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Miami Pedestrian Bridge, Part XV
4

Miami Pedestrian Bridge, Part XV

Miami Pedestrian Bridge, Part XV

(OP)
Please allow me to continue the previous thread (Miami Pedestrian Bridge, Part XIV) as the prior is getting unwieldy.

For those who have lost track of the discussion, my summary is that we have most recently moved into a discussion of the progression of nodal region deterioration of members 11, 12 and the deck as it pertains to the physical placement of the structure in its permanent location, then detensioning of PT rods in member 11, and then prior to retensioning of same. Epoxybot was able to connect the timeline of texts sent by Kevin Hanson inquiring of necessary supplies prior to detensioning with the timestamp on photos indicating significant deterioration prior to detensioning. I was reviewing documentation trying to narrow down on this timeline to confirm this critical detail with the implication that analysis contributed to the NTSB review has conflated events and attributed them to post-detensioning occurrences thus leading further analysis astray.

I have also posted what I consider evidence of compression failure of member 11 as the leading event of the collapse immediately after completion of retensioning PT rods in member 11. This includes questionable reinforcing design and deformation patterns in exposed rebar post-collapse.

To forward my own hypothesis, it is that two failure mechanisms were at play, one was the nodal region degeneration, and the second was the member 11 degeneration as it came into the nodal region. Although they played into each other, the weaker nodal region allowed the deck to detach from the node but the structure was able to rely on the connection of the diaphragm with the repurposed member 12 (i.e. a connection not including the deck). Meanwhile, the demand on the flawed member 11 grew and the structure collapsed when 11 failed just above the node.

P.S. With this new and more nuanced timeline, it allows the identification of three significant events to member 11:

1 - Overloading upon removal of shoring followed by release when mounted on transporters,
2 - Overloading upon setting on piers followed by release with detensioning,
3 - Overloading upon retensioning of PT rods followed by collapse.

RE: Miami Pedestrian Bridge, Part XV

(OP)
Testimony from DENNEY PATE of FIGG Bridge Engineers, Inc. (See NTSB Docket HWY18MH009, 21 FIGG Interview Transcript) indicating that he was aware of likely degradation of the structure from the time that FIGG representatives left the construction site after the move and prior to the detensioning of the PT rods in member 11.

Page 260


Page 261


Page 262 and 263


Page 264

RE: Miami Pedestrian Bridge, Part XV

(OP)
See NTSB Docket HWY18MH009, 98 Bridge Factors Attachment 73 – FHWA Assessment of Bridge Design and Performance

In sections 3.5 SETTING OF BRIDGE SPAN ON PIER 2 SHIM STACKS and 3.6 POST-TENSIONING FORCE IN MEMBER 11 IS REMOVED (starting at Page 19), the FHWA report starts setting out the aforementioned timeline complete with imagery before suddenly making the following incongruous statement and worse, immediately relying upon it to forward a concluding hypothesis:

Page 24


Although it is possible that significant progression did occur post-detensioning, there is an evidence gap because the north face of the diaphragm was not photographed by Alexis Molina and member 11 was not photographed by Kevin Hanson. The evidence only indicates that significant progression occurred prior to detensioning yet this fact seems to be dismissed.

RE: Miami Pedestrian Bridge, Part XV

(OP)
Text scroll from Kevin Hanson's mobile device (HWY18MH009, 48 Bridge Factors Attachment 23 – Texts from Mr. Kevin Hanson of Structural Technologies on his mobile device dated March 10, 2018).

If we use the timeline from Alexis Molina's images that detensioning had not yet started even at 3:17 p.m., and that detensioning was completed at 6:30 p.m., that allows over three hours to complete the task and another thirty minutes (or so) to examine the north face of the diaphragm. On hind sight, Hanson may just be reiterating concerns of Molina.

RE: Miami Pedestrian Bridge, Part XV

(OP)
I can only wonder why the work crew would stay on the decrepit structure after the Molina images were taken unless they truly believed they were safe and their work necessary for the good of the project. All indications at that point were that the structure could collapse at any moment let alone five days down the road. At that time, the road was already closed and it wouldn't have been a back and forth to irritate the public. They could have just left the road closed and said "Sorry, we're just being safe."

