told USA Today. “Something triggered the system to quickly go through disproportionate collapse.”
The $14.2-million cable-supported structure was a design-build collaboration of Munilla Construction Management (MCM), South Miami, and FIGG Bridge Design, Tallahassee, that began in early 2017. Fabricated on temporary shoring adjacent to the highway, the 174-ft long stretched “I-beam” section included a bottom flange for the walking surface, a top flange for the canopy and a truss-like web between the two, according to the university.
Structure Set a Record For Construction Method
Early on the morning of March 10, Memphis-based Barnhart Crane and Rigging used self-propelled modular transporters to rotate the span 90 degrees onto permanent supports. No problems were reported during the installation, which reportedly took only a few hours. FIU claimed the operation was the largest pedestrian bridge moved via the method in U.S. history.
A statement from Barnhart Crane and Rigging noted the firm was "saddened by the news of the tragedy," and added: "Barnhart was contracted to move the bridge into place and was not involved with the design or construction of the bridge. Our scope of work was completed last week without incident and according to all technical requirements. Barnhart crews and equipment demobilized from the site on Saturday, March 10, and were not at the location at the time of the incident. We will fully cooperate with authorities as they investigate the cause of the collapse."
Construction was to continue into next year, with the final structure set to be 298 ft long, including stairs and elevators on each end, and a 109-ft tall center pylon. Doubling as a 9,900 sq ft public plaza, the structure was to be the first in the world built entirely with “self-cleaning” concrete, with a titanium dioxide additive that would capture particulates from the air when exposed to sunlight. The deck’s longitudinal and transverse post-tensioning was designed to last at least a century and withstand a Category 5 hurricane.
In a statement, MCM said that the company “will conduct a full investigation to determine exactly what went wrong and will cooperate with investigators on scene in every way.”
MCM ranks at No. 235 on ENR's list of the Top 400 Contractors, reporting about $328 million in 2016 revenue, about 68% in transportation work. Barnhart ranks at No. 58 on ENR's Top 600 Specialty Contractors list, reporting about $329 million in revenue 2016 revenue.
A FIGG statement expressed a similar commitment, adding that in the company’s 40-year history, “nothing like this has ever happened before.”
The firm later provided an additional statement to media, which read, in part: "This pedestrian bridge collapse is truly tragic. The focus right now is on the first responders’ recovery operations. Multiple agencies have already begun an extensive review to determine what caused the collapse, and we will work closely with all appropriate authorities in this effort."
With an enrollment of more than 55,000, FIU is Florida’s second-largest public university. The school is home to the Accelerated Bridge Construction University Transportation Center, established in 2010, according to its website, to “reduce the societal costs of bridge construction by reducing the duration of work zones, focusing special attention on preservation, service life, construction costs, education of the profession, and development of a next-generation workforce fully equipped with ABC knowledge.”
Editor's Note: This article has been updated since its original posting on 3/15/18. ENR will continue to update this page as new information is obtained.
VIEWER COMMENTS (section is now closed)
March 16, 2018
I'm hoping the structural engineers here can comment, but if the final structure needed the cables to hold up the bridge platform, how can the platform be placed post-and-lintel style without the cable attachments? The tower hadn't been built yet, so it was apparently planned to be cable-free for a while.
March 16, 2018
It looks like this unfortunate accident may result from a combination of a/ an insufficient reinforcement of the upper beam (the canope) around the point of junction with the truss to resist puncture and b/ an insufficient cohesion of the concrete; Both of these factors seem to appear on the pictures.
March 16, 2018
As a practicing private civil engineer, I have never been a proponent of the design-build process. This process can subject the engineer to make inappropriate decisions to the purpose of the contractor's profit line. It also removes the engineer from one of the most important components of the design process which is a direct line of communication with the project owner. The design-build process makes the engineer subject to the direction of the contractor. The roles are are completely reversed of what is best for the safety of the public.
March 16, 2018
Or, a possible future academic example of "Lean" Construction gone bad?
