Florida International University pedestrian bridge collapse

FIU-Sweetwater UniversityCity Bridge
FIU-Sweetwater UniversityCity Bridge
Coordinates	25.7612°N 80.3728°WCoordinates: 25.7612°N 80.3728°W
Carries	Pedestrian traffic
Crosses	Tamiami Trail
Locale	University Park and Sweetwater, Florida, U.S.
Characteristics
Total length	320 feet (98 m)
Longest span	175 feet (53 m)
History
Engineering design by	FIGG Bridge Engineers
Constructed by	Munilla Construction Management
Construction cost	$14.2 million
Collapsed	March 15, 2018
	; University Park Location in Florida Show map of FloridaShow map of the USShow all

FIU Sweetwater UniversityCity pedestrian bridge collapse
	; National Transportation Safety Board members inspecting the collapsed pedestrian overpass on March 16
Date	March 15, 2018
Time	1:47 p.m. EDT
Location	University Park and Sweetwater, Florida, U.S.
Type	Overpass section collapse
Deaths	6
Non-fatal injuries	9

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On March 15, 2018, a 175-foot-long (53 m), recently-erected section of the FIU Sweetwater UniversityCity pedestrian bridge collapsed onto the Tamiami Trail (U.S. Route 41).

The initial construction on the pedestrian bridge was located in front of the campus of Florida International University (FIU) in University Park, a suburb west of Miami, Florida, United States, and was in the process of post-tension cable adjustment when it suddenly failed.^[1] The road beneath it had been opened to traffic. Several occupied vehicles were crushed underneath, and six deaths and nine injuries have been reported.^[2]

Background[edit]

The FIU Sweetwater UniversityCity pedestrian Bridge, located just west of the intersection of Tamiami Trail (Southwest 8th Street) and Southwest 109th Avenue^[3], was planned to connect the FIU campus to student housing neighborhoods in Sweetwater.^[4] It was intended to improve pedestrian safety, as the busy crosswalks at this wide, busy intersection had been identified as a safety hazard and the site of fatal collisions.^[5] The $14.2 million project was funded with a $19.4 million Transportation Investment Generating Economic Recovery (TIGER) grant from the United States Department of Transportation in 2013, along with state agencies.^[6] The bridge itself cost $9 million to construct.^[7]

The main companies behind the construction project are Munilla Construction Management (MCM), a Miami-based construction management firm, and FIGG Bridge Engineers, a Tallahassee-based firm.^[8] Unlike most bridges in Florida, the design for this project was overseen by the university itself, not the Florida Department of Transportation.

Florida International University is known for its expertise in accelerated bridge construction, and has attracted international scholars as PhD students. It is home to the federally-funded Accelerated Bridge Construction University Transportation Center, which sponsors industry conferences and seminars.^[9]^[10]

Bridge layout[edit]

The full 320-foot-long (98 m) pedestrian overpass was to cross both a major roadway and a parallel water canal with two separate spans connected at a faux cable-stay tower. The main, roadway-crossing span was 175 ft long, the shorter canal span was to be 99 ft long. An elevator and stairs at the south end added 31 ft, and at the north end, 15 feet, for a total bridge length of 320 feet. ^[11] At the bridge site the Tamiami Trail roadway has six lanes of traffic plus two turn lanes. The main span was rolled into place and set on support columns on March 10, five days before the collapse. The canal span, access ramps, and faux cable-stay tower had not yet been built. Pedestrian use was to begin when the whole project was complete. The school was on spring break at the time of collapse.^[1] The section of the bridge that collapsed weighed 950 short tons (860 metric tons) and fell onto several vehicles on the roadway below.

Bridge construction and design[edit]

Construction of the bridge began in March 2016 and was scheduled to be completed in December 2018.^[7]^[12] The bridge's main span was assembled adjacent to the highway using accelerated bridge construction (ABC), a technique promoted at the university.^[1] It was lifted into place on the morning of March 10, five days before the collapse, during a weekend closure of the highway.^[13]^[14]

The new pedestrian bridge was designed to connect the campus to student housing in a dramatic sculptural way, and also to showcase the school's leadership in the ABC method of rapid bridge construction.^[15]^[16]

The bridge was a post-tensioned concrete structure.^[15] Concrete structures are generally 10 times heavier than equivalent steel designs.^[17] The bridge was made using a new formulation for concrete intended to stay cleaner than standard concrete formulations.^[18]In the main bridge span, the concrete floor deck, roof, and diagonal struts each contained post-tensioning cables whose compressive effect on the concrete could be adjusted after the concrete was cured.^[15] Adjustments were expected each time the span was moved onto a different foundation with different support points.

