How did Miami Pedestrian Bridge collapse?

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Dr. N. Subramanian

On March 15, 2018, a pedestrian concrete truss bridge in Miami, Florida, USA collapsed during construction. The failure of this bridge caused several casualties and raised serious concerns about the design and construction of the bridge, in addition to the use of accelerated bridge construction (ABC) technique. 

Fig. 1 Collapsed FIU-Sweetwater University City pedestrian bridge

A partially constructed 53 m long, 862 tonne Florida International University (FIU) pedestrian bridge collapsed on March 15, 2018, at about 1:46 p.m. This bridge, crossing an eight-lane roadway in the city of Miami, in Miami-Dade County, Florida, fell  from a height of about 5.64 m on to SW 8th Street (US Route 41), crushing eight vehicles, killing one bridge worker and five vehicle occupants. In addition, another employee was permanently disabled; four more bridge workers and five other people were also injured (see Fig.1).

The collapsed bridge was a single concrete truss spanning 53 m and placed over its piers just five days prior to the collapse. Work on the adjoining span of this truss over an adjacent canal (to make it a continuous bridge of span 88m after completion), access ramps, and faux cable-stay tower had not yet commenced (see Fig. 2). The concrete bridge was first cast at a nearby off-site location as per the Accelerated Bridge Construction (ABC) technique and then transported to its final location. ABC was chosen for this project as it provides minimal traffic disruption.

Structural details



F
ig.2 Elevation of the proposed Pedestrian Bridge at FIU

As shown in Fig. 2, this two-span pedestrian bridge was designed by FIGG as a continuous, two-span single truss concrete bridge with a main span of 53 m (cast at an off-site location) and a back span of 29.26 m (to be cast in-situ), and scheduled to be constructed in 8 stages (Ayub, 2019). This bridge had 9.65 m wide bottom deck and a 4.88 m wide canopy. Architecturally, the structure was made to appear like a cable stayed bridge with a central pylon. 

The cross section of this single plane central, open truss, comprised of narrower top chord that served as a canopy over the wider bottom chord, which was considered as the walkway (see Figs. 3 and 4). The height between the deck and the canopy was 4.55 m. The overall height was 5.49 m. The pylon was 33.22 m tall with inclined steel pipes connected to the top nodal points of the truss canopies of the main and back spans. The connection points were located in a concrete pad called blisters. The use of concrete trusses in bridges is rather uncommon because it requires pre-stressing of truss members carrying tension, as concrete is weak in tension.

The concrete deck had two-way post-tensioning tendons. The concrete truss members including the canopy were pre-stressed with high-strength steel cable and bars. The construction of the entire bridge was scheduled to be finished by early 2019. Hence to reduce the time, it was built using the Accelerated Bridge Construction (ABC) technique. The main span bridge superstructure, including deck and canopy, were precast near the actual site, but in a direction perpendicular to the axis of the bridge. 

During construction, the concrete deck, trusses, and canopy were cast separately and in a sequence, i.e., the deck and diaphragm concrete were cast first, then the truss and the canopy. Because the concrete batches were cast at different times, construction joints or cold joints were left at the interface between the truss members and the slabs. Finally the blisters over the canopy were concreted to provide connection to the future sloping steel pipes.  

After all the PT tendons (12 longitudinal strands and 65 transverse tendons in the deck and four in the canopy) and PT bars in the diagonals were post-tensioned, the main span was moved to the final location. It was transported, rotated by 90o, and placed on the bridge piers by two transporters on March 10, 2018 (see Figs 3 and 4). On March 15, 2018, 5 days after the relocation, the main span of the bridge collapsed onto US Route 41.

Diaphragm II experienced a blow-out of concrete at the junction of diagonal 11 and column 12 creating a hole. As a result, column 12 lost support over the pylon and failed with the top tilting approximately 80 degrees towards the south, as shown in Fig.1 (see also Fig. 5). The base of column 12 shifted a few feet towards the north but remained on the top of the pylon. The collapse of the canopy, diagonal 11 and the deck soon followed.


Fig. 3 Main span of the Pedestrian Bridge at FIU before, during, and after transportation

Fig. 4 NTSB diagram shows diagonal member 11 and vertical member 12 (NTSB, 2019)

Probable causes

After the bridge collapsed, the Federal Highway Administration (FHWA) collaborated with the NTSB to test and evaluate the construction materials collected from the collapse site, including samples of concrete, steel bars, and one of the post-tensioning rods. NTSB identified that a combination of the following factors caused the bridge to collapse: design errors, inadequate peer review of the bridge design, poor engineering judgment, and response to the cracking that occurred in the joint region which led to eventual failure, and lack of redundancy in the bridge design. NTSB concluded that the failure of the nodal region of truss members 11 and 12 triggered the bridge to collapse. 

