Juggy done a bridgedozy

[buzby]

I say telescopic dampers I mean rigid staaays

They stays could well be puely to act as dampers for wind-  or loading-inducsd oscillations as it was supposed to withstand a category 5 hurricane.

[black_betty]

Not sure how I managed to read 'Judge Judy' in the headline the first time.

[Shoulders?-Stomach!]

Judgy done a Dredddozy

[madhair60]

Jungy done a psychedozy

[Replies From View]

According to a brochure on the university website, it was designed to withstand a Category Five hurricane and last 100 years

Rookie error.  Should always build bridges to withstand the next 100 years.

[Shoulders?-Stomach!]

Jungy done a psychedozy

Yesss

[Bhazor]

The real reason it collapsed? WOMAN

https://www.snopes.com/fact-check/was-all-female-responsible-bridge-collapse/

[bgmnts]

That's the first time i've read Snopes busting a myth I had no idea existed.

[buzby]

The real reason it collapsed? WOMAN

https://www.snopes.com/fact-check/was-all-female-responsible-bridge-collapse/

I saw some of that while I was looking for data on the structure, it was on the Reddit MGTOW nutcase pool.

Some posters over on the Eng Tips forum have put up a few good explanatory graphics.
The forces on the span while it was on the crawlers getting transported. The higer the forces in each member, the darker the colour Note that the arrows on the two end diagonal truss members point inwards - they are in tension, using the post-tension rods to basically hold the deck up during the positioning operation:

Here's what it would look like once placed onto the piers. Note that the lower deck is in tension (it was cast with pre-tensioned steel cables running through it to counteract this) and the upper deck is in compression, which is then brought down to the piers through the end diagonal trusses which are now in compression (arrows pointing inwards):

Once in position, there should have been no need for any tension to be on the post-tension bars on the north end truss (in fact, in the original design it didn't have any, so they were added during construction purely to support the end of the span to allow the position of the crawlers for the move to be changed). Any tensioning operation on that truss member would have just added an additional compression load to the concrete, which may have caused it to overload and fail.

They also did a good job of stabilising and cleaning up the dashcam footage:
A close-up view of the poor guys who were doing the post-tensioning

A frame by frame that shows it took about 160ms from the first indication of failure (a puff of concrete dust from the base of the final north-end truss member they were tensioning - either the end of the rod snapping off or the anchor plate in the concrete failing) to the shear failure of the decks. The projectile at 160ms is the post-tensioning rod and cylinder flying out of it's channel


There's some clearer pictures turned up of the final diagonal truss member too, which show how it failed.
In this one you can see it's connections to both the upper and lower decks have completely fractured:

and a closer view shows the concrete surface on the lower face has completely spalled off, exposing the reinforcing bars::

This member had 2 post-tensioning bars running down though it to the deck, one above the other. The spalling is on the lower side, which is the rod they were tensioning at the time of the failure. to me, that says the concrete has experienced a large asymmetric compression force (when the lower rod was being tensioned), which bent the truss enough to blow the outer face of the concrete off.

The thing nobody can understand so far is why they were putting additional tension onto a bar in a member that was already under a lot of compression, as it would just increase the compression load. If you need to loosen the post-tensioner nut at the end of the bar you usually have to tension the bar a little further to remove any clamping force so you can spin the nut, but it shouldn't be anywhere near the about of force to require the truss member to fail like it did.

[Isnt Anything]

Another brilliant post, buzby, thank you.

Once in position, there should have been no need for any tension to be on the post-tension bars on the north end truss (in fact, in the original design it didn't have any, so they were added during construction purely to support the end of the span to allow the position of the crawlers for the move to be changed)

How does that work, then ? Surely they couldnt have drilled a channel the whole length of the truss for the bar on the site ?! So would i be right in assuming that maybe these things are always cast with a channel, in case its final resting place in a structure requires a bar to be added ?

[buzby]

How does that work, then ? Surely they couldnt have drilled a channel the whole length of the truss for the bar on the site ?! So would i be right in assuming that maybe these things are always cast with a channel, in case its final resting place in a structure requires a bar to be added ?

Yes, it would have had channels cast in situ inside the truss member by inserting a foam core or plastic tube into the reinforcing steelwork, along with the anchor  and bearing spreader plates for the ends of the rods to act against before the concrete was poured. It means the transport plans must have been changed early enough in the design for them to include a means to counteract the cantilever effect on the north end of the deck from moving the crawlers inboard. One thing the investigators may be looking at is if the design of that member and how it tied into upper and lower decks was changed to cope with the reduced internal cross section from the channels and anchor plates being added to it (the NTSB investigators have apparently marked those areas with blue paint on the scene).

There's a press video out there from when the lift was completed with the operations manager saying one of the things they have left to do was release the tension on two of the post-tension rods at the top of the deck. That would have been the rods in that truss member on the north end, as once it was in position it would be in compression and so wouldn't need any tension adding to it. It does make me wonder if the reported cracking was around the points where that truss tied into the decks, and they were tightening the post-tensioning rods to try and close the cracks. The guys who work in that field are stunned that anyone would attempt to do any tensioning work with live traffic running under it though.