March 16, 2018
I am absolutely astonished that dynamic testing and cable (tendon) tensioning took place with the public directly exposed to any failure. I also reject any suggestion that this is an "accident". The consequences may not have been intended but they flowed directly from the actions of the designer and builder. I've been teaching safety regulation to engineers since 1975. Have tehy ignored the canons of Ethics of the ASCE? CANON 1. Hold Safety Paramount Engineers shall hold paramount the safety, health and welfare of the public a..... a. Engineers shall recognize that the lives, safety, health and welfare of the general public are dependent upon engineering judgments, decisions and practices incorporated into structures, machines, products, processes and devices.
March 16, 2018
This accident reminded me of the Hyatt bridge collapse in Kansas City back in the early 80s. It may be awhile before we know what and how this happened. Vigilance in this business never ends.
Dr. Saty Satyamurti
March 16, 2018
In my opinion there was a major error in computer program that analyzed the structure. Also materials of construction was not up to standard, from the manner in which the bridge collapsed. material test records and concrete test records should by reviewed and verified.
March 16, 2018
The death of 6 people is a significant tragedy. Engineers and builders will learn a great deal from the investigation results. However we must not lose sight of the 40,000 people who are killed each year using facilities designed by civil engineers to maximize the flow of automobiles through communities. For the profession to ignore or brush off this death toll is, in my opinion, gross negligence.
March 17, 2018
The first failure is on the first interior roof panel point from the stream side support . The cracks should be there first. When they noticed the cracks, that is why some bridge personnel was standing (running) there when they failed. The canopy deck cracked through like a Kitkat and then followed by landing ending with punching through of the second panel zone of the canopy deck.. It is possible the tightening of the cables in the floor deck resulted in over compression of the deck hence reversing the forces in the webs, leading to the failure of the first interior joint of the canopy cracked like a kitkat. The sound profiles should have 1 break sharp then 3 thump dull impact sounds represented by the three punch through on impact.
March 18, 2018
Here is the original design for that bridge: http://facilities.fiu.edu/projects/BT_904/MCM_FIGG_Proposal_for_FIU_Pedestrian_Bridge_9-30-2015.pdf My initial guess is that the design should have required solid web to avoid point load on the bridge deck (lower slab), or use other material instead of concrete. Even better would have been to use a totally different bridge design, for example real trusses (not made of concrete), or prestressed concrete box cross section with deck on top which was apparently rejected to allow students to climb less stairs.
March 18, 2018
The selection of concrete for a truss bridge is inexplicable. The truss/frame failed in the joints between the webs and chords. If this had been a steel bridge, there would have been sufficient ductility to prevent such a brittle collapse. The pipes which looked like cable stays were mostly for aesthetics, with some intended benefit in damping vibration. So the precast structure was intended to span full length. It failed. The NTSB, probably with some assistance from specialists, will get to the bottom of what happened.
March 18, 2018
Apart from this being a horrifically tragic way to die for eight people... I believe that the civil and structural forensic data will show not only the engineering deficiencies, but will also point to a much broader scope including the lean ABC design process. That process has been proven around the world in many projects. However, other than seeing what we only want to see... maybe it’s time for the actual design process to be re-examined, and to have the structural robustness that used be be engineered into a project, put back in. Yes, even if it takes longer and costs more. I hope the ABC center at the University takes the time to critically self examine its role not just in this tragedy, but globally - with lessons learned baked into every one of these types of lean projects. One does not have to look very far to find many many examples of iron foot bridges that are over 100 years old and still in use. They were engineered and constructed using basic construction techniques that worked!
March 18, 2018
The design constraints pose an interesting design problem. Possible explanations for choosing concrete may include that steel requires more maintenance in that climate, and the local economy has over-capacity to produce concrete. Brilliant advances have been made in concrete construction, for example the box girder design of the Long Key Bridge also by Figg. But that puts the bridge deck higher. As this article notes, the design intent here was to design a large I-beam. I-beams are widely used in concrete bridge construction, although with solid webbing, not that big, and with flaring inside the lower flange to distribute load. Here the flaring seemed to be underneath, outside the I-beam, and the webbing was not solid.
