Miami Pedestrian Bridge, Part VIII 80

[JAE]

A continuation of our discussion of this failure. Best to read the other threads first to avoid rehashing things already discussed.

Part I
thread815-436595

Part II
thread815-436699

Part III
thread815-436802

Part IV
thread815-436924

Part V
thread815-437029

Part VI
thread815-438451

Part VII
thread815-438966



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[Sym P. le]

I'm curious as to the scrutiny the #11 PT rods and accompanying hydraulic equipment is getting. They were obviously up to the task of destroying what was left of the bridge.

 

[dik]

From Part 1:

We'll wait and see what the 'official' cause of the collapse was; hopefully it will be similar to this thread. It would be great to have copies of photographs taken of the 'crack', just to see how insignificant it was. The EoR was likely doing 'damage control' in the event others brought up the crack.

It will be interesting to see if the project goes ahead, and, if there is any 'political spin'. It would be nice to see if the funding is challenged.

I was really surprised that the collapse was 'instantaneous', without warning, other than maybe the earlier crack. The EoR will likely carry his statement about the insignificance of it to his grave...

Dik

 

[jhardy1]

Wow!

I may have missed it before (very possible, given that we have now extended to 8 threads of commentary), but I hadn’t appreciated that the photos of the cracks that we have seen for some time (e.g. scroll up to chris snyder’s post above, dated 11 July) were actually taken on or about 24 February – a couple of weeks BEFORE the bridge was moved.

I had been under the impression that these were the cracks that were mentioned in the phone calls AFTER the move (for a transcript, see bimr’s post above, dated 9 August) – but no, the cracks that Denney Pate was saying “we don’t see that there’s any issue there so we’re not concerned about it from that perspective” are the ones in the photos dated 13 and 14 March, as shown in the latest NTSB report:

https://www.ntsb.gov/investigations/AccidentReports/Reports/HWY18MH009-investigative-update.pdf

I’m surprised (but not totally gob-smacked) that they concluded the bridge was safe to move, given the cracking that they had seen BEFORE the move.

But I am absolutely astounded that anybody could possibly conclude that there was no safety issue after the move, with cracks like that?!

http://julianh72.blogspot.com

 

[hokie66]

Based on those photos, the structure had failed at least one or two days before it collapsed. Despite knowledge of that, traffic was allowed to proceed below. Beyond comprehension.

 

[thebard3]

This is beyond stupid! That may be harsh with the advantage of hind sight...

I think you're wrong about that (being harsh, I mean). A crack like that in my home foundation would keep me awake at night.

Brad

It's all okay as long as it's okay.

 

[wiktor]

I do not like to sound to harsh, but these cracks are beyond shrinkage (Dik - you were joking right?), or result of wrong sequence or lack of post-tensioning - for any engineer with a little common sense these were signs of imminent catastrophic failure. Whoever seen and inspected this bridge when cracked, should immediately stopped any intervention, post-tensioning or keeping this already failing structure over traffic lanes without immediate shoring.
It is a long overdue result of playing with the load factors (see my previous posts - and the link to an excellent article on the subject), stringent adherence to codes and design guides, but without slightest understanding of the meaning, and in general promotion of pushy but not so engineering-smart administrators to a position of responsibility.
The initial cracks are apparent result of the post-tensioning, and lack of strands at the center of the lower deck - as the structure cracked when it was still on the casting bed, prior to lifting for movement. When the structure was loaded with self weight, it cracked more - as cracked concrete has no capacity for shear - and all stresses at the #11 and #12 top and bottom nodes were carried by "undernourished" steel, but sized accordingly to the code, or design guide for pedestrian brides. It seems, that more rigorous analyses of the stresses in the critical nodes were not performed - or not understand for the implication for the safety of the structure, as this was very innovative design. Which brings another question - what was the role and input of the peer review duds - and were they informed about cracks?
I somehow understand the secrecy by NTSB in publishing these photographs (these are just beyond my imagination as these were sings of imminent failure) - but I will immediately suspend the usage of "Design Guide for Pedestrian Bridges", and seriously question the approach towards design for the shear stresses in the crucial nodes of non-redundant structures, by both ACI and ASHTO. It may save some lives down the road.
The report by NSTB is fine, finally publishing the evidence of the total neglect by all involved parties - because whoever seen this pictures, should ordered immediate shoring of the span, pending safe removal.

 

[hokie66]

I wouldn't call it an "innovative design". Maybe you meant "ignorant".

 

[jhardy1]

@wiktor:

You referred to:
"undernourished" steel, but sized accordingly to the code, or design guide for pedestrian brides [sic]
and
I will immediately suspend the usage of "Design Guide for Pedestrian Bridges"

I will bet $100 to the charity of your choice that when the NTSB report and other peer reviews are complete, the root cause has little (if anything) to do with the design codes and design guidelines, but are instead due to fundamentally flawed analysis and design which were not in compliance with those codes - or indeed, in accordance with undergraduate-level structural engineering principles.

