Ep 49 Florida Pedestrian Bridge Collapse
Engineering News – Florida Condo Laws (0:51)
This week's engineering failure is the Florida Pedestrian Bridge (5:50). The bridge collapsed (15:15) during construction with traffic on the roadway underneath. The collapse led to investigations (20:44) by the National Transportation Safety Board and the US Department of Labor.
Miami Herald Summary of Champlain Towers Collapse - https://www.miamiherald.com/news/special-reports/surfside-investigation/article256633336.html
Florida Pedestrian Bridge Collapse
Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole
And I’m Brian. And we’re both from Calgary, AB.
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This week in engineering news, update on Florida condo legislation.
This article is from mid march, so it’s a couple months old, but still interesting. And keeping with our Florida theme this episode.
The Florida Senate and House of Representatives were unable to agree on a bill that would require inspections of ageing condo buildings and mandate condo boards to conduct reserve fund studies.
We get it, it’s a balance. You want to give condo owners some flexibility in how they set fees and how they manage their building. But you also need to protect people from themselves. There is apparently a loophole that allows condo boards to avoid putting money into reserve funds to keep their fees lower, but then how does one plan for repairs with no budget. And everyone hates special assessments.
We also wanted to mention that the Miami Herald has a fantastic and interactive summary of Champlain Tower Collapse. We talked about Champlain a little right when it happened on episodes 27 and 28 as part of our news segment. We still plan to go back and cover it in a full episode once the formal report comes out. The Miami Herald summary has video and photos and audio clips and drawings, as well as written explanations. We’re going to put a link in the show notes. - https://www.miamiherald.com/news/special-reports/surfside-investigation/article256633336.html
Now on to this week’s engineering failure; the Florida pedestrian bridge collapse.
The bridge is located on a Florida University campus to connect student housing neighbourhoods in Sweetwater Florida to the campus in University Park – in Miami – that crosses Tamiami Trail and Tamiami Canal
The intersection at this location was identified as hazardous and one student was already struck and killed by a vehicle
Bridge design was overseen by the university not the Florida Department of Transportation
FIU was known for its accelerated bridge construction and attracted PhD students from all over the world for industry conferences and seminars
The National Transportation Safety Board (NTSB) found that FIU has no professional engineers on staff for this project and relied solely on the expertise of the hired engineers and contractors
98m total length with 2 spans and a faux cable stayed tower in the middle – was a mono truss bridge with self-supporting spans - no other design like it has been used
The concrete walkway deck makes up the bottom flange of a wide I-beam and the roof of the bridge acts as the top of the I-beam, with diagonal members between the upper and lower decks to make up the web of the beam, running down the middle of the bridge. So essentially, the bridge itself makes up one large I-beam. This allowed the bridge to be lower since the walkway wasn’t sitting on top of a structure, it was the structure itself, reducing the number of stairs up and down from the bridge.
The bridge was constructed in three separate concrete pours - deck, truss members, canopy - creating cold joints at the connection of each pour.
Important to note that the bridge did not contain redundant members, failure of an individual truss member would result in a collapse of the bridge.
The bridge was post-tensioned, which always makes for some interesting challenges, and used a new concrete recipe that was said to be self-cleaning. This created a much lighter bridge than would be under conventional methods.
The post tension specifications were different from when it was being moved into place vs its permanent position. There were even post tension cables that were only required for transport due to the span being cantilevered until the final piers were in place. This is because the bridge was longer than the transport device and the ends of the bridge were cantilever over the device while the span was being moved. To counteract this, trusses 2 and 11 were post tensioned for the move.
Was intended to last 100 years and withstand category 5 hurricanes
Construction started in March 2016 and the cost was $14.2 million USD
The bridge span was in place by 1130am on March 10th
Walkway surface was poured concrete and hardened before the truss braces were poured above it.
Truss members were connected to the deck by steel reinforcing rods embedded in the deck and in the concrete of the truss
Following inspection, the OK was given to release the temporary post tension cables on trusses 2 and 11. When they did this “it cracked like hell”. The cracks were in the area where the walkway contacts the support pier. These weren’t the only cracks, but were the most concerning.
On march 13th, the engineer of record was notified of the cracks and began working on a remediation plan. He also contacted the Florida Department of Transportation by voicemail, but they were away and didn’t hear the message until the day after the collapse. The engineer didn’t think the cracks were an immediate issue, but something that would have to be dealt with later.
At 9am on march 15th on the day of the collapse, an employee of FIU heard a “loud whip cracking” sound while waiting for a red light and sitting under the bridge.
A site meeting to discuss the remediation plan was also scheduled for March 15th at 9am – the engineer of record and another engineer, managers of the engineering firm and the contractor were present and used a lift to closer inspect the cracking
The results of the engineering inspection were that structural integrity of the bridge was not compromised and that there were no safety concerns. Boy were they wrong! They also didn’t want to address the cracks until the post tension was complete and the bridge was stabilised. They also wanted to expedite the construction of the pylon at the north end of the span
The contractor immediately got to work re-tensioning, but the engineer of record left the site and the post tension inspector was not available on short notice.