March 19, 2018
As already pointed out by Al Austin, "The selection of concrete for a truss bridge is inexplicable". There are other mysteries to elucidate : - What is the purpose of such a complicated scheme ? Why combine truss and suspension cables ? One of two things : either the truss beam is self supporting and it must be so designed (which obviously was not the case) or it needs the cables to be stable and it should be temporarily propped up until completion. - Is the miracle self-cleaning concrete as resistant as needed ? - Isn't there confusion of functions and responsibilities for FIU being both the Client and the sponsor of construction methods ? - Is there any relation between this last question and the inexplicable absence of road closure ?
March 19, 2018
I found this video showing top slab may have broken first as Michel Villaneau mentions: https://www.youtube.com/watch?v=p6L20i6_gzE
March 19, 2018
Looks like it is a construction sequence method problem. Was there a method statement for what they where doing with a risk analysis. I highly doubt it was a design or quality issue.
Bruce A. MacMahon
March 20, 2018
This was not a "bridge" that collapsed. It was ONE span of a two-span cable-stayed bridge. Cable-stayed bridges rely on the tension cables running from the deck to the pylon to support the span. The "truss-like" web this article mentions was not a truss designed to bear the load of the structure. The diagonals were to connect to the tension cables. The tension cables were to take the load of the bridge (plus live loads) and carry them to the pylon where they'd become the compression loads bearing down on the pier/footing. Whoever made the decision to allow traffic to flow under that 950-ton piece of concrete (that was never designed to support its own weight) is about to have his or her life ruined by criminal charges and civil lawsuits.
March 20, 2018
Construction sequence could have been a problem. I was wondering why the transport vehicle was not lined up under the north end of the span as in the original design. Today it was reported FDOT required design change to lengthen span to allow for future road widening, without actually putting in the extra lane yet, forcing the transport vehicle away from the load points. Seems strange the original design did not plan on road widening below the bridge. Plus, such an odd bridge design may be difficult to change: should be built as prototypes first to test its characteristics. Better to have gone with box girder with deck on top, like the Dave Pelz Memorial Bicycle Bridge in Davis, California (see also the ped bridge across I-80 just west of I-5 in Sacramento).
March 21, 2018
Looks like we have here an accumulation of odd decisions leading to a catastrophy. A bizarre design combined with an unusual construction method should have led to extra safety precautions in the precast bridge handling. Instead, as pointed out by Carlos Portela, the construction team accepted that one of the transport vehicles be placed in such a way that the constraints pattern was locally reversed, resulting in cracks, apparently in the deck at the northern junction with the web. These cracks, initially declared non threatening, were probably amplified by the stress test, hence the unfortunate decision to tighten tendons in an already compressed member which apparently failed, leading to the collapse. A road closure should normally have been ordered during the whole process, but it would have defeated the purpose of The Accelerated Bridge Construction method …
March 21, 2018
When modelling, the designer should have accounted for moments to develop at all joints, including external PT, of this complex concrete frame. If they didn't, that's a problem. For a proper review it's necessary to see how the designer modeled the structure in their finite element model. Axial dead load stresses in the end diagonal -- member 11 (the one pictured in the article above)-- are already high. Now add to this some undetermined moment load, and flexural stresses could become critical. Then also combine this with the fact that minimal shear reinforcement seems to exist in member 11, and things could become dire (there seems to be minimal observable shear reinforcement in collapse pictures seen elsewhere). Having said all that, the following is a hypothetical cascading collapse sequence based on concerns with design of member 11. To date, I've seen nothing in the limited, grainy, heartbreaking video to contradict this proposed sequence of events. 1) First member to fail - member 11: Diagonal, under high axial and shear loads, becomes critical with the additon of moments at member ends [This may or may not be connected to the external PT that has been so much talked about. Destressing of the PT rod in 11, however, as some speculate was the operation being performed at the time of collapse, would actually reduce shear capacity of section]. The integrity of the end(s) become compromised, pins develop as a behavior mode, with final result being brittle shear failure at one, or both, ends. Note that from pictures it does not appear that the diagonal buckled, as the center zone of member appears to be relatively intact. Also, the "zippering" along the bottom side of 11 is likely the result of PT bar being ripped out during collapse. 