The full bridge project was styled to look like a cable-stayed bridge with pylon tower and high cables for dramatic effect. But functionally and structurally it was actually a truss bridge, with the spans being fully self-supporting.^[15]^[19]^[20]The bridge spans used a novel concrete truss design invented for this project, a "re-invented I-beam concept". Concrete truss bridges are rarely constructed, and few exist. ^[21]The vertical web of the beam was a series of triangulated concrete diagonal struts along the centerline. The diagonal angles of the struts varied across the bridge so they would align with the eventual cable-stayed appearance - a feature unique to this truss bridge. Conventional truss bridges use regular repeating angle triangles. A bridge redesign late in the project required moving the bridge’s main pylon tower 11 feet (3 meters) north to allow for future highway widening, increasing the gap between the bridge end supports and requiring some new structural design. It is unknown at this time if the angles of the concrete diagonals or the length of the spans were changed in the redesign.^[22]

The concrete walkway deck was to act as the horizontal bottom flange of a wide I-beam, and the concrete roof canopy was to function as the horizontal top flange of the I-beam. ^[15] The walkway was thus nearer to ground level than in a standard design where the walkway is placed on top of the structural support system.^[15] This reduced the number of steps to climb. The deck carries the entire weight of the bridge span, as a tension load. The canopy carries the structure's main compressive loads. The diagonal struts carry either compression or tension forces, depending on their angle and position. The struts contain post-tensioning rods that connect the flanges, and are adjusted as needed.^[23] VSL was the company contracted for the post tensioning (PT) of the new bridge. Post-tensioning of a diagonal member may be one of the reasons for the structural failure of the FIU bridge. Concrete is much more prone to crack than steel in trusses. ^[24]Certain cracks could compromise the cable/rod anchorings, making failure inevitable when the post-tensioning operation began.^[25] The bridge was meant to last more than 100 years, and to withstand a Category 5 hurricane, in a statement by the university.^[26]

Collapse[edit]

Schematic of the bridge. Green: collapsed parts, Blue: not installed at the time of the collapse. The diagonal beam that was undergoing post-tension cable/rod adjustment at the time of the collapse is highlighted in red.

Reports of pre-collapse cracking[edit]

On March 13, the third day after lifting of the main span, the project's lead engineer discovered cracks at the north end of the span (the end that later broke). He reported this by voicemail to a Florida Department of Transportation (FDOT) employee. He thought this was not an immediate safety issue, merely something that would need to be repaired later. The FDOT recipient was away for several days and did not hear this message until the day after the collapse.^[27]^[28]

At 9 a.m. on March 15, a university employee heard a loud "whip cracking" sound while under the bridge span, waiting for a red traffic light.^[29] At the same time, the design-build team met for about two hours at the construction site to discuss the cracks discovered on March 13. Representatives from both FIU and the FDOT were present. The FIGG lead engineer's conclusions were that the structural integrity of the bridge was not compromised and that there were no safety concerns raised by the presence of the crack.^[30]

The mayor of Miami-Dade County, Carlos A. Giménez, said that workers conducted a stress test on the morning of March 15.^[31]

Collapse[edit]

At 1:47 p.m., the north end of the installed bridge span sagged deeply as the first diagonal fractured, folded, and immediately dropped the heavy full span onto the roadway below. A surveillance video shows the collapse sequence took only a few video frames.^[32]

United States Senator and FIU adjunct professor Marco Rubio tweeted that engineers were tightening loosened cables on March 15:^[31] Workers were adding more tension to the steel rod (tendon) inside a concrete diagonal strut at the north end.^[27] The National Transportation Safety Board, who are investigating the collapse, stated that crews were applying "post-tensioning force" on the bridge before the collapse.^[33]

A witness reported that at the moment of collapse, a blue box fell loose from the hook of a crane. His impression was that its fall triggered the collapse.^[34] It dropped onto the roof of the bridge next to workers, and where the roof and span broke apart.