RE: Miami Pedestrian Bridge, Part XV

(OP)
In the video, from 9 sec +22 to 12 sec +28 I can see someone inspecting the deck, Members 1/2, and 11/12 while there is a lull in the activity on the canopy. Detensioning work appears to start on Member 11 at 17 sec +4 and finishes by 18 sec +4. This time frame does not seem to support gradual alternate release of the upper and lower PT rods. The work then shifts to Member 2 and completes by 23 sec +27. At the tail end of the video after everything appears to be tidied up, someone gets back on the man lift and goes up to inspect the north face of the diaphragm one more time.

I was not able to correlate the time stamps from the FIGG images (even though the time stamps are rough extrapolations from FIGG's presentation) with the video. Perhaps the cadence of the time lapse varies throughout but I'm not familiar with this aspect of video recording. The frames that I have identified above are accurate, I just don't know if I've assigned the correct time stamp.

It does seem that the video supports the above evidence of Molina and that Hanson was only the first to see the diaphragm because until detensioning was complete, the man lift was prioritized for that work.

RE: Miami Pedestrian Bridge, Part XV

(OP)
I've attached a cropped and zoomed video of the detensioning work on Member 11 (size 11.0 MB). The upper PT rod was detensioned first. If any work was done to detension a PT rod prior to Hanson requesting oil for the jack, and hence, Molina taking his photos, it was to the upper PT rod. Very little time was spent on the lower PT rod just at the end of the video. Again, I don't know the cadence of the time lapse or if it is steady through the whole video.

Early Frame


Late Frame


Animated GIF of the two frames



RE: Miami Pedestrian Bridge, Part XV

It's terrible that this project was badly designed and failed catastrophically... I thought it was quite an attractive structure... and likely one that will not be duplicated.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

(OP)

Quote (dik)

It's terrible that this project was badly designed and failed catastrophically... I thought it was quite an attractive structure... and likely one that will not be duplicated.

I agree.

I'm also disappointed in the NTSB/FHWA/OSHA reviews as the closer I look, the more amateurish the design seems and the more disappointing the reviews seem, at least so far as identifying the trigger for the collapse. That's not to say that their findings of responsibility were necessarily wrong. It is patently obvious that Member 11 failed in compression below the last full column tie and above the 11/12/deck node.

A few more images to illustrate the point. In the 3D perspective, I've erased the far side longitudinal rebar (west side and bottom face, including the lower PT rod), hopefully to reduce the confusion of how much rebar exists and to illustrate how vulnerable the upper surface of Member 11 is. Again, the skewed U shaped tie is practically useless.










):

RE: Miami Pedestrian Bridge, Part XV

Thanks for digging up Denny Pate's interview. It really is stunning that he was aware that the bridge was failing under its own weight but didn't seem troubled with what was happening. I suppose he chalked it up to the concerns of the Peer Reviewer, regarding the expeditious detentioning of the PT bars, perhaps not occurring quickly enough.

The PT Hydraulic Jack had a separate reservoir/pump. One of VSL's junior field techs during his interview, stated it was his & usually another junior field tech's job to mobilize the equipment to the location where work was to be done. So it is entirely possible, they had the jack mounted on the PT bar before they realized the reservoir needed hydraulic oil.

RE: Miami Pedestrian Bridge, Part XV

(OP)
The timing of the detensioning/jacking could use a definitive answer. I suspect they would have to give a little tug on the PT rod in order to release the nut. Is it possible that they engaged the jack before realising oil was required and if so did that trigger the cracking? Why did Molina do his inspection when he did? Did something raise concerns?

As I reviewed Molinas images some more, it became clear that the deck surface cracks that he caught at the corners of his images could only be part and parcel of the north face cracking later imaged by Hanson.

It all seems that the investigators were steering the investigation to a predetermined conclusion.

RE: Miami Pedestrian Bridge, Part XV

(OP)
Denny Pate appears rather defensive in his interview yet all the while admitting he knew what he knew when he knew it.

RE: Miami Pedestrian Bridge, Part XV

Just read Pate's interview and the interview of Jason Stauffer after Denney's. Is it a strategy to come into these interviews without any info so you can play dumb? The Stauffer guy could answer almost no questions including who provided him the loads to design the substructure. How do you not remember that or at least do enough refresher work to know that answer?

RE: Miami Pedestrian Bridge, Part XV

Quote (Rabbit12)

Is it a strategy to come into these interviews without any info so you can play dumb

Yes, it absolutely 100% is.

There are some people who won't email critical information because they don't want a subpoena-able record to ever be available. If you are under subpoena, anything in writing is obtainable.