March 15, 2018, same day as the engineer and team inspected the cracks, @ 1:47 pm eastern - 53m longest section collapsed while trusses 2 and 11 were being re-tensioned
At the time of collapse, six of the eight lanes of traffic below the bridge were open - no request was submitted during bridge construction for a full road closure - school was on spring break, the canal span, access ramps, and the faux cable stayed tower between the two spans had not been built
Collapsed section weighed 860 metric tons
6 deaths (one worker, five motorists), 10 injuries, 8 crushed vehicles underneath the collapsed bridge
The collapse initiated at the cold joint between truss members 11 and 12 and the deck of the span; on the northern end of the bridge. When this tendon let go, it sheared the rebar in an adjacent vertical member. The collapse happened over a few seconds, the unsuspecting drivers sitting below had no idea it was coming and no opportunity to get out of the way.
Investigation & Cause
The National Transportation Safety Board (NTSB) initiated an investigative team almost immediately. The US department of labour also conducted an investigation.
The responsibility to identify safety issues was on the FIU design build team and more specifically the engineer of record. At no point were any concerns flagged. In fact the voicemail left on March 13th, two days before the collapse, had it been heard, noted there were some concerns that needed to be addressed but nothing from a safety perspective.
The official probable cause was the load and capacity calculation errors made by the bridge engineers in the design of the main span truss members 11 and 12 and their connection to the bridge deck. Contributing to the collapse was the inadequate peer review which failed to detect these calculation errors or the errors in redundancy. Further contributing was the failure to recognize the significance of the cracking and failure to obtain a peer review of the repairs to the cracks. And lastly, failure on the part of all involved parties to stop bridge work and close the street.
The concrete and steel materials were not a factor in the collapse.
The hydraulic jack used to post tension truss 11 was operating as expected
The bridge design was nonredundant, as we mentioned if any member of the bridge failed, the bridge would collapse. But then the engineers used poor judgement by labelling the bridge a redundant structure and using a redundancy factor of 1. So they treated it like a redundant bridge, except it wasn’t; leaving it more at risk than if they had called it non redundant from the start.
The bridge engineers made significant design errors in determining the load and overestimated the effects of the clamping forces resulting in a significant overestimation of capacity.
The area near truss members 11 and 12 contained non structural voids for the post tension cable sleeves and a drain pipe (damn plumbers) which further impacted the capacity in this area.
The rate of premature cracking should have alerted the engineers of an significant issue and progression towards failure
The initial peer review process did not follow Florida Department of Transportation requirements for a qualified, independent peer review. And then the repairs and re tensioning of member 11 was not subject to peer review.
Everyone involved is responsible to some degree. The rules were changed to allow pretty much anyone on this team to close the road if there was a safety concern, yet no one did.
Then US Department of Labour investigation concluded
The engineer of record failed to recognize the potential for collapse and imminent danger when inspecting the bridge hours before. The cracks should have initiated a re-shore plan immediately and the collapse occurred while contractors were re-tensioning tendons at the direction of the engineer of record
The cracks occurred due to deficient structural design
The cracks were growing in size daily, and the engineering of record was notified of this. But since he couldn’t replicate them in his calculations and wasn’t sure what was causing them, he didn’t think they were a safety concern.
The cracks alone should have closed the road below to allow re-shore to be installed until remediation could be completed.
The independent construction engineer and inspector failed to recognize the danger of the cracks and exercise their own independent professional judgement to sound the alarm.
The contractor deferred the decision on what to do about the cracks and the bridge to the engineering of record, rather than exercising their own professional judgement as well.
The evaluation of cracks and remediation work was not part of the original design and therefore should have been subject to peer review.
The peer review engineer didn’t check the structural integrity of the bridge during different construction stages and only reviewed the structure in its final state when all segments would be installed and complete.
The truss that failed was a non-redundant member, which the engineer of record should have known. Failure of this truss or similar other trusses could have caused the bridge to collapse, which the engineer of record also should have known. These two factors further support the notion that the engineer should have closed the roadway and installed shoring under the bridge until it could be fixed.
The NTSB also put our a list of recommendations to the following groups
Federal Highway Administration, Florida Department of Transportation, American Association of State Highway and Transportation Officials, and the bridge engineering firm.
Develop a requirement for concrete bridge structures to be designed with more reasonable estimates related to shear capacity and clamping forces
Strengthen the requirements and verification of qualified peer review, including a prequalification letter.
Revise local agreements to close the bridge and road underneath at the first sight of structural cracks.
Florida Department of Transportation personnel should monitor and inspect all bridge projects with uncommon designs
Add a section in the structures design guideline on redundancy in uncommon bridge designs
The roadway was closed until March 24th while they cleared the area and performed a full investigation
On May 6th 2020, the Florida Department of Transportation said they planned to rebuild the bridge, but with input from the national transportation safety board this time.
Demolition of the old bridge started in September 2021, from there they will re-survey the land and design the new bridge which is expected to be complete in 2025.
So there you have it, the university pedestrian bridge collapse. A completely preventable collapse, not only from the original design stage, but by several parties during construction. Yes, the engineer of record was at fault for the original design mistake and not recognizing the significance of the cracks. But everyone else had blood on their hands too for not stepping up and making their own judgement calls based on what was occurring on site.
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