The tensioning rod that was being tightened at the time of the collapse, with a hydraulic tensioning machine (blue) still attached to its end, extends from the canopy in the upper left of this image.

At this time, it is not clear why the post-tensioning operation by VSL company was being carried out at the time of the collapse as the bridge diagonal member support they were working on would have already been in compression. A possibility, as reported by New Civil Engineer, is they were trying to put an additional compression into the member to close cracks in the bridge superstructure.The bottom joint of diagonal member #11 (which contains the anchor nut for the post-tensioning rod) 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.^[35] A driver who survived the collapse reported small rocks fell onto her car just before the car got smashed.^[36] A worker saved himself when he heard cracking and locked his safety harness just before the collapse.^[37]

The span that collapsed weighed 950 short tons (861 tonnes).^[38]

At the time of the collapse, the roadway was open and there were multiple cars stopped at a traffic light under the span.^[39]^[40]^[41] Eight cars are reported to have been crushed. Six people were killed and nine others were injured; while early reports said that the death toll was expected to rise,^[2] authorities said on March 18 that all bodies had been accounted for.^[42] An employee of VSL, Navaro Brown, 37, worked for the company contracted to apply post-tensioning and died in the collapse. Two other employees of the company were hospitalized. The other deceased victims of the tragedy were identified as Alberto Arias, 53, Brandon Brownfield, 39, FIU student Alexa Duran, 18, Rolando Fraga, 60, and Oswaldo Gonzalez, 57.^[43]^[44]^[45]

Aftermath[edit]

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 would be closed indefinitely while debris removal and investigations were conducted.^[46] A team of 15 people from the National Transportation Safety Board (NTSB) began their investigation that morning.^[47] 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.^[48]

Inquiry[edit]

NTSB investigates Miami bridge collapse IMG 0009 (39942593765)

On March 15, 2018 the National Transportation Safety Board (NTSB) launched a Go-team to investigate the bridge collapse incident. The accident number assigned is HWY18MH009. ^[49] On March 16, 2018 the NTSB Investigators held their first press conference to discuss the inquiry into the bridge collapse. Noteworthy points from the meeting included a statement that the inquiry was in the very early stages, that cracks in the bridge superstructure did not necessarily make the bridge unsafe, that on-site investigations would take about a week, that preserving perishable evidence was crucial, and that bridge workers were applying a "post-tensioning force" on the bridge before the failure.^[50]On March 21, 2018 the NTSB sent out the following press release:

Investigators secured a contract Tuesday to have a company remove components from the bridge that we believe warrant additional examination and testing. These components include sections of the floor, the canopy, a vertical member and a diagonal member; all from the north end of the structure. These components were in the area of where the failure occurred. In addition to these components, we obtained additional core samples from this area to supplement the core samples we obtained earlier. Shipping the core samples along with some recovered rebar and tensioning rods to the Turner Fairbank Highway Research Center, in McLean, Virginia, for testing and evaluation. The investigative team has obtained an exemplar tensioning rod and hydraulic unit used by the construction crew to make tension adjustments to the tensioning rods. These items are also being shipped to the Turner Fairbank Highway Research Center for evaluation and testing.The larger bridge components the investigative team is removing, are being stored at a secured Florida Department of Transportation facility, under the control of the NTSB. The additional examinations and forensic deconstruction of these components will occur there. The investigative team has confirmed that workers were adjusting tension on the two tensioning rods located in the diagonal member at the north end of the span when the bridge collapsed. They had done this same work earlier at the south end, moved to the north side, and had adjusted one rod. They were working on the second rod when the span failed and collapsed. The roadway was not closed while this work was being performed.

On March 21, 2018 U.S. Transportation Secretary Elaine Chao asked the department’s inspector general to probe whether a federally funded UniversityCity pedestrian bridge complied with all rules.^[51] A subsequent internal memorandum from the Inspector General of the U.S. Department of Transportation, dated March 22,2018 included the following statements^[52]:

The Florida International University pedestrian bridge was funded in part through the Department of Transportation’s (DOT) Transportation Investment Generating Economic Recovery (TIGER) discretionary grant program.1 The Federal Highway Administration administered the grant funds for the project. On March 19, the Secretary of Transportation requested that we initiate an audit to evaluate whether the project complied with Federal requirements and specifications. On March 20, Senator Bill Nelson, Ranking Member of the Committee on Commerce, Science, and Transportation, requested that we also review the implementation and oversight roles of all parties on the project. We share these concerns about the collapse, and we are committed to supporting the Department’s efforts to improve transportation safety. Accordingly, the objective of our audit will be to assess whether the Florida International University pedestrian bridge met Federal and DOT requirements for the TIGER application, approval, and grant agreement processes.