Go read Manual Feliciano's interview where he states he was instructed after the collapse not to do any engineering evaluation or perform any calculations. I can almost guarantee that was direction all the way from the top of their leadership and/or legal counsel, to avoid any internal documentation creating a paper trail that indicated they were aware of mistakes that may have been made.

RE: Miami Pedestrian Bridge, Part XV

Quote (Sym.p.le)

I suspect they would have to give a little tug on the PT rod in order to release the nut.

They would- but done correctly, this would apply no more load to the PT rod than had been applied during the initial tensioning. If there was any creep of the concrete loaded by the rod, then the retensioning process would apply (very slightly) less strain then the first tensioning did. This all assumes correct use of the tools to tension the rods, which is difficult/impossible to confirm after the fact.

Quote (Sym.p.le)

Is it possible that they engaged the jack before realising oil was required and if so did that trigger the cracking?

Based on what I know about this type of equipment, no. The jack can't apply the correct tension to the rod if the pump runs out of oil; in that case they would need to either completely detension the rod to fill the reservoir and then re-apply tension, or they would need to stop tensioning and fill the reservoir in the middle of the process. Which of the two depends on the specific design of the hydraulic ram being used, and whether or not it has high pressure isolation valves on the supply connection. In either case as far as I can tell, the rods would stay within the limits of previously applied tension conditions (again assuming correct use of the tools which is impossible to truly confirm).

RE: Miami Pedestrian Bridge, Part XV

I agree with SwinnyGG's comments on hydraulic stressing.

I will just add a few comments:

As I recall from the collapse photographs back in 2018, the hydraulic system used for tensioning/de-tensioning the PT bars on this project was an electrically-operated SPX POWERTEAM pump with 4-way (3 position) valve and a double-acting center hole ram.

This was a very basic hydraulic setup where only one PT bar was stressed at a time, so no manifold was required for two or more rams. With only one pump and one ram, the valve would have been a 'tandem' center (as opposed to an 'open' or 'closed' center).

To add hydraulic fluid to a partially stressed tendons you would usually return the ram to zero (therefore re-engage the nut to the PT bar anchorage back to its previous load) fill up the reservoir and re-commence stressing, OR, park the pump valve in center HOLD position and add oil. You would NOT disconnect the hoses and remove the pump from the work area, fill up the pump and return it to service.

To de-tension a PT bar using a manual method, we usually conduct a lift-off test. The ram is first advanced by a distance greater than the bars elongation, under no pressure. With a bar wrench installed on the PT bar nut (inside the stressing stool) and whilst operator #1 is on the pump, operator #2 is applying a nominal force on the wrench, and as the ram takes up load the nut will partially turn when lift-off occurs, as indicated by operator #2. Operator #1 will be watching the pressure gauge and determine the magnitude of lift-off, and this value would usually be recorded. From there, the nut can be manually backed-off, upon which the ram will be returned and the PT bar de-tensioned.

RE: Miami Pedestrian Bridge, Part XV

(OP)
Just out as of 2021/02/02 the NTSB's Executive Summary (from the investigation web site).

Needless to say, I find their continued efforts to restate their flawed findings rather disappointing.

RE: Miami Pedestrian Bridge, Part XV

Quote (REPORT)

Revise local agency program agreements to specify
that when structural cracks are initially detected during
bridge construction, the engineer of record, construction
engineering inspector, design-build firm, or local agency
that owns or is responsible for the bridge construction
must immediately close the bridge to construction
personnel and close the road underneath; fully support
the entire bridge weight using construction techniques
that do not require placing workers on or directly under
the bridge during installation; and restrict all pedestrian,
vehicular, and construction traffic on the bridge until the
complete support is in place and inspected.

What is a structural crack exactly? (as opposed to a non-structural crack) Does anyone have a good definition? What sort of cracking would trigger this requirement to declare a structural emergency?

RE: Miami Pedestrian Bridge, Part XV

Quote (Tomfh)

What is a structural crack exactly? (as opposed to a non-structural crack)

This is why I emailed the Applied Technology Council, referencing this project and suggested they look at ATC-20 and prepare a program that addresses this point, regardless of cause. Too many engineers, it seems, simply don't see enough of the remedial side of concrete construction to know what they are looking at.

RE: Miami Pedestrian Bridge, Part XV

They should have defined it if it forms one of their recommendations. I had heard somewhere that it is cracking due to loads or support movement rather than the various other causes, but can't recall where.

The full report says "...structural cracking (beyond what sound engineering judgment considers acceptable)..."