[Neville Chamberlain]

Fascinating stuff, buzby - I like a man who knows his concrete!

[Isnt Anything]

Thanks for the extra info, buzby.

The guys who work in that field are stunned that anyone would attempt to do any tensioning work with live traffic running under it though.

Absolutely and that, no doubt, is what the inevitable lawsuits will be targeting more than anything else, i would expect. Truly astonishing decision, especially with cracks already showing.

[Blumf]

...It does make me wonder if the reported cracking was around the points where that truss tied into the decks, and they were tightening the post-tensioning rods to try and close the cracks.

So we don't (publicly) know where the initial cracking was spotted?

Also, is it possible the work was being performed on the wrong member? i.e. miscommunication between engineers and work crew.

[buzby]

So we don't (publicly) know where the initial cracking was spotted?

Also, is it possible the work was being performed on the wrong member? i.e. miscommunication between engineers and work crew.

No as yet. There would have presumably been photos taken but given the consequences for the future of the design agency Figg and/or the construction contractor MCM (who teamed together to bid as a 'design & build' consortium) nobody's been inclined to leak them (if they exist, the NTSB will have subpoenaed them by now though). I'm pretty certain it would have been in either the final north end diagonal  (which is referred to as Member #11 in the original design drawings) or one of the decks around the area where the it joins with them.

In their latest update, the NTSB have said that at the time of the collapse, crews were working one of the diagonal members at the north end of the structure "applying post-tensioning force that is designed to strengthen the diagonal member.". Given that member would already have had a compressive load on it, adding post-tension to it would have meant adding to the compressive load, which is counter-intuitive. I can only assume they were trying to close up the cracks by cranking up the tensioning rod.

Here's a page from the design proposal that shows the configuration of post-tensioning rods in each diagonal member and also what tension should be applied to each rod:


All of the other post-tensioning rods in the other diagonal members would have been tensioned to their final specification once the concrete had reached a specific hardness, before the move had taken place and so usually wouldn't have needed to be adjusted (from the photos, it looks like the access points for the tensioning rods in all the other stay blisters on the canopy other than the ones at the tops of the two end members #2 and #11 have already been capped and grouted over).

The cutaway shows the configuration of the rods in the southern end diagonal members (Members #2 and #3). One of the rods was tensioned from below. The rods added to Member #11 were both accessible from the top of the blister.

As shown in the drawing, Member #11 originally didn't have any rods. The ones that were added were only required to be in tension during the move when that end of the deck was being cantilevered. If the rods in the other members had already been capped and grouted, it's the only one they could have been working on.

[Blumf]

Thanks. Seems like we're going to have to wait for the report for those last puzzle pieces.

[gib]

I saw some of that while I was looking for data on the structure, it was on the Reddit MGTOW nutcase pool.

Some posters over on the Eng Tips forum have put up a few good explanatory graphics.
The forces on the span while it was on the crawlers getting transported. The higer the forces in each member, the darker the colour Note that the arrows on the two end diagonal truss members point inwards - they are in tension, using the post-tension rods to basically hold the deck up during the positioning operation:

typo?

[buzby]

typo?

Yes, it should have been outwards (i.e. in tension, which the force from post-tensioning would then be acting against). I only spotted it after the edit timeout had expired.

[Replies From View]

How on earth does buzby know so many things?  He's essentially a one-man CaB.

[buzby]

The NTSB have put out a new press statement:

The investigative team has confirmed that workers were adjusting tension on the two tensioning rods located in the diagonal member at the north end of the span when the bridge collapsed. They had done this same work earlier at the south end, moved to the north side, and had adjusted one rod. They were working on the second rod when the span failed and collapsed. The roadway was not closed while this work was being performed.

It's also been discovered that in 2016, late in the design process, the Florida DoT requested that the pylon at the north side be moved 11 feet further north, further away from the inside lane of the road. This required the span to be redesigned to accomodate the increased length. It's possible that in doing so, Figg's engineers did not run all the calculations and simulations as thoroughly as when it was originally designed. It also was probably the reason why the transport plan had to be changed (if the pylon was nearer the road, the crawlers would be nearer the north end of the bridge), and necessitated the inclusion of the post-tensioning rods in Member #11.

The NTSB have also released some video (which since seems to have been taken down, as it possibly revealed too much detail) and images of their investigation at the site. In particular, the spalling on the face for Member #11 was likely not due to asymmetric stress, but due to the other post-tensioning rod being ripped out from the centre through the concrete and reinforcing bars as the deck collapsed (it's two ends are still connected to the spreader plates in the deck and the cable stay blister in the canopy). You can see it here, still attached at the deck end and sheathed in the plastic tube that was cast into the concrete to accomodate it:

Here's the other end, still passing though the remains of the top end of Member #11:

[Bhazor]

Well thats the problem right there. That tape measure is clearly malfunctioning.

