In the early afternoon of 15 March 2018, a partially constructed pedestrian bridge at the Florida International University collapsed. It fell 5.6 m to the highway below and crushed or partially crushed 8 vehicles. Five vehicle occupants died, as well as one worker who was on the bridge at the time of the failure.
In October 2019 the National Transportation Safety Board (NTSB) released their report into the incident. While I’ve read many NTSB reports over the years, I’ve never read one quite like this.
It uses language like it was “just plain sloppy” and “every organisation absolved themselves of responsibility”. It talks about behaviour that was “disingenuous and unconscionable”. It describes how “not one of the organizations involved was willing to take the essential and unpopular step to call a halt”, even when the bridge was “screaming at everyone that it was failing”.
It says: “Engineering schools will use this as a landmark case study for years”.
In this series of articles, we will step through the failure, examine the events surrounding the collapse itself, as well as explore the organisational, regulatory and technical reasons for the tragedy.
This is the story of how all the protections we erect as an engineering and construction profession can be eroded, ignored and dismissed. This is the story of what happens when we forget why these protections were put there in the first place.
Only the main span of the bridge was in place at the time of the collapse. It was still under construction, but if it had been completed it would have also included a back span crossing the Tamiami Canal, as well as a pylon and steel pipes. The final structure was meant to resemble a cable stayed bridge.
I use the term resemble because the structure was, in fact, not a cable stayed bridge. And in the days following the incident this caused confusion: a true cable stayed bridge requires a pylon and cables to support the span. (The bridge span is supported by the cables, which in turn are supported by the pylon, which then carries the load down into the ground.) Given that only the main span of the bridge was in place at the time of collapse, questions were asked as to how this span was supported in the absence of the rest of the structure.
And the answer was that the bridge was only designed to mimic, as opposed to be, a cable stayed bridge – the main span was intended to span the highway without the assistance of cables, it was a simply supported structure. The pylon and cables, which in this case were steel tubes, did not carry any load. Instead they had two functions: 1) to improve the bridge’s dynamic performance (they increased the natural frequency of the structure), and 2) to make the bridge look more architecturally impressive.
This main span was a post-tensioned and reinforced concrete truss structure. The bottom of the structure was a concrete deck, the top was a concrete canopy, and down its centreline were diagonal and vertical truss members connecting the deck to the canopy.
A few points about this design are worthy of note.
The first is that the NTSB says truss bridges, constructed entirely of concrete, are rare. And there are sound engineering reasons for this. Concrete is good at carrying compressive loading – it’s incredibly strong when you try and squash it. (This is why concrete arch bridges are so common – arch bridges remain in compression, which suits concrete perfectly.) But concrete is very poor at carrying tensile loading – loading that tries to pull the concrete apart. In a truss bridge, some of its members will be in compression, while others will be in tension, and the members in tension need to be carefully designed to ensure the concrete doesn’t tear apart. In this bridge, reinforcing steel was designed into the members to carry the tensile load – in essence, the steel is now preventing the concrete pulling apart.
In addition to reinforcing steel, the bridge also had post-tensioning tendons included in some of its members. Post-tensioning tendons are fed through tubes embedded in the concrete, and once the concrete has cured, the tendon is clamped at one end and a jack applies tension to the other end. This tendon is now squeezing the concrete member into compression, and it keeps it in compression – even when tensile loads are applied to it.
The second point is that truss bridges typically have two rows of truss members running along each side of the structure. This provides some redundancy – if one member fails on one side of the bridge, there are other members on the other side that may carry load and prevent collapse. But this bridge had only one row of diagonal and vertical members – if one were to fail, total collapse could ensue.
The main span was initially constructed off site, and then on the night of 10 March 2018 was moved into position over the 8 lane highway. As part of this move, Members 2 and 11 (see figure) had been post-tensioned, but afterwards, as was planned, this tension was released – it was only required during transport.
Fast forward five days, to the day of the failure, and Member 11 was being re-tensioned by workers located on top of the bridge canopy.
The reason this member was being re-tensioned was that cracks had appeared in the bridge. These cracks were located at the joint where Member 11 and 12 joined the bridge deck. Member 11 was being re-tensioned to ‘pull’ this joint back together and close the cracks.
This re-tensioning would apply significant forces to the bridge, but despite this, the freeway was not completely closed. Traffic flowed freely in 6 of the 8 lanes.
What happened next was captured on a video camera mounted in the interior of a pickup truck. At 43 seconds and 881 milliseconds past 1:46 in the afternoon (13:46:43:881), the first sign of failure was evident. There appeared to be a blowout of concrete at the Member 11 and 12 deck joint. This joint failed catastrophically because of the forces being applied by the re-tensioning of Member 11.
Then just 165 milliseconds later the canopy fractured at the top of Member 11, and the span began to hinge downwards.
The span struck the freeway, with only Members 1, 2, 3 and 4 remaining intact.
The time was 13:46:44 and 310 milliseconds. The collapse had taken just 429 milliseconds.
What began as an exercise to close cracks, ended in collapse.
In Part 2, we’ll examine the cracks, from when they first became evident, right through to when they were documented and discussed by all key parties involved in the project.
We’ll follow how, day-by-day, they grew in size until they became more than 40 times the acceptable limit for cracks in concrete structures. And we’ll look at why, despite this, the project wasn’t halted.
Understanding these cracks, along with why no one did anything about them, is key to understanding this failure.
Courtesy of Miami Dade Fire Rescue