It also quotes the Florida DOT specifications: "In general, nonstructural cracks are cracks 1/2 inch or less deep from the surface of the concrete; however, the Engineer may determine that a crack greater than 1/2 inch deep is nonstructural. In general, structural cracks are cracks that extend deeper than 1/2 inch. As an exception, all cracks in concrete bridge decks that are supported by beams or girders will be classified as nonstructural"

RE: Miami Pedestrian Bridge, Part XV

Those sort of definitions are what I’m wondering about. They’re extremely vague and boil down to saying that cracks in the structure are structural unless a structural engineer says they’re not. Which means what exactly? It still leaves open the question of what is a structural crack?

As an example - a regular flexural crack, or diagonal tension (shear) crack could satisfy all those definitions of structural cracking. They’re due to loads. They penetrate right through the structure. When does an engineer have to raise the red flag and deem them structural cracks?

RE: Miami Pedestrian Bridge, Part XV

Quote:

In general, structural cracks are cracks that extend deeper than 1/2 inch.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Miami Pedestrian Bridge, Part XV

Is there a critique of the NTSB's handling of this tragedy?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

Quote (Tomfh)

Those sort of definitions are what I’m wondering about. They’re extremely vague and boil down to saying that cracks in the structure are structural unless a structural engineer says they’re not. Which means what exactly? It still leaves open the question of what is a structural crack?

As an example - a regular flexural crack, or diagonal tension (shear) crack could satisfy all those definitions of structural cracking. They’re due to loads. They penetrate right through the structure. When does an engineer have to raise the red flag and deem them structural cracks?

Therein lies the rub, imo.

No matter how the NTSB or OSHA or ACI or ASME or ASHRAE or SAE or ANS or SPE or SAME or NAE or ASNT or NACE defines the term 'structural crack' ultimately, at some point, someone who knows what they're doing has to get boots in the mud and visually inspect, and make a call.

In my opinion the primary failure in this case was and is as much about communication and engineering ethics as it is or ever was about correctly estimating loads and accurately calculating their effects on a structure. That's still an 'engineering failure' very worthy of study and of action, but I don't believe it's something you can address completely with a change in regulation or definitions of terms.

RE: Miami Pedestrian Bridge, Part XV

Quote (Sym.P.Le)

Needless to say, I find their continued efforts to restate their flawed findings rather disappointing.

Maybe I'm missing something because I haven't studied every single post in this thread.. but are you really that confident that a group of us doing many months of internet sleuthing have a better explanation than the NTSB does after they've spent an equal amount of time reviewing actual physical evidence and interviewing all parties?

RE: Miami Pedestrian Bridge, Part XV

(OP)
Yes.

It's not so much the sleuthing as the openmindedness. The NTSB set out to explain how the node failed, full stop. They straight up say that is their sole objective in their report.

It's really hard to find obviously missing rebar (right next to the node) if you're not going to look for it.

Promise not to swear, but there are no hoop ties through the entire three feet of compression lap at the base of Member 11. It really is that obvious.

RE: Miami Pedestrian Bridge, Part XV

The initial crack photos of the day in question should have run up a 'really big' red flag...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

Quote (but are you really that confident that a group of us doing many months of internet sleuthing have a better explanation)


I would suggest that could be a real possibility...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

Quote (Sym.Pe.Le)

Promise not to swear, but there are no hoop ties through the entire three feet of compression lap at the base of Member 11. It really is that obvious.

Yeah... and the NTSB report states several times that insufficient reinforcement is a major component of the root cause of the failure...

I guess I just remain unconvinced that the failure mode you're pushing is undoubtedly correct.

RE: Miami Pedestrian Bridge, Part XV

Quote (dik)

The initial crack photos of the day in question should have run up a 'really big' red flag...

Totally agree. My point was that changing a definition of a term in the code doesn't resolve the issue of someone in the field recognizing a problem AND having the balls to make the call that no one wants to be made.

I'm not saying that the discussion about defining what a 'structural crack' is isn't valuable or necessary; it's both. But upgrading or clarifying that definition doesn't solve the real problem that resulted in the deaths here.

RE: Miami Pedestrian Bridge, Part XV

Viewing the pre-collapse photos, it always seemed clear to me that the node connection with the bridge deck was not cracked when it was inspected before the collapse. It was clearly already broken.

Brad Waybright

The more you know, the more you know you don't know.

