 In the March Floridian weather of 2018 investigators are picking through the results of a disaster. Six are dead and ten more injured but the only question on the investigators minds is how does a new uncompleted bridge fail in such a way that no one picked it up until it was too late. This is what we'll be looking at today, the Florida International University pedestrian bridge collapse. Our story starts with a safety concern. Students crossing the 8th Street and 109th Avenue intersection. Numerous reports from students at Floridian International University have come over the years of near misses with traffic across the eight lane highway. The university concerned with the risks to their students began to put into motion a plan for a new pedestrian walkway. Finances were secured in 2013 from the Department of Transport for the bridge across the highway between the FIU and the town of Sweetwater. Sweetwater was a popular place for students to find accommodation outside of the university campus and as such the crossing of the multi-lane highway is frequently used by both students and faculty members alike. The main organisations to be involved in the project were Manila Construction Management, a Miami based construction management firm and FIGG Bridge Engineers, a Tallahassee based firm. The project was different to other similar structures in the state as the Florida Department of Transport did not oversee the design, instead this task was taken on by the university itself. But the unusual plan did on the face of it makes sense. The university was known for its expertise in accelerated bridge construction. As a side note, this is a technique that allows bridges to be installed with minimum disruption to traffic, often in the space of a few days. This is done by constructing the bridge near to its final installation location and after assembly placing the structure in its final location. The bridge was to be 320 feet long and span the roadway and a canal that runs parallel to the street. This is formed of two sections of the bridge, 175 foot long and a 99 foot long section for the roadway and canal spans respectively. To tie it all together, a faux tower and cable arrangement was intended for the structure to make the bridge look like a cable staid arrangement. The total structure length was added to further by an escalator and lift on the southern end and a lift on the northern end. The design of the bridge was intended to have a 100 year plus life expectancy and could apparently withstand a category 5 hurricane. Although looking like a cable staid bridge, the structure's strength actually came from its truss construction. The bridge had a 30 foot wide deck and a 16 foot wide canopy, 15 feet above. The canopy and deck were connected by a single row of concrete diagonal and vertical supports in the centre. The canopy and deck were designed to act like an eye beam with the concrete walkway deck acting as the horizontal bottom flange of a wide eye beam and the roof canopy was to function as also a horizontal but narrower top flange of the eye beam. Having the walkway on the bottom of the truss allowed for less steps up to the bridge and due to it only using one truss it was a non-redundant design as the whole structure's strength came from the single collection of supports. The concrete was to be a post tensioned design. This is achieved by casting the concrete with a protective sleeve or duct within it for still cable tendons. Once the concrete is cured these tendons are tensioned and held in place with a lock off wedge adding strength to the beam. The deck and canopy had tending cables which were permanently tensioned and while the mainspan was still located in the staging yard before it even got installed on the bridge. Later the diagonal truss members had two to four post tensioning rods that could be tightened or loosened allowing for alterations to be made by the engineers to allow for the extra stresses expected during installation. These were to be de-stressed after the mainspan was in place. The plan for the bridge was to have it pre-built in a staging area next to the bridge site and then lifted into situ. Construction began in 2016 but just over a year later on a Sunday in August 2017 the death of a student Alexis Dale vindicated the need for a pedestrian bridge. She was crossing the very busy intersection at South West 8th Street and 109th Avenue like many students do any day of the week. In February 2018 the mainspan of concrete including the walkway, the canopy and diagonal and horizontal members were cast. Once it had cured inspections of the mainspan showed some cracking around truss member 11 and 12 at the truss members intersection with the deck. Cracking is not uncommon in concrete casting and this was to be monitored during the installation process. Thus the mainspan was prepared for installation which would involve movement from the fabrication yard to the bridge site. In order to get the assembled bridge in place it had to be lifted to its final location using self-propelled modular transporters. And that took place on the morning of 10th March 2018. Traffic on the South West 8th Street was detoured during the planned installation period leaving the entire roadway closed. Because the whole structure was vulnerable to twisting for transport and installation a maximum tolerance of plus or minus 0.5 degree was set. Beginning at 4.30 am on the 10th March the mainspan was moved by the SPMTs and placed on the permanent pylon and south pier supports. The movement was completed by 12.30pm. During the operation the twisting tolerance was exceeded twice, occurring for about 4 minutes as the bridge came in contact with one of the bearing pads on the south pier. After it was in its final place the mainspan's diagonal members 2 and 11 were detentioned as part of the original plans. And photographs were taken of the cracks at the 11th member to see if they were active or dormant. On the 12th March more photos of the cracks around the 11th and 12th nodes were taken and were now worryingly getting larger. Manila Construction Management viewed the photos and indicated that the cracking was not a safety issue and recommended that plastic shims be placed underneath the diaphragm the part that attaches to the pier. On March the 13th and faced with growing cracks MCM and FIGG building engineers decided that retentioning the 2 and 11 supports may lessen the cracking but FIGG determined that the cracking was still not a safety issue. On the 14th March the cracks still causing concern were discussed between the contractor companies. A day later on the morning of the 15th an inspection team went to the bridge to evaluate the cracking in preparation for a 9am meeting to discuss potential remediation action. It was then decided that member 11 which at the bottom end formed the 11th and 12th node Amazon the pylon pier would be restored to its