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Florida update | Engineers dismissed crack in bridge hours before collapse

@miami dade

Engineers had observed, discussed at length and ultimately dismissed the significance of a crack in a Florida pedestrian bridge just three hours before it collapsed killing six, it has been revealed.  

The meeting of bridge designer Figg Bridge Engineers and contractor Munilla Construction was revealed in a statement from Florida International University (FIU) on Friday. The statement details how the bridge design and build team had met for two hours on Thursday morning before concluding that the crack did not compromise the structural integrity of the bridge. The bridge collapsed three hours later.

“On Thursday morning (March 15, 2018), at 9am, the Design Build Team of MCM and Figg, convened a meeting at the MCM trailer, located on the construction site, to discuss a crack that appeared on the structure,” says the statement. ”The Figg engineer of record delivered a technical presentation regarding the crack and concluded that there were no safety concerns and the crack did not compromise the structural integrity of the bridge. This meeting lasted approximately two hours and included FIU and FDOT [Florida Department of Transportation] representatives,” it concludes.

FIU said the investigation to determine the collapse of the bridge was ongoing and that it was fully cooperating with all authorities to understand what happened.

Meanwhile FDOT has released its own statement clarifying that it had, “at no time, from installation until the collapse of the bridge”, received a request to close the entire highway.

It also said that it was not made aware by FIU’s design and build team of any scheduled “stress testing” of the bridge following installation and has no knowledge or confirmation from FIU’s design build team of “stress testing” occurring since installation.

Florida senator Marco Rubio has been using social media to claim that engineers had been stressing post-tensioning cables in diagonal members at the time of collapse. But official investigators from the National Transportation Safety Board said that this had not yet been confirmed.

FDOT has said that had this been the case then the road should have been closed to traffic before any work was carried out.

“Per standard safety procedure, FDOT would issue a permit for partial or full road closure if deemed necessary and requested by the FIU design build team or FIU contracted construction inspector for structural testing,” it said.

FDOT also released the transcript of a voicemail left on a landline on Tuesday,13 March by Denney Pate, Figg’s lead engineer responsible for the FIU pedestrian bridge project. The voicemail message confirms the presence of cracking near the north end of the span but was not heard until Friday, after the collase, because the FDOT engineer was out of office on site visits.

Says Pate: “Hey Tom, this is Denney Pate with FIGG bridge engineers. Calling to, uh, share with you some information about the FIU pedestrian bridge and some cracking that’s been observed on the north end of the span, the pylon end of that span we moved this weekend.

“Um, so, uh, we’ve taken a look at it and, uh, obviously some repairs or whatever will have to be done but from a safety perspective we don’t see that there’s any issue there so we’re not concerned about it from that perspective although obviously the cracking is not good and something’s going to have to be, ya know, done to repair that. At any rate, I wanted to chat with you about that because I suspect at some point that’s gonna get to your desk. So, uh, at any rate, call me back when you can. Thank you. Bye.”

FDOT also confirmed that a representative was at the Thursday morning meeting, but that it was not notified of any life-safety issues, need for additional road closures or requests for any other assistance from FDOT. It stressed that the responsibility to identify and address life-safety issues and properly communicate them was the “sole responsibility” of the FIU design build team, adding that: ”at no point during any of the communications above did Figg or any member of the FIU design build team ever communicate a life-safety issue. Again, Figg and the FIU design build team never alerted FDOT of any life-safety issue regarding the FIU pedestrian bridge prior to collapse.”

A statement from Figg said: “We are heartbroken by the loss of life and injuries, and are carefully examining the steps that our team has taken in the interest of our overarching concern for public safety. The evaluation was based on the best available information at that time and indicated that there were no safety issues. We will pursue answers to find out what factors led to this tragic situation, but it is important that the agencies responsible for investigating this devastating situation are given the appropriate time in order to accurately identify what factors led to the accident during construction. We are committed to working with all appropriate authorities throughout this process.”

ntsb bridge inspection

NTSB staff inspecting the bridge

Source: NTSB

The cable stayed bridge was being built in the city of Sweetwater by FIU as a safety measure so that students did not have to cross the busy highway to get to and from its Modesto A. Maidique campus.

The bridge was to be cable stayed with one central pylon. The cables were to support a 862t deck post tensioned prestressed concrete truss structure with a bottom flange serving as the walkway and a top flange providing cover from bad weather. At time of collapse on Thursday afternoon the 53m long deck structure had been installed over the main highway, but the back span, spanning over a river, the pylon and the cables were yet to be installed.