RE: Miami Pedestrian Bridge, Part XV

(OP)

Quote (SwinnyGG (Mechanical))

... the NTSB report states several times that insufficient reinforcement is a major component of the root cause of the failure

That could almost be a direct quote from the report, but again, the NTSB's only frame of reference for that statement is a lack of rebar within the node. They then go on to state that you couldn't possibly put enough rebar in the node to remedy the issue which is of their concern (the cold joint). They are distracted with their preconceptions and that is why their explanations are continually oddball and contradictory.

By carrying on with their chosen path, the educational aspect of this disaster their review is completely lost. The discussion about cracks is a case in point.

RE: Miami Pedestrian Bridge, Part XV

Quote (My point was that changing a definition of a term in the code doesn't resolve the issue)


Just NTSB smoke and mirrors...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

Quote (...was not cracked when it was inspected before the collapse. It was clearly already broken.)


Concur wholeheartedly...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

"It will be interesting when the report on the collapse is completed, how many of the comments in this thread 'ring true'."

A quote from this forum, from nearly 2 years ago...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

Quote (Sym P.Le)

They are distracted with their preconceptions and that is why their explanations are continually oddball and contradictory.

By carrying on with their chosen path, the educational aspect of this disaster their review is completely lost. The discussion about cracks is a case in point.

Well at one point you were posting heavily in agreement with their final analysis, and I have yet to see anything that warrants your arrogance. I don't really understand what your motivation is, to be honest. The path of their analysis leads to a pretty clear conclusion, while yours, to me, does not.

I don't know if it's the truth or not, as I don't know you personally- but your posts come across as determined to prove that the NTSB could not possibly be right, and that the answer to the question about what caused this bridge to collapse is, quote, 'patently obvious'. I submit that if the answer was as plainly obvious as you say, we wouldn't still be talking about it 15 (!) threads and just shy of three years later.

RE: Miami Pedestrian Bridge, Part XV

I thought the NTSB report was a little lacking, too. If "They straight up say that is their sole objective in their report." was the total objective they may have achieved that, albeit a little 'light' from my perspective. I don't not recall what their sole objective was.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

Reading through the interviews with the FIGG folks the NTSB seemed to focus a lot on the re-stressing of the node. A lot of the answers talk about how the bridge was supported after it was cast and before it was moved. Was the support after it was cast identical to the support over the roadway?

RE: Miami Pedestrian Bridge, Part XV

(OP)
I doubt that I've posted anything heavily in agreement with the NTSB final analysis. They apportioned plenty of blame to go around, which I have to agree with. Even at that, if an engineer can't spec a column or column rebar properly, can the construction crew bear much responsibility for building that design?

Aside from the usual concern of human tragedy, my attention was piqued when reports said that crews were restressing Post Tensioning elements at the time of collapse, and that sounded a little off. It sounded more ridiculous when I realized they were adding compression to a critical compression element (that by some estimation looked quite thin). I have never seen the idea of 11/12 sliding off the north end as a reasonable explanation, nor can I believe that a little bit of air chisel to a patch of interface less than four foot square, or "more rebar than can fit" should be an adequate solution to keep 900 tons from collapsing.

My curiosity kept drawing me back until I happened to make a connection (which I posted in Part XIV, 26 Nov 20 01:39) precisely locating the stub of Member 11 to the formwork/rebar photos and that's when I finally realized what I was looking for. That this explanation (compression failure) was hanging in plain sight for all these years does not lessen its validity. We all missed it even though we were all looking at the same deficient design drawings.

I realize that many are tired of me pointing to alternate explanations for what the photos show, but the pieces that I've put together add up. Unfortunately they are hit and miss with dead end theories and spread out over fourteen pages but the conclusion is unmistakable. The NTSB did not examine the adequacy of Member 11 because they already had their answer.

/end of rant, sorry

RE: Miami Pedestrian Bridge, Part XV

(OP)
The deck is sagging and moving south relative to 11/12 thus causing the diaphragm to twist. The heel of 11 was not capable of punching out the slab thus it is a very weak argument that somehow the toe of 11 punched out the base of 12. The NTSB essentially posits that the toothpick (11) when pressed against the 2x4 (12) caused the 2x4 to fail which in turn caused the tip of the toothpick to break off.

Does anyone have a take on how much 11 would compress longitudinally at its critical load (Edit: or should I say the critical load of the unreinforced concrete since the rebar was free to pop out)?

The interface shear cracks (below, fuchsia) become vertical, encompassing 12.

