In the March Floridian weather of 2018 investigators are picking through the results of a disaster.
6 are dead and 10 more injured but the only question on investigators minds is how does
a new uncompleted bridge fail in such a way that no one picked up until it was too late.
Well this is what we will be looking into today, the Florida International University
pedestrian bridge collapse.
Background
Our story starts with a safety concern…
Students crossing the 8th street and 109tg avenue intersection, numerous reports from
students at Florida International University have come in over the years of near misses
with traffic across the 8 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 sweetwater.
Sweet water was a popular place for students to find accommodation outside of the university
campus, as as such the crossing of the multilane highway is frequently used by both students
and facility members alike.
The main organizations to be involved in the proposed project: were Munilla Construction
Management (MCM), 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 Transportation (FDOT), did not oversee the design; instead this task was taken on
by the university itself.
But this unusual plan did on the face of it make 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.
This allows traffic to flow during all the bridge construction as it’s not being built
over the road itself.
The bridge was to be 320feet long and span the roadway and a canal that runs parallel
to the street.
This was formed of two sections of the bridge one 175 ft long, and a 99 ft long for the
roadway and canal spans respectively.
To tie it all together a faux tower and cable was intended for the structure to make the
bridge look like a cable-stayed structure.
The total structure length was further added to by an escalator and lift on the southern
end and a lift on the northern end.
The design of the bridge was to have an intended 100 year + life expectancy and could withstand
a category 5 hurricane.
Although looking like a cable-stayed bridge the structure's strength actually came from
its truss construction.
The bridge had a 32 foot wide deck with a 16ft wide canopy 15 feet above.
The canopy and deck were connected by a single row of concrete diagonal and vertical supports
in the center.
The canopy and deck were designed to act like an i beam.
With The concrete walkway deck acting as the horizontal bottom flange of a wide I-beam,
and the roof canopy was to function as the horizontal but narrower top flange of the
I-beam.
Having the walkway on the bottom of the truss allowed for less steps up to the bridg eand
due to using only 1 truss it was non redundant as the whole structure's strength came from
the single collection of supports.
. Where the bridge also differed from the more
standard steel truss design was weight, the new bridge was roughly 10 times heavier than
that of a more conventional design.
Also different was the concrete mix, which was intended to be self cleaning.
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 steel cable tendons.
Once the concrete has cured these tendons are tensioned and held in place with a lock
off wedge adding strength to the beam.
The deck and the canopy had Tendon cables that were permanently tensioned and while
the main span was still located in the staging yard before it even got installed in the bridge.
However the diagonal truss members, had two to four PT 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 destressed after the main span was in place.
The plan for the bridge was to have it pre built in a staging area next to the bridge
site then lifted into situ.
Construction began in 2016 but just over a year later on a Sunday in august 2017 the
death of student, alexis dale vindicated the need for a pedestrian bridge.
They were crossing the busy intersection at SW 8th Street and 109th Avenue like many students
do any day of the week.
Dale had joined the university in June and had lived on campus but decided to live in
tower 109 across the street.
In february 2018 the main span of concrete including the walkway, the canopy and diagonal
and horizontal members was cast, once it had cured inspections of the main span showed
some cracking around truss member 11 and 12 nodal region at the truss member 11s intersection
with the deck.
Cracking is not uncommon in concrete casting and this was to be monitored during the installation
process.
Thus the main span 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 10 March
2018.
///
The Disaster
Traffic on SW 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 +/-0.5 degree was set.
Beginning at 4:30 a.m. on March 10, the main span was moved via the SPMTs and placed on
the permanent pylon and south pier supports.
The movement was completed by 12:30 p.m.
During the operation the twisting tolerance was exceeded twice, once at 0.65 degree and
another of 0.75 degree 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 main spans diagonal members 2 and 11 were de tensioned
as part of the original plans and photographs were taken of the cracks at the 11th meber
to see if they were active or dormant.
On the 12th of march more photos of the cracks around the 11/12 nodes were taken and were
now worryingly getting larger.
Munilla 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 13th and faced with growing cracks MCM and FIGG Bridge Engineers decided that
re tensioning the 2 and 11 may lessen the cracking, but FIGG determined that the cracking
was not a safety issue.
On the 14th 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 11/12 node and was
on the pylon pier would be restored to its original tension pre installation of 280 kips
or 280,000 pounds.
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 engine sump or valve cover as to even out
the load during tightening.
If you have ever owned a honda and forget to tighten a 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 restressting, and ahead they went.
The re- tightening 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 the cars came to a stop at the SW 8th Street and 109th Avenue junction.
At 1346 the northern end of the bridge at the joint of the 11/12 supports blew out under
the applied tension on the tendons.
Within a faction of a second the bridge sagged at the northern pylon 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.
5 were immediately killed after the collapse and one would die in hospital, the victims
were Navaro Brown, aged 37 a worker on the bridge and Alberto Arias, 53, Brandon Brownfield,
39, FIU student Alexa Duran, 18, Rolando Fraga, 60, and Oswaldo Gonzalez, 57.
a further 10 were injured including passengers of the 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 13:47…. 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 to
Kendall Regional Medical Center, although one person self-transported to the hospital.
Investigation
What is perplexing is how could something so run if the mill as a footbridge fail so
catastrophically, well the NTSB would find out that the bridge was more complex in design
than it possibly needed to be prioritizing form over function, and that the warning signs
seemingly were ignored during the bridges installation.
The NTSB dispatched a15 person team to pick over the wreckage.
On the 16th march the Beuro in a press conference stated that tensioning was being undertaken
and that 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 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 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&2 and 11 & 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 1 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 redundancy factor of 1.00, which is commonly used for structures with redundant
load paths.
This hints that 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 re tightening 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 of acceptability when
discovered after casting in February.
Even after this disaster could have been averted… even after the bridge was in situ by stopping
and diverting traffic during the re-stressing 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 sate dot took a backseat and did not
discover any of the safety issues.
Aftermath
The first of the legal cases were brought On March 19, 2018, against FIGG Bridge Engineers,
MCM, Bolton Perez & Associates, the project's consulting engineer, Louis 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
report and to add more evidence Osha cited multiple contractors for safety violations.
On March 1, 2019, Munilla 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, that
would pay up to $42 million.
The remnants of the bridge's piers remained on the site as a gruesome reminder of the
disaster, and the risks to padetreans 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 starting 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
dot and ntsb.
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https://www.google.co.uk/amp/s/www.local10.com/news/2018/03/23/police-release-911-calls-from-fiu-pedestrian-bridge-collapse/%3foutputType=amp
https://www.youtube.com/watch?v=vxwprt-BwOA
https://www.osha.gov/sites/default/files/2019-12/2019_r_03.pdf