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Experts cite explosive joint failure as cause of Florida bridge collapse

The collapse of the bridge in Florida was probably caused by an explosive failure of a key joint in the bridge’s concrete truss, structural engineers have told New Civil Engineer.

The 862t concrete bridge over a highway at the Florida International University (FIU) in Miami collapsed last Thursday killing six people. Investigations are now underway to determine the exact cause.

But structural engineers who have spoken to New Civil Engineer have studied multiple photographs of the scene and believe the collapse was caused by an explosive failure of the north end, bottom joint – a critical connection in the bridge’s structure.

In its final state, the FIU bridge was supposed to be a two span, concrete truss and cable stayed bridge – the main span over an eight lane highway and the backspan over a narrower waterway. However, at the time of the collapse, it was in a temporary configuration having only been partially constructed. Only the main trussed span was in place, with the backspan, central mast and cable stays yet to be installed.

The 53m concrete truss had been installed six days before its collapse. It had been built off site, wheeled into place and then lifted onto its supporting piers. At the time, the bridge appeared stable.

Florida bridge collape diagrams permanent and temporary

Florida International University (FIU) bridge collape diagrams permanent and temporary

Independent bridge consultant Simon Bourne said something would needed to have changed within the structure to have triggered a collapse.

“For a bridge which is sat there under self-weight, there’s something which has got to happen to cause it to collapse,” he said.

Florida senator Marco Rubio has said on social media that on the day of the collapse the contractor, Munilla Construction, was carrying out post tensioning of cables within one of the diagonal members of the bridge structure.

However, official investigators from the American National Transportation Safety Board said these reports had yet to be confirmed. Highway owner and operator Florida Department of Transportation (FDOT) also reported that it had at no time received a request to close the highway and was not aware of any scheduled “stress testing” of the bridge at the time of its collapse.

However, in photographs of the scene, a blue, post tensioning jack can be seen still attached to a bar which appears to run down the centre of diagonal member 10, the last diagonal closest to the middle pier at the north end.

Collapsed end of florida bridge close up of blue jack pa

Collapsed end of florida bridge close up of blue jack pa

A blue jack still attached to the end of a bar feeding into diagonal member 10 can be seen on the collapsed bridge.

Bourne said this would indicate the team was carrying out tensioning work at the time.

“I think they probably were carrying out jacking works,” said Bourne. “You only have a jack connected to the bar on for the few minutes you’re stressing and it’s still on in the collapsed condition. If they weren’t stressing it, it wouldn’t be there.”

It is this additional force being put into the diagonal member during the jacking operation that Bourne thinks could have caused failure of the critical end joint.

Florida bridge collape diagrams before and after

Florida International University (FIU) bridge collape diagrams before and after

In its temporary condition, diagonal member 10 (see diagram), was carrying half the dead load of the bridge. The joint at the bottom end of the member was critical as it had to transfer all of the load into a horizontal force in the bottom chord of the truss and vertical force in the supporting pier.

Bourne estimated the stresses in the concrete at this joint would have been within its limits, and noted failure of the concrete in compression was unusual. Therefore, he surmised the concrete in the joint may not have been properly compacted due to the high density of reinforcement needed to transfer the forces (the cross section of the diagonal is estimated to be around 700mm by 700mm).

This, combined with the additional tension from the jack could have triggered the joint to explode.

“The contractor looks to be stressing a bar which is only capable of putting 0.5MN into a member which is already carrying 8MN,” he said. “It’s not a significant increase, but if it was close to the edge of its capacity or the concrete hadn’t been compacted properly, then that could cause it to collapse.”

With the joint and diagonal member 10 failed, Bourne said there would have been nothing to carry the weight of the bridge. This would have then rapidly caused the whole top corner section to rotate and the bottom chord to collapse, as seen in the video footage of the collapse and photographs of the wreckage.

“In any event, the bottom node clearly exploded, in tri-axial compression, causing the rotation of the whole top corner section, and the fall of the bottom tensile flange,” he said. “Normally, concrete is much stronger in tri-axial compression with loads close to the cube strength being possible – so, that’s odd too,” said Bourne.

“Note that the tri-axial effects come from the diagonal 10-bearing reaction vertically, the bottom flange prestress longitudinally and the top compression in the end beam laterally.”

It is not clear why the jacking operation was being carried out as the member would have already been in compression. One theory is that the contractor was trying to put an additional compression into the member to close cracks which had appeared in the concrete.

Cracks in the concrete at the northern end near the middle pier were reported to FDOT by the bridge’s designer Figg Bridge Engineers two days before the collapse but not deemed to be of “safety concern”.

The exact location of the cracking is not known but, said Bourne, despite the diagonal member 10 being in compression, unexpected bending moments induced during the installation phase, could have caused one of its faces to crack.

Figg Bridge Engineers issued a statement saying it was stunned by the tragic collapse of the bridge.

“Our deepest sympathies are with all those affected by this accident,” it said. “We will fully cooperate with every appropriate authority in reviewing what happened and why. In our 40-year history, nothing like this has ever happened before. Our entire team mourns the loss of life and injuries associated with this devastating tragedy, and our prayers go out to all involved.”

Munilla Construction said it was “devastated” by the event and was doing everything it could to assist with the investigation.

“The new University City Bridge, which was under construction, experienced a catastrophic collapse cause injuries and loss of life…. ,” it said in a statement. “We will conduct a full investigation to determine exactly what went wrong and will cooperate with investigators on the scene in every way.”

Readers' comments (2)

  • You may wish to see the following video which I hope is genuine; but includes the suggestion that the temporary supports were not positioned as planned, increasing the force in one diagonal member.
    https://www.youtube.com/watch?v=JvmvFQKTrMY
    It includes a dash-cam video of the actual collapse, and casts some (confusing) light on why the diagonals were pre-stressed.

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  • This article seems to pre-date the NTSB press conference where they stated that the cable-stayed configuration was purely cosmetic and that the bridge had been designed as a truss bridge. (NTSB video https://youtu.be/XIqeSkdxPdM )
    In view of trusses' susceptibility to the single-point of failure mode of collapse, this form of construction should not have been chosen. The recent US history of the I-35 bridge collapse in Minneapolis should have been a reminder of this. https://www.ntsb.gov/investigations/AccidentReports/Reports/HAR0803.pdf
    The single-point of failure problem has been made worse by lack of ductility. There seems to have been a serious lack of reinforcing steel in this structure.
    (See this NTSB video starting at 1:30 https://www.youtube.com/watch?v=aeJKqojmHgY )
    I agree that "explosive joint failure" seems to be the most probable failure mode, but I think it was more likely to have occurred at the bottom end of the strut - in the zone of your diagram labelled "Critical Joint" - not at the top of the strut.

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