FIXED IT

NTSB's Executive Summary (from the investigation web site)

RE: Miami Pedestrian Bridge, Part XV

(OP)
NTSB Final Accident Report HAR1902, under Executive Summary, Safety Issues page xiv, "The investigation of the collapse of the FIU pedestrian bridge focused on the performance of the northernmost nodal region (11/12 node) of the 174-foot-long main span. The failure of this nodal region was the triggering event for the bridge collapse.

It's rather difficult to move away from that if you're focused on finding that conclusion.

RE: Miami Pedestrian Bridge, Part XV

Quote (It's rather difficult to move away from that if you're focused on finding that conclusion.)


concur... I think you've summed it up very nicely. The beginning of Part II is a great read...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

(OP)

Quote (Rabbit12 (Structural))

Was the support after it was cast identical to the support over the roadway?

Two primary differences to the support over the roadway: 1. the formwork provided continuous support under the diaphragms as opposed to the pads supporting the structure on the piers when over the road, 2. The structure slopes downhill to the south whereas it may have been cast horizontally level with a tilt built into 1 and 12 and the diaphragm ends to accommodate the sloped installation (at least that's my take on the drawings)

I think the interview questioning was aimed at examining the dependability of the first scenario.

RE: Miami Pedestrian Bridge, Part XV

For the slight tilt, I don't think it would make any difference.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

Quote (Sym P. Le)

"The investigation of the collapse of the FIU pedestrian bridge focused on the performance of the northernmost nodal region (11/12 node)

Yeah you keep saying that... but the north end compression cracks in member 11, which you keep talking about as the 'real' root cause, are shown in all the diagrams in the NTSB report that outline the failure.

Your whole position on the NTSBs perceived incompetence is that they evaluated the joint between the deck and the node and completely ignored the concrete three inches away in member 11. Their documents don't indicate that, at all. You seem determined to find any reason why they're wrong and you're right.

RE: Miami Pedestrian Bridge, Part XV

(OP)
If the NTSB failed to recognize compression failure in a concrete column, that would be a big problem.

RE: Miami Pedestrian Bridge, Part XV

Right... and they didn't fail to recognize it. This is why your point isn't clear.

RE: Miami Pedestrian Bridge, Part XV

(OP)
Can you point me to the documents where they identified member 11 failing in compression.

RE: Miami Pedestrian Bridge, Part XV

Quote (Sym P. Le)

Can you point me to the documents where they identified member 11 failing in compression.

It's very clearly stated in the probable cause defined in the full report. You can google it if you want.

Their conclusion is load calculation in the 11/12 nodal region - which includes the section of member 11 you claim they ignored - AND the interface to the deck. As in, they are identified as separate calculations which both contain errors.

RE: Miami Pedestrian Bridge, Part XV

I didn't think they had identified the actual failure member... have to take a gander again...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

(OP)
SwinnyGG, You are correct that I have erred in the use of the term "nodal region" as it clearly was a term coined by the review to include the lowest portion of Member 11. This, however, is an exercise in semantics as the review then narrowly focuses on the cold joint interface and attributes all of the visible deterioration to a failure at that surface. In turn, their calculations are narrowly focused on that surface.

What I am pointing to is a design flaw which I consider part of Member 11. I differentiate from the nodal block based on the rebar design which includes a cage that extends only about 8" above the deck or the height of the filet beneath 11. In Part XIV, I posted a sarcastic sketch illustrating the problem, and I will post a comparison here. No one in their right mind would consider the detail on the right side, yet this is essentially the detail used at the base of Member 11.

**** Note: Member 11 does not have any corner rebar ****


sketch from STRUCTURE magazine


The precise location of the failure in Member 11 is easily seen in the following photos. Note that the skewed U shaped tie (called for in the design) is doubled and the lowest full hoop tie is substituted with double U shaped ties. Unfortunately, Member 11 needs to carry 450 xxx tons through to the deck/diaphragm but the design is clearly inadequate. Calculations are not required and this clearly is not part of any design node, it is the bottom of Member 11 above the node.









RE: Miami Pedestrian Bridge, Part XV

Quote (Sym Pe)

No one in their right mind would consider the detail on the right side, yet this is essentially the detail used at the base of Member 11.

They used overlapping U bars didn’t they? Not great, but hardly analogous to a single U that can easily open.

RE: Miami Pedestrian Bridge, Part XV

(OP)
As seen in the rebar drawing photo, they did use overlapping U bars. I did the sketch based off the design drawing before I understood as built photos.