These investigations resulted in several findings including the following (see NTSB, 2019):

  1. Concrete and steel materials used in the construction of the bridge were not defective and the hydraulic jack used to post-tension the steel rods in member 11 was operating as expected at the time of the collapse. The samples of concrete taken for testing showed satisfactory strength at or above 58.6 MPa.
  2. FIGG Bridge Engineers (FIGG) and the Engineer of Record (EOR), failed to recognize that the bridge was in danger of collapsing when they inspected it hours before the collapse. The concrete truss had developed numerous wide and deep structural cracks jeopardizing the integrity of the bridge. The EOR did not issue instructions for shoring the bridge at appropriate locations and closure of the SW 8th Street. At the time of collapse, the post-tensioning bars were being re-tensioned at the specific instructions of the EOR.
  3. The bridge had structural design deficiencies that contributed to the collapse during construction stage III. FIGG Bridge Engineers made significant design errors in the determination of loads, leading to a severe underestimation of the demands placed on critical portions of the pedestrian bridge; and significantly overestimated the capacity of the member 1/2 and 11/12 in the nodal regions. The cracks on the bridge occurred due to deficient structural design. FIGG Bridge Engineers should have considered all critical construction stages when designing the bridge and correctly determined the governing interface shear demands. But according to FIGG Bridge Engineers Inc., the construction joint between the deck and truss members 11/12 was not roughened by the contractors as required by standard construction specifications and hence is the main cause of failure.
  4. M/s Louis Berger was not qualified by the Florida Department of Transportation to conduct an independent peer review and failed to perform an adequate review of the FIGG Bridge Engineer’s design.  M/s Louis Berger were of the opinion that they were contracted to do the final check only and not the design check at intermediate stages. 
  5. Despite the admissions and the knowledge that the cracks were growing in size, EOR stated more than once that the cracks did not present any safety concern.
  6. The evaluation of the cracks by EOR, and his recommendation to re-tension the post-tensioning bars of diagonal 11, were not included in the original design and therefore should have been subjected to peer review.

The member 11/12 nodal region contained non-structural voids (four hollow vertical pipe sleeves and the horizontal drain pipe) within the concrete and this might have resulted in overstress and the subsequent collapse of the main span.

Cao et al., 2020 considered four important construction stages (pre-stressing, transportation, relocation, and re-tensioning) in their computer re-analysis of the bridge and found that the horizontal component of the re-tensioning force overcame the resistance of the joint and caused it to slide with respect to the deck. As the sliding progressed, dowel action between the deck and joint became fully mobilized, crushing and damaging concrete locally within the joint and the deck. This damage (of the cold joint, adjacent joint and deck concrete) produced more sliding leading to the ultimate collapse of the entire bridge. 

Conclusions

The collapse of the Florida International University (FIU) pedestrian bridge on March 15, 2018, which claimed six lives and injured 10 people, could have been avoided. The failure is attributed mainly due to errors of the designer, who failed to consider the loading and conditions of the truss bridge during the construction stage III, which resulted in simply supported condition, instead of the final continuous bridge. Moreover, the contractor did not roughen the cold joint as specified in standards, thus reducing the shear-friction capacity of the joint. In addition, though several cracks were found during construction and erection, which kept on increasing in size, all the people involved in the project neglected them and considered that they were not catastrophic.

This article is based on the report of the U.S Department of Labor (Ayub, 2019) amd the illustrations used in this paper are from the references cited.

References:

Ayub, M., Report on the Investigation of March 15, 2018 Pedestrian Bridge Collapse at Florida International University, Miami, FL, U.S Department of Labor, Occupational Safety and Health Administration, Directorate of Construction, July 2019, 115 pp.

Cao, R., El-Tawil, S., and Agrawal, A.K.,”Miami Pedestrian Bridge Collapse: Computational Forensic Analysis”,  Journal of Bridge Engg., ASCE, Vol.25, No.1, Jan. 2020.

NTSB Accident Report NTSB/HAR-19/02,PB2019-101363, Pedestrian Bridge Collapse Over SW 8th Street Miami, Florida March 15, 2018, 152pp. Accessed December 17, 2019. https://ntsb.gov/investigations/AccidentReports/Reports/HAR1902.pdf 

Party Submission to the NTSB-FIU University City Prosperity Pedestrian Bridge Construction Accident Miami, Florida, March 15, 2018, Submitted by – Figg Bridge Engineers, Inc. Sept. 20, 2019, 344 pp. https://dms.ntsb.gov/pubdms/search/document.cfm?docID=476566&docketID=62821&mkey=96877 

Author bio

Dr. N. Subramanian is an award winning author, consultant, researcher, and mentor, currently based at Maryland, USA, with over 45 years of experience in Industry. He was awarded with ‘Life Time Achievement Award’ by the Indian Concrete Institute for his contributions to Structural Design. He had also served as the Vice President of the Indian Concrete Institute and ACCE (India).

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