References[edit]

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^ Jump up to:a b Shapiro, Emily; Doom, Justin (March 16, 2018). "Bridge collapse death toll likely to rise as investigators sift through rubble". ABC News. Retrieved March 16,2018.
Jump up^ "FIU Bridge Collapse: Four Dead, 8 Cars Trapped Underneath". CBS Miami. March 15, 2018. Retrieved March 17, 2018.
Jump up^ Ashaboglu, Selin (March 15, 2018). "Miami Pedestrian Bridge Collapses". ARCHITECT. Retrieved March 15, 2018.
Jump up^ Rodriguez, Marybel (August 22, 2017). "Pedestrian Bridge In The Works For FIU Students To Cross Busy 8th Street". CBS 4 Miami. Retrieved March 15, 2018.
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^ Jump up to:a b Harris, Alex (August 24, 2017). "A walking bridge for FIU students is coming soon. But it's too late for one student". Miami Herald. Retrieved March 15, 2018.
Jump up^ Gomez Licon, Adriana; Replogle, Josh (March 16, 2018). "Tearful families wait as bodies remain under failed bridge". Associated Press. Retrieved March 17, 2018.
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Jump up^ General Plan and Elevation|url=https://www.ntsb.gov/investigations/Documents/HWY18MH009-3-16-press.pdf |access-date=March 23, 2018|publisher=Florida International University}}
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Jump up^ Viglucci, Andres (March 10, 2018). "'Instant' bridge aims to make a dangerous crossing safer for thousands of students". Miami Herald. Retrieved March 15,2018.
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^ Jump up to:a b c d e f MCM. "MCM design-build proposal" (PDF). Retrieved March 18, 2018.
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Jump up^ News, F. I. U. (March 10, 2018). "#DidYouKnow the new pedestrian bridge that will connect our FIU and the @CitySweetwater is the first in the world to be constructed entirely of self-cleaning concrete? #WorldsAheadpic.twitter.com/lQVJh09Pv2". @FIUnews. Retrieved March 17, 2018.
Jump up^ Jansen, Bart (March 16, 2018). "Miami bridge that collapsed was a truss design, despite the cosmetic tower, support cables". USA Today. Retrieved March 17,2018.
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^ Jump up to:a b "Rubio says engineers were tightening loose cables when the FIU bridge collapsed". miamiherald. Retrieved March 17, 2018.
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Jump up^ Cochrane, Karen (March 16, 2018). "Pedestrian Bridge FAQ" (Press release). FIU. Retrieved March 17, 2018.
Jump up^ Svrluga, Susan; Siddiqui, Faiz (March 15, 2018). "New pedestrian bridge collapses at Florida International University, crushing eight cars driving underneath". The Washington Post. Retrieved March 15, 2018.
Jump up^ Batchelor, Amanda; Suarez, Carlos (March 15, 2018). "Several dead after newly-installed portion of FIU pedestrian bridge collapses". Local 10 News. Retrieved March 15, 2018.
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Jump up^ Mazzei, Patricia; Robles, Frances; Dickerson, Caitlin (March 16, 2018). "They Were Heading Home, to Lunch, to Work. Then a Bridge Came Crashing Down". The New York Times. Retrieved March 18, 2018.
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External links[edit]

NTSB Chairman Robert Sumwalt briefs media on the Miami, FL bridge collapse investigation, March 16, 2018, NTSB
Computer animation showing the assembly of the FIU pedestrian bridge, MCM - Munilla Construction Management
Rendering of FIU Pedestrian Bridge, MCM - Munilla Construction Management
Time-lapse of the FIU-Sweetwater UniversityCity Bridge construction, Florida International University
An initial rendering of the bridge. Actual location is an opposite side of Southwest 109th Avenue, Florida International University
Accelerated Bridge Construction, University Transportation Center, Florida International University

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