It's hard to make allowances for flaky designs with the results produced. I can see in the bottom photo that the two east side longitudinal rebars were sprung free along with the east top face longitudinal rebar. Combined with excessive spacing, this should be seen as something not to do. The substitution for the lowest full hoop seemed to work.

RE: Miami Pedestrian Bridge, Part XV

The U bars would have ripped open freeing the longitudinal bars during the collapse - yes?. Or are you saying that confinement failure of the longitudinal bars was already underway?

RE: Miami Pedestrian Bridge, Part XV

(OP)
The confinement failure was already underway. When the PT rods were fully retensioned, 11 failed in compression, springing the shorter exposed bar free while the rest, still fixed in the deck/12 are deformed.


For further thought.

left photo from gharpedia.com right photo from Alexis Molina prior to detensioning.

RE: Miami Pedestrian Bridge, Part XV

(OP)
The NTSB did not core the truss members for evaluation. Although all other testing confirmed the adequacy of the concrete, I felt that they were presumptuous in skipping that opportunity.

RE: Miami Pedestrian Bridge, Part XV

If 11 failing in compression was the instigator of the failure, the base of 12 would not have been driven north out of the deck, 12 would have failed in bending somewhere midspan, instead of remaining mostly whole as it was post collapse.

I don't believe that the configuration of the various components as they appeared after the collapse supports your theory.

The photo you've posted above makes it very clear that the node has already migrated north when that photo was taken; member 11 is cracked, but the totality of information is not consistent with a lack of capacity in the last 24" of member 11 being a singular cause for the collapse.

I honestly find it pretty ridiculous that you're leaning into this so hard. The photos don't lie, and they disagree with you.

RE: Miami Pedestrian Bridge, Part XV

(OP)
The cracks on the east and west faces could be complicated by the interface movement but I can't see the underside longitudinal cracking being affected by that movement at all. Another complication when making comparisons is that member 11 doesn't have any rebar in the corners whereas most examples, if not all, do.

RE: Miami Pedestrian Bridge, Part XV

The root crack at the base of 11 which runs east-west, and is open by 1/4" or more, cannot open unless the base of 11 has moved north relative to the deck. Cracks can only open if there is localized crushing (i.e. material is removed) or if there is relative movement between components on opposite sides of the cracks.

All the other cracks on 12, in the deck, between the deck and the node areas, are all consistent with the 11/12 node moving north. They are not consistent with compression failure in 11 being the sole trigger.

The video of the collapse also very clearly shows that at the moment of collapse, debris was blown north from the north side of the 11/12/deck joint, and that the canopy hinged at or near the 10/11 node. None of that is consistent with a compression failure in 11 until after the node has already detached, the collapse is in progress, and 11 is compressed dynamically by the movement of the other members.

Was 11 underdesigned? Without any doubt. But you seem to be ignoring everything else which was underdesigned.

RE: Miami Pedestrian Bridge, Part XV

SwinnyGG,
You cannot say definitely that the member 11/12 node moved north. There was relative movement between those members and the deck, but you are discounting the shortening of the deck. There was a lot of PT in that deck, which combined with drying shrinkage meant the deck had to shorten, and the frame members tried to restrain that shortening. The crack had initiated even before the frame was moved to the site.

RE: Miami Pedestrian Bridge, Part XV

2
(OP)
From the Security Video Factual Report (item 106 from HWY18MH009 Docket), the GoPro frames 21 to 24 offer the best indication of the events immediately after Member 11 retensioning was completed. Also the FDOT frames offer support. In both cases I've tuned up the colours to offer more clarity.

The GoPro frames are aligned vertically at the top of the west end of the pier and horizontally at the southeast corner of Member 12. This gives us the best opportunity to observe relative movement of concern. The scale has some issues since the camera platform was moving (increasing frames magnify the subject vertically and horizontally) but over 4 frames, this just needs to be taken as advisement. I've simply magnified the images for processing but have not rotated them or manipulated them otherwise because the vague detail deteriorates quickly.

Frame 22 gives the first indication of collapse and, of interest, in frames 22 and 23, Member 12 does not move from its base. In frame 24, Member 12 shows severe bending which I take to indicate that it's still firmly attached at its re-purposed base, it's connection with the diaphragm. Of further interest is the vertical decent of the banner graphic (or the 10/11 node at the canopy). These two observations offer proof that Member 11 is being reduced. From a fixed point, any other point along a hypotenuse of length 'c' that is following a downward vertical trajectory sees c1 > c2 > c3 etc. So we have all the proof we need to show Member 11 failed in compression.