original tension pre-installation of £280,000. This was to be done in increments alternating between the top and bottom rod. The way I understand it is similar to how you install an engine sump or valve cover as to even out the load during tightening. If you've ever owned a Honda and forget to tighten the tappet lock nut properly then you'll get a lot of practice doing this on valve covers. A 6 man team was given the go ahead for the restressing and a head they went. The retightening of member 11 would begin on the 15th all while traffic still flowed below the structure. The contractors thought that the operation couldn't compromise the structural integrity of the bridge due to the contractors insistence that the structure was still safe. How wrong they were. At roughly 1345 the traffic lights changed and cars came to a stop at the south west 8th street and 109th avenue junction. At 1346 the northern end of the bridge at the joint of the 11th and 12th supports blew out under the applied tension of the tendons. Within a fraction of a second the bridge sagged at the northern pile on pier. Cars on the road below continued to wait under the stricken structure. A driver of one of the cars under the bridge would later say that small rocks had landed on her vehicle moments before the bridge failed. The bridge immediately dropped the heavy full span onto the roadway below crushing anything underneath. Five were immediately killed after the collapse and one would die later in hospital. The victims were Navarro Brown aged 37 a worker on the bridge, Alberto Arias 53, Brandon Brownfield 39, FIU student Alexa Durran 18, Ronaldo Fraga 60 and Oswaldo Gonzalez 57. A further 10 were injured including passengers of cars and workers on the bridge. In total eight cars were crushed during the collapse. Emergency 911 call handlers would be inundated with calls starting at 1347. The dispatcher sent multiple emergency responders to the bridge location with the first police officers reaching the site at 1352. Miami-Dade fire rescue was sent to the scene and assisted in transporting the injured Kendall Regional Medical Center, although one person self-transported to the hospital. What was perplexing is how could something so one of the mills of Footbridge fail so catastrophically? Well the NTSB would find out that the bridge was more complex in design than it possibly needed to prioritizing form over function. And that the warning signs seemingly were ignored during the bridge's design and installation. The NTSB dispatched a 15 person team to pick over the wreckage. On the 16th of March the bureau in a press conference stated that tensioning was being undertaken and they were aware that there had been cracks. But this did not necessarily mean it was the cause of the failure. Originally the NTSB team suggested that the investigation of the wreckage would take around a week. After this another press conference was held in which it was told that certain items of the bridge required further investigation at the Turner Fairbank Highway Research Center in McLean, Virginia. In May a preliminary report was released but investigations were still being undertaken on the failure mode of the bridge. In 2019 the NTSB released their final report and the results were mixed but at the same time all too familiar. The root cause was a miscalculation in overestimating the strength of supports 1 and 2 and 11 and 12 which was compounded by underestimating the loads which would be put on critical portions of the structure. The design was also flawed in that it was non-redundant meaning if the main portion failed then the whole thing would go. Which conjures up similarities with the silver bridge collapse. You see when compared to a more traditional truss bridge design two trusses are used and this spreads out the load across the whole structure. The design of the FIU bridge only had one truss section and thus concentrated the load on the nodes where the supports met. The NTSB discovered that when the loads on the bridge were calculated FIGG erroneously used a factor redundancy of 1. Which is commonly used for structures with redundant load paths. This hints at FIGG were assuming the bridge had a redundant design. The distressed and increased cracking of the concrete was the main indicator of the poor design of the main span. Although cracking is not uncommon in concrete just look at this path. But the location and increased severity should have provoked a more worried response than retightening the diagonal supports in the hope of it fixing itself. No one involved in the bridge's assembly not Florida International University, MCM, FIGG bridge engineers, nor Bolton, Perez and associates the consulting engineers took the responsibility for declaring that the cracks were beyond any level acceptability when discovered after casting in February disaster could have been averted even after the bridge was in situ by stopping and diverting traffic during the restressing of support 11. Multiple opportunities to catch the poor design were missed by all involved and due to the university taking the oversight of the construction Florida State Department of Transport took a back seat and did not discover any of the safety issues. The first of the legal cases were brought on March 19th 2018 against FIGG bridge engineers MCM, Bolton, Perez and associates the project's consulting engineer Lewis Berger and network engineering services for reckless negligence. Needless to say the case against the companies didn't go too well with such a damning NTSB reports and to add more evidence OSHA cited multiple contractors for safety violations. On the 1st of March 2019 Manila Construction Management the main contractor of the pedestrian bridge construction announced a chapter 11 bankruptcy petition plan of reorganization. The company reached a settlement with the victims and their families in May 2019. The remnants of the bridge's peers remained on the site as a gruesome reminder of the disaster and the risks to pedestrians continued with loads more near misses between cars and students reported. The original problem still existed and as such a bridge is still needed on site. The need will be started to be fulfilled as the remnants of the walkway were removed in September 2021. A replacement is pegged to be built by 2025 this time under the guidance of the Florida Department of Transport and the NTSB. Plain difficult videos are produced by me, John, in a currently sunny but windy suburban southeastern corner of London, UK. All videos on the channel are creative commons attribution share like license. Help the channel grow by liking commenting and subscribing. Check out my Twitter for all sorts of odds and sods as well as hints on future videos. I've got Patreon and YouTube membership as well, so check them out if you fancy supporting the channel financially. And all that's left to say is thank you for watching.