FIU Bridge: Why did it collapse?

FIU Bridge: Why did it collapse?

Bridge Construction and Design   

     On March 15, 2018 the bridge that would have connected Miami campus if Florida International University to the student residential neighborhood of Sweetwater collapsed onto 8 lanes of traffic, killing 6 people and injuring 9 more. Construction on the bridge began in March of 2016 and was scheduled to be completed in December 2018. The main companies responsible for construction were Munilla Construction Management and FIGG Bridge Engineers and unlike most bridges in Florida, the construction of the this bridge was overseen by the university itself. The purpose of the bridge was to alleviate traffic and improve pedestrian safety, as the busy crosswalks on the roadway had been identified as a safety hazard and the site of many fatal collisions.
     The bridge was a post-tensioned concrete structure, meaning it would be 10 times heavier than equivalent steel designs. It was designed to be 950 tons and span 320 feet, crossing both a major roadway and a parallel water canal with two separate spans connected at a faux cable-stay tower. The main span that would cross the roadway would be 175 ft long and the shorter span would be 99 ft long. Engineers designed an elevator and stairs at the south end added 31 ft and at the north end, 15 feet, for a total of 320 feet. The main span would have a concrete deck, roof, and diagonal struts, each containing post-tensioning cables which could be adjusted after the concrete was cured. The main span of the bridge was set in place on support columns only 5 days before the collapse; the canal span, access ramps, and faux cable-stay tower had not yet been built.
     The full bridge was styled to look like a cable-stayed bridge, but functionally it was a truss bridge, with the spans being fully self-supporting. The engineers for the project used a "re-invented I-beam concept," with a novel concrete truss design. The most complex obstacle, being that concrete truss bridges are rarely constructed, and few exist.

The Collapse    

     On the third day after lifting the main span of the bridge, the project's lead engineer discovered cracked at the north end of the span. He reported this by voicemail to a Florida Department of Transportation employee. The engineer did not believe this to be a serious issue, and because the FDOT employee was away at the time, he did not receive the message for several days. At 9 AM on March 15, a university employee heard a loud cracking noise while he was waiting at the red light under the bridge. FIGG, FIU, and FDOT representatives investigated the cracks and concluded that the structural integrity of the bridge was not compromised and that there were no safety concerns raised.
     At 1:47 PM, the north end of the installed bridge span sagged deeply as the first diagonal fractured, folded, and immediately dropped onto the roadway below.
A video captured by surveillance cameras caught the collapse of the bridge: http://www.dailymail.co.uk/news/article-5508397/Miami-bridge-collapsed-stress-testing-killing-four.html#v-3035966362207731933
     Senator Marco Rubio tweeted that engineers had actually been tightening loosened cables at the time of the collapse, while applying "post-tensioning force" to the steel rod in an attempt to test the strength of the bridge.

The Investigation
     
     The precise cause of the bridge collapse is still currently being investigated, but experts have been dissecting the collapse, and have some information about what may have caused it. The project used a novel design to create a "wow" factor, but contained many hidden structural risks and vulnerabilities that were possible compounded by a last-minute design change, all brought a catastrophic conclusion by engineering and construction miscalculations.
     According to the Miami Herald, "The span had been fabricated by the side of the road over several months under what’s known as accelerated bridge construction...Lifting the span into place took only a few hours." The truss design was chosen because of its quick, one-day installation. But experts believe that it was an unorthodox design. Truss bridges normally consist of parallel sets of bracing along each side. However this bridge boasted only on asymmetric arrangement of concrete beams in a zig-zag across center of the span. That deign may have accentuate the vulnerability of the truss bridge, which are known to fail when a single structural support is damaged.
     Independent researchers have begun investigations of their own, and some overlapping conclusions have been discovered. One Canadian researcher phrased "the smoking gun" of the failure as "steel tension rods that popped out of place after last-minute onsite changes to the engineering plans. Other small groups performing unofficial investigation through observation of the videos that captured the collapse. One group found that a particular beam, member 11 specifically, may have been the member that caused the collapse. Further research by professional engineers concluded that the bottom joint of diagonal member 11 apparently failed explosively during the post-tensioning operation, causing the bridge to collapse. Currently, it remains unknown why the joint may have failed, although post-tensioning remains a possible contributor. The failing member is circled below in the proposed blueprints of the bridge.























Aftermath and Plans for Future

     On March 16, the Florida Highway Patrol announced Southwest 8th Street between Southwest 107th and 117th Avenues and the eastbound Southwest 8th Street exit from the Homestead Extension of Florida's Turnpike were closed indefinitely while debris removal and investigations were conducted. Debris from the bridge was cleared by March 24, allowing Southwest 8th Street to be reopened. A team of 15 people from the National Transportation Safety Board (NTSB) began their investigation on the morning of March 16. On March 19, 2018 the first civil lawsuit was filed against FIGG Bridge Engineers, MCM, Bolton Perez & Associates, the project’s consulting engineer, Louis Berger, and Network Engineering Services for reckless negligence.
     Engineers who are investigating the bridge collapse believe their analysis is simply speculation because Manu key pieces are missing. Photographs, construction plans, and records have not been made public and are not completely available for use in the investigation. It is likely that this bridge collapse will be studied by students in the future, as it has been deemed one of the most fatal bridge collapses.

Sources:
http://www.miamiherald.com/news/local/community/miami-dade/west-miami-dade/article207358659.html
https://www.forbes.com/sites/berniecarlson/2018/04/30/the-fiu-bridge-collapse-do-we-need-slow-engineering/#67a477663abf
http://www.miaminewtimes.com/news/fiu-bridge-collapse-mystery-explained-by-foul-mouthed-canadian-youtuber-10186504