To further follow my theory, Member 11 is also pushing out to the east and as it gives way, the structure descends. The canopy and slab both show indications of failure in frame 23, the canopy on its upper surface and the deck on its lower surface. Both are fully fractured in frame 24. Through these frames, Member 12 is held in its vertical alignment by the rotational trajectory of the north south portion of the structure.

As Member 12 is held in place and the slab starts to descend at the 9/10 node, the slab rotates the diaphragm off of the base of 12. Torsional failure is super efficient at turning concrete into small chunks, thus explaining the gaping void and surface spalling that wrapped around the base of the diaphragm.

And before I forget, the big blue arrow in frame 21 which the NTSB identified as a concrete blowout can plainly be seen as the bundle of pull cord or flagging that is hanging off of the corner of the deck. All of this has been covered before but I've reordered it to make my point.

Thanks.



RE: Miami Pedestrian Bridge, Part XV

Where's the breakage in member 12 where that video shows it bending, because there is no way it could bend that much without being damaged. The failure damage in member 12 is at the deck joint end, not in the middle. If you consider that it's a video artifact and that member 12 was actually relatively straight the bottom was moving north.

Also, I see a funny step artifact in the deck line below or just a little to the left of member 12 in those 2 frames.

Personally, I have yet to see picture or video evidence that proves what exactly happened either way, other than it seems obvious the failure started at the bottom of that 11-12 node.

RE: Miami Pedestrian Bridge, Part XV

(OP)
Barrel distortion in the lens or rolling shutter effect from the video camera do not negate the fact that the base of Member 12 had not gone anywhere significant by frame 24. They still support the fact equally as strong as an exquisite video system. You're suggesting that a propeller is not attached to an airplane if it looks weird in a video.

RE: Miami Pedestrian Bridge, Part XV

Sadly. the traffic lights hide the east edge of the deck and the crane hides the top of 11, but it is clear that the north end of 11 descends while the deck does not. The bottom of 11 must be damaged while 12 manages to remain.
PS the rolling shutter effect can be seen in 11 horizontal with the "bend" in 12.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XV

You completely ignored the question I asked and instead posted some stupid comment about a propeller and airplane. How about you answer how 12 could bend like that and yet not have any damage where it bent?

RE: Miami Pedestrian Bridge, Part XV

Quote:

LionelHutz (Electrical)20 Feb 21 02:21]...how 12 could bend like that and yet not have any damage where it bent?
It did not bend. What appears to be a bend is the the camera recorded the top half (of the frame) later than the bottom half. Please note that this camera artifact extends all the way horizontally across the frame.
Thanks

SF Charlie
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RE: Miami Pedestrian Bridge, Part XV

Quote:

In frame 24, Member 12 shows severe bending which I take to indicate that it's still firmly attached at its re-purposed base, it's connection with the diaphragm.

It not actually bending is why I don't agree with this. The angle of the canopy in frame 24 and the fact 12 stayed attached to the canopy after the collapse together strongly suggest the bottom of 12 was moving north, not still attached.

RE: Miami Pedestrian Bridge, Part XV

I agree that 12 appears tilted in frame 23, The canopy appears to till slightly before. We have after collapse photos that show damage to the 12 - canopy joint even if it did not separate.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XV

Quote (Sym P.le)

long post

11 is not translating purely vertically- it is rotation about the end attached to the node, along with the node.

Your own markup shows this clearly. The displacement relative to the north end is roughly linear between the sign you've highlighted and the node- clear indication of rotation.

Note also that the canopy and deck remain parallel through this rotation, and that 12 remains pretty much vertical the entire time. This cannot happen unless 12 is detached from the deck.

I'll say this again, in an attempt at clarity:

11 was without a doubt underdesigned. But a lot of other parts of this bridge were underdesigned as well.

Saying 'the node failure looks like a major factor but I think they ignored the contribution that compression failure of 11 at the root may have played in the collapse' is VERY different than '11 failed and caused the collapse, the NTSB has no idea what they are talking about'.

RE: Miami Pedestrian Bridge, Part XV

anything on the litigation of this collapse and anything on the 'real' lessons learned?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Miami Pedestrian Bridge, Part XV

1. Don't let academics determine bridge characteristics criteria without demanding they analyze the design in detail?
2. Don't let the marketing or Art departments dictate the design requirements!
3. Build trusses (or a least their joints) out of steel even if you make it look like concrete (if you can have fake suspenders, you can have fake concrete).

SF Charlie
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