2) Second member to fail - top cord/flange above member 11: After member 11 fails the structure is "theoretically" still viable, assuming we have frame behavior. However, this is obviously not sound. So next, the top cord near intersection with 11 fails quickly in shear and flexure; top cord is much weaker than bottom so top fails first. It's also possible then that the longitudinal PT force in the top cord, combined with this new instantaneous frame bending of structure in this corner, causes the end vertical -- member 12 -- which intersects member 11 above the pier, to fail in bending at its base in the diaphragm area. And it's even conceivable that member 11 has become "detached" from the bottom slab and is driven along the top surface of the slab, impacting 12 and causing additional damage to its base. This could explain why the final resting place of 11 & 12 are on top of the pier. 3) Bridge collapse: After top cord fails the only section remaining is the bottom slab. Location where loads are highest is where the bottom slab intersects the next diagonal -- member 10. Bridge now hinges at this point and falls. At some point during fall the longitudinal PT force in bottom slab pulls this now free bottom cord (section beneath 10 & 11) off its bearings, and that entire end of bridge plunges to ground. Punching out of anchorage blocks on top (seen in the picture above) is likely the result of impact from the falling bridge. >>> An alternate, albeit unlikely, failure mechanism: With diagonal member 11 carrying all dead load back to bearings, the zone of shear transfer at the intersection with member 12 and the bottom slab is crucial. The load is entering the bottom slab, and all that wonderful longitudinal PT spread out across the entire cross-section, at a single point--midspan. Adequate shear and confinement reinforcement must be provided to ensure that the horizontal component of load from 11 can be distributed back into the slab. If not, the "triangular frame" of members 11-12-top cord could conceivably pop out of end diaphragm above pier in a prying action-like behavior, leading to catastrophic collapse. The video, however, doesn't appear to show this, although that could be merely a trick of the eye. And there's also the fact, seen in pictures elsewhere, that 12 appears to have failed in flexure at its base, ABOVE the surface of the deck.
March 21, 2018
A horrible tragedy and loss of life. In looking at the design posted, it seemed that the center pylon and cables were going to pick-up the truss panel points through roof top connection nodes at the roof or top flange of the wide flange or "I" beam mentioned. without these in place wouldn't shoring under each of the truss panel points be needed to assist in carrying the loads until the upper cables were in place and tensioned?
March 22, 2018
The level of speculation on cause is similar to that we see in aircraft crashes, especially those involving engine failure. Generally, the root cause is categorically related to design, manufacture, maintenance, or operation outside of design parameters, which would include external events such as a bomb or ultra-severe weather. With the exception of the last category, there is almost always more than one influence because design parameters deliberately seek to prevent loss of aircraft from single point of failure. This may be as simple as 2 or more engines or as complex as a mechanical control system as backup for a primary electronic system. While civil engineering is not my field, it seems civil structures rely more on factors-of-safety (weight is not a major issue like it is in aerospace) than they do on redundancy for safety. Beyond that, it seems the design, manufacture (including the on-site portion), and maintenance (minimal opportunity in this case) are categorically the same.
March 22, 2018
It is already understood that discussion is theoretical. I recommend developing all possible theories, in order to better prepare to evaluate possible future information. To recap a possible theory, when the span was transported, the north end was sticking out past the transport vehicle, making it a cantilever instead of a beam, although it became a beam instead of a cantilever after placement. For example, members that were in tension during transport were in compression after placement, and vice versa. Steel trusses and concrete box girders may be dead loaded in reverse like that if the trusses or girders are designed for that. Might not have been possible with this design.
March 24, 2018
Based on videos and photos, it looks like the collapse started with the failure of diagonal member 11 whose lower end at its junction with pillar 12 seems to have exploded. This could be explained by the disastrous decision to tighten the tendons in this heavily compressed member, all the more since it looks relatively slender compared to less loaded intermediate diagonals of the same section and to member 2, similarly loaded but of a much larger section. If this is true, was there also a design error to start with ?
March 25, 2018
Statistically, early collapse could be indicative that there were many errors. The bridge may have eventually collapsed anyway absent some of the errors, perhaps not right away. Regarding geometry, besides a thicker south end diagonal, that end had better cantilever geometry, because the next member was more vertical, thereby using 2 triangles instead of 3 in the cantilever. Think of it as like building a picket fence gate supported by its hinges. It is better to have a vertical side with a diagonal that reaches end to end, instead of discontinuous diagonals forming an upside-down V with skewed support.