The report is on a pedestrian bridge at Florida International University, Uni Park, Florida, US. This bridge is a 174-foot-long bridge connecting university and student accommodation which is crossing Tamiami trail (US Route 41) with an estimated cost of 14.2 million USD. On 15th March 2018, this bridge collapsed causing 6 deaths, 8 injuries and crushed 8 vehicles.
Case Study and Research
Background
FIU footbridge was being constructed on a main road line which makes people easy to cross the big road without disturbing vehicle traffic, using the road. This bridge connects university campus and student housing towards the near areas of Sweetwater, FL, US. This construction drew an achievement of an accelerated construction method by FIU officials which was alleged to cut back risks for pedestrians and minimize traffic disturbance.
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The bridge is designed to be a single continuous concrete truss bridge with 31’8” wide and 16’ wide canopy. The main span started over from South pier to north pylon and back span located over a canal from north pylon to north pier. The overall height is measured 18’ whereas the height between deck and canopy is approximately 15 feet. The steel pipes from the sides connected to pylon gives a look of cable.
Nature of the Case
This looked and seemed to be a structural failure. After the collapse the National Transportation Safety Board (NTSB) didn’t blame anyone straight but formed an investigation team to detail all the issues. According to the initial two-page report released by the NTSB – “The cracks were reported and documented weeks before the collapse as marked consistent with the identified errors”. NTSB released a second investigative update stating that this chaos is caused by structural failure.
This is not a single level of failure but there is a sequence of issues which together caused bridge to collapse. The main issue considered to be the cause is overestimating the strength of critical section and underestimating the load of the bridge. Designers also underestimated the structural load on the north end of bridge while overestimating the strength. In this project the samples of steel and concrete met the minimum requirements specified in the project plan.
There are some of the indirect causes which mainly show the negligence of the management as the design engineer from FIGG Bridge Group noticed a crack at one end of concrete span and tried to report the same to the Florida Department of Transportation and left a voicemail as he did not think that as a safety issue. This voicemail came into limelight after the incident happened.
Collapse
On March 13, 2013 9:00 AM, some employee who works in FIU while waiting at a traffic light under the bridge listened a load ‘Whip Cracking’ sound. At the same time, officials from FIU and FDOT were in meeting with the design team. In this meeting, the design team explained how uncompromised they are with the structural strength of the bridge. In this meeting, the design team also said that there were no safety issues raised on the cracks which were observed.
On the same day at 1:47 PM, the northern end of the structure which was already installed slipped suddenly as the first diagonal member fractured, folded and then quickly the bridge collapsed onto the road. Just before the incident happened workers from VSL Company were working on PT on diagonal member support number 11. More tension was applied to the steel bar in the concrete diagonal member by workers. The structure member which collapsed was estimated to be weighing 851 tons.
Reasons for Collapse
Construction Joint and Shear Transfer
In a large structure, construction joints are the basic unavoidable structures. Design engineers should report these as per industrial standards. In the investigation it was reported that the construction joint for truss members 11 and 12 is not good therefore creating an evaluation for stress. The shear transfer between the two members was not same exactly as designed.
Redundancy
Redundancy is a critical factor in structural design. This is also considered by the industry. In this project the span was determinate and non-redundant. Forces in the members of span truss can be known by static and equilibrium equations with some assumptions which makes a structure non-redundant. In this structure’s design there were no multiple load paths to distribute load to the diagonal members instead there is only a single line of truss at center, this became so critical as even if one diagonal truss fail the whole bridge will collapse. After the collapse, the steel rods and tendons were taken to inspection and result showed clearly that design was given with underestimation of load.
Location or Drain Pipe
The key flaw seemed to be making a 12-inch circular cut at the center of the deck to give room to an 8-inch drain pipe reduced cross sectional area of deck and made impossible to install post tensioned tendons or reinforcement to take the horizontal load. There were some 4” and 4 1/2” Diameter plastic pipes near the columns. Which were the main causes for crack initiation.
Transferring Tensile Force to Deck
In the structure, there were no observable mechanisms to transfer horizontal tensile forces from diagonal section member 11 to deck. In this structure, the diagonal structural members 2 and 11 were the primary members to carry the majority of dead load for main span truss. After de-stressing the post tension bars in diagonal structure member 11, the axial compressive force is about 1321 kilopascals. This load can’t be transferred as there are no viable load transfer mechanisms. The remaining members also had loads which were to be transferred into deck and the post tensioned tendons from D1 to D6 are placed away from the junction of diagonal members and deck due to the location of drain pipe. Thus, made the structure weak and developed cracks. When the horizontal tensile forces increased to 1595 KPa due to PT steel bars in member 11 the bridge collapsed.
Conclusion
- The bridge is so big with bigger dimensions and there is only one supporting member at the center for this sort of structures it’s better to use steel truss rather than using a concrete truss.
- The design engineer did not take cracks as work-life safety issue which actually are.
- Consultant whom did the peer review didn’t check structural design of truss as required by standards of FDOT.
- The loads were underestimated so the structural design is not accurate to the actual s standards. This caused the cracks, which led to structure collapse.
References
- Jennifer Kay. (2018). Report Finds Key Design Flaws in Florida Bridge That Collapsed Bridge Collapse. Retrieved from https://www.claimsjournal.com/news/southeast/2018/11/19/287178.htm
- Mohammad Aruba. (2019). Investigation of March 15, 2018. Pedestrian Bridge Collapse at Florida International University, Miami, FL,. Retrieved from https://www.osha.gov/doc/engineering/pdf/2019_r_03.pdf
- Christopher O’Neil. (2018). NTSB Issues 2nd Investigative Update in FIU Bridge Collapse Investigation, NTSB Media Relations, Retrieved from https://www.ntsb.gov/news/press-releases/Pages/NR20181115.aspx
- Jansen, Bart (2018). Miami Bridge That Collapsed Was a Truss Design, Despite the Cosmetic Tower, Support Cables. USA Today. Retrieved from https://www.usatoday.com/story/news/2018/03/16/miami-bridge-collapse-suspension-cables-support-tower/431418002/
- Madison Park, Jason Hanna, Joe Sutton, Steve Almasy (2018). Engineer Advised of Cracking on Bridge Two Days Before Collapse, CNN. Retrieved from https://edition.cnn.com/2018/03/16/us/bridge-collapse-florida/index.html
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