[Johnny Yesno]

if the pylon was nearer the road, the crawlers would be nearer the north end of the bridge

I'm a bit confused. Do you mean 'if the pylon was further away from the road...'? It was 11 feet nearer to the canal than originally intended, right? Also, how come that didn't give them more scope to put the crawlers in the right place?

[buzby]

I'm a bit confused. Do you mean 'if the pylon was further away from the road...'? It was 11 feet nearer to the canal than originally intended, right? Also, how come that didn't give them more scope to put the crawlers in the right place?

The original plan had the centre pylon a few feet from the kerb on the north side of the road. The Florida DoT objected due to future plans to add an extra lane on that side, so the plans were changed to move the pylon right to the edge of the canal and lengthen the span by 11 feet.

As a result, the crawlers on the north end couldn't be placed near the end of the deck as was originally intended due to the extra 11 feet being over the pavement and soft verge, so the design of member 11 also had to be changed to add post tension rods to hold that end of the deck up during the move.

[Johnny Yesno]

The original plan had the centre pylon a few feet from the kerb on the north side of the road. The Florida DoT objected due to future plans to add an extra lane on that side, so the plans were changed to move the pylon right to the edge of the canal and lengthen the span by 11 feet.

As a result, the crawlers on the north end couldn't be placed near the end of the deck as was originally intended due to the extra 11 feet being over the pavement and soft verge, so the design of member 11 also had to be changed to add post tension rods to hold that end of the deck up during the move.

Ah, thanks. I got confused about which end of the bridge was the pivot point.

[Isnt Anything]

https://www.enr.com/articles/44210-lawsuit-claims-post-tensioning-triggered-bridge-failure

nothing much here that webuzby didnt already know but may fill in a few minor specifics

[Isnt Anything]

bumpy was looking for more insight on the recent Genoa bridge collapse and found this new update on the Florida one in this thread

https://www.enr.com/articles/44991-investigators-big-cracks-but-no-cause-yet-for-florida-bridge-collapse

no cause yet ?!  okaaay

[Bazooka]

These bridges have been getting away with it for way too long, who can even keep them in check?

[buzby]

bumpy was looking for more insight on the recent Genoa bridge collapse and found this new update on the Florida one in this thread

https://www.enr.com/articles/44991-investigators-big-cracks-but-no-cause-yet-for-florida-bridge-collapse

no cause yet ?!  okaaay

Fucking Hell.
Here's the link for the full NTSB report:
https://www.ntsb.gov/investigations/AccidentReports/Reports/HWY18MH009-investigative-update.pdf
It's pretty clear that they were now tensioning the rods in Member #11 in a desperate attempt to stop the bridge collapsing. How someone can have inspected those cracks and not immediately called for the area to be evacuated, never mind working on it without stopping the traffic beneath, defies belief. It's a  criminal act, simply put. W. Denny Pate may be looking at jail time with regards to his comments about the severity of the cracks in the voicemail he left on the morning of the collapse.

Going back and checking the Eng Tips forum for updates before this latest one, it seems there was a document pack released by the NTSB in May containing photos and memos sent between the design and construction firms about cracks forming as soon as the formwork was removed after the concrete was cast and the initial post-tensioning was taking place (which was probably rue to inadequate amout of steel reinforcement in the joint between the diagonal members and the deck), which presumably got exponentially worse during the move of the deck and once it was under the load of it's own weight once mounted on the pylons.

By that point the concrete was so fractured it was basically only the rebar and PT members holding the north end of the deck together. The additional compression force by the post-tensioning operation to 'stabilise' the cracks was the straw the seems to have broken the camel's back.

[Dex Sawash]

These bridges have been getting away with it for way too long, who can even keep them in check?

[MojoJojo]

Sorry for the bump, but I want to respond to posts in this thread.

So the full report on this bridge collapse has just come out and "Well There's Your Problem" just released an episode on it which I half watched yesterday.

Yes, cable stayed. After I looked at the design proposal it said the trusses were supposed to be capable of supporting themselves (at least when unloaded, otherwise there's no point going to the expense of adding the cable stayed element)  and the cable stays were mainly to add additional defection support when the bridge was loaded, or for purely fro aesthetics

Apparently just decorative.

By that point the concrete was so fractured it was basically only the rebar and PT members holding the north end of the deck together. The additional compression force by the post-tensioning operation to 'stabilise' the cracks was the straw the seems to have broken the camel's back.

The timeline that I understood from my half watch was that some of trusses were put under tension for the move. These then had the tension released and it's then that the cracks were seen. The decision was made to put the trusses back under tension, to put it back in the state it was in before the cracks had appeared. Since they were "resetting" back to a previous state it was thought this was safe and didn't need a review process (or closing the road, presumably). This was wrong and re-tensioning the trusses caused the cracked areas to break completely.