The $14.2M (£10M), 53m long main span was built offline using a method being dubbed Accelerated Bridge Construction (ABC). It was moved into position by a rig in just six hours on Saturday 10 March.

The bridge was designed by Figg Bridge Engineers and was being built by Munilla Construction company (MCM). Barnhart Crane and Rigging operated the self-propelled modular transporters that placed the bridge on its permanent supports.

UK bridge experts who have spoken to New Civil Engineer have suggested compression failure caued by poor concrete in the top flange could be a cause. The Guardian newspaper has obtained and posted CCTV footage that captures the moment of the collapse which appears to shows the failure beginning in the top flange, close to the central pier.

Independent bridge consultant Simon Bourne said: “It has clearly failed in an un-stayed condition, but this was also (very clearly) a condition that it must have been designed to accommodate. So, with the back span, tower and cables not even built yet, it must either be a compression failure in the top flange due to poor concrete or a temporary works failure related to one of the temporary bearings.”

“While the CCTV footage is not hugely clear, it does seem to show a possible failure starting in the top flange. But the location is quite close to where the central tower is to be built, which is odd, as the stresses would be relatively low there.

“Overall, it looks like the top flange is prestressed as well as the bottom flange. But globally at this stage, the self-weight is clearly compressing the top flange, but any prestressing is actually compressing the bottom flange and reducing compressions in the top. I would estimate that a truss of this nature would probably have higher compressions in the bottom flange at this stage, which then makes the top failure look more odd, unless there’s some poor concrete in the top,” he added.

“Collapses are invariably due to poor concrete in the highly stressed zones, or poor concrete anywhere, in fact. So, poor concrete does look to be the main culprit, but I wouldn’t rule out the fact that the whole span might simply have slipped off its temporary bearings.”

He added that it would not be unusual for the contractor to be stressing some more prestressing cables in the bottom deck at the time, and that such activity is always tricky and potentially dangerous, as the loads are enormous - hundreds of tonnes per cable). But he said that this rarely causes a collapse.

”Even though you can get cable failures on rare occasions, it never usually causes a collapse,” he said. ”So, I get back to the same basic issues - it’s either a major compression failure in the concrete of the span somewhere, or it fell off its temporary bearings.

Bridge designer Ian Firth said it was too early to draw any firm conclusions, but one area to examine would be how far the construction methodology had been scrutinised by the designers.

He pointed out that following the collapse of a steel box girder bridge under construction in Milford Haven, South West Wales in 1970, the investigation by the Merrison Committee of Inquiry recommended that designers should supervise construction. This rarely happens today and may be a factor here, he said.

“Maybe it was something in the construction stressing sequences. Some have said that stressing was taking place at the time. How was the bridge being stressed? Did the stressing system fail? Did the jack fail? When you stress a strand, you first take the load on a jack, and the temporary system needs careful checking,” he said.

Firth added Figg was a well-known firm in the bridge design community.

florida bridge during construction

Florida bridge during construction

Source: FIU

Bridge during construction

The ABC method is being advanced at FIU’s Accelerated Bridge Construction University Transportation Center (ABC-UTC). 

MCM said the bridge was still under construction and it would undertake a full investigation.

Florida bridge artists impression

Florida bridge artists impression

FIU has previously said that the bridge was the first in the world to be constructed entirely of photocatalytic “self-cleaning” concrete. When exposed to sunlight, titanium dioxide in the Italian-developed cement catalyses chemical reactions that convert surface pollutants to soluble compounds that are washed away by the rain, reducing maintenance costs.

New Civil Engineer technical editor emeritus Dave Parker said this unusual concrete could also be a factor if its properties were not properly understood.

Figg Bridge Engineers said in a statement: “We are stunned by today’s tragic collapse of a pedestrian bridge that was under construction over Southwest Eighth Street in Miami. Our deepest sympathies are with all those affected by this accident. 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.”

Florida bridge artists impression close up

Florida bridge artists impression close up

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Readers' comments (1)

  • The failure is definitely something to do with the crane, and a concrete compression failure in the top flange, at the truss node so my guess would be that the crane was taking weight of the structure off its temporary supports, increasing the effective span.
    If you zoom in on the dash cam footage you can see a couple of workmen falling by the crane and then the jib just swinging after the collapse.

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