The biggest mystery to me is how a team of apparently well-credentialed bridge designers and constructors got it so wrong, at so many points of the whole project - from the fundamental design concept of a faux-cable-stayed bridge, to design detailing of the concrete connections, to not recognising the significance of the cracking observed before the bridge was moved into position, to most baffling of all - not acting on the major cracking observed after the move.

There may well be cause for revision of the various codes to try to capture and prevent these sorts of systemic errors - but just how you can codify protection against "wilful ignorance" by qualified professionals is frankly beyond me.

When you asked "what was the role and input of the peer review duds" [sic], I first thought that it was an apt Freudian slip, but in retrospect, it would seem that the whole peer review process was indeed a "dud" in this case.

http://julianh72.blogspot.com

 

[dik]

Dik - you were joking right?

... could be...

The type of crack is a clear example of something overstressed. A shrinkage crack does not in any manner resemble what has been shown.

Dik

 

[Retiredat46]

Looks like Figg's insurers are trying to get out of paying:

https://www.law360.com/articles/1058149/insurers-want-out-of-coverage-for-miami-bridge-collapse

They say they weren't made aware of the lash-up between Figg and MCM for this project, and therefore have no responsibility to provide coverage. It's easy to see why they want out after reading this article about the insurance aspects of the collapse:

https://www.justinziegler.net/fiu-bridge-collapse-lawsuit-settlement/

It's interesting that the NTSB report cannot be used as evidence in a lawsuit, but can be used in negotiations for a settlement.

 

[dik]

Has there been any activity from the Florida's professional engineering association?

Dik

 

[JohnRBaker]

This even made the national news tonight, at least it did on the 'NBC Nightly News with Lester Holt'. They talked about the report being released and showed pictures of the cracks and the video of the collapse.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[Sym P. le]

To me the real tragedy is that no one needed to die on that Thursday afternoon. Regardless of any shortcomings of the design or construction process to that day, the imperfect structure stood no matter the improbability of that reality. Yet on that Thursday morning someone decided to play a deadly game of chance and no one had the mettle to stop the insanity. Someone knew or ought to have known and everyone knew or ought to have known that the activities of that day were reckless. Did the Engineer need to hold everyone's hand? The construction principal or site supervisor or foreman? The hydraulic operator? Who said "We shouldn't be doing this!" and who said "Never mind!"

The big players could have argued for fifty years whether they threw good money away on an absurd or whimsical project but no one needed to die.

 

[LittleInch]

Wow,

Some of those cracks are deep, 4-6 inches min when someone stuck their tape measure in.

The testing of the bars and concrete look like standard testing to be able to rule out

Snapping of the tendon
Excessive force applied by accident by the jacking ram
Poor concrete mix

But how those cracks could be essentially dismissed as not being a safety concern is bordering on criminally negligent. IMHO.

I think it is now pretty clear that the 11/12 joint either had already failed and the bridge was being held up by the stiffness of the beams or was so close to failure that it didn't take much. Fig 4 is possibly the worse showing the level of stress and movement between no 11 and the base.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TehMightyEngineer]

Well then, now that there's further preliminary official reports out I feel there's enough information for me to comment without indiscriminately disparaging the parties involved or without criticizing without sufficient knowledge (for me personally; you folks carry on, I enjoy reading all these comments. I'm being deliberately conservative in not commenting until now given how much public scrutiny this failure has received).

Knowing nothing else, those cracks should have junked the bridge IMO. A flexural, shrinkage, or other crack from casting, moving, or in-service stresses should be partial depth cracks of a hairline nature at worst. Plus, per most standards I've had to design pedestrian bridges to, and per the PCI design handbook which I assume applied to this project, tensile stresses that exceed the modulus of rupture and cause cracking on exposed members should be prevented during moving.

Without mentioning the likely design errors by the EOR, this is troubling from all aspects of the project.

I designed a slab a few years back to span over a trench at a farm. I goofed and forgot to check lifting and handling stresses on the panels for tensile rupture. The panel was completely safe and serviceable but did not meet PCI standards due to these hairline cracks that formed after lifting. The cracks were only 0.010" wide. This is standard industry practice for precast items where I work, this rejection was done on my call with no oversight.

If my cracked panel was a state project then this would have been similarly been rejected right away by the state inspector and I wouldn't have been able to brush it off even if I wanted to. Hairline cracks are not permitted without repair or at least a non-conformance report in our bridge work. I can only imagine that on this bridge there were government inspectors or at least some third-party there inspecting for the owner. This should have instantly been something that the owner and/or the governmental organization overseeing it should have been made aware of, and required a written report from the contractor/engineer on how these cracks should be addressed.

I can't believe that any third-party inspector or overseeing engineer stood by and allowed this to proceed. In my opinion, unless these were hidden from them, they need to share some liability in allowing this to proceed. The engineer should have been required to submit documentation showing that there was no structural issue, that this could be repaired, and that the bridge was not unsafe. I can't see how any of that was done, as it should have identified the impending failure and no work should have been done with traffic under the structure. If these cracks were hidden or their effects minimized in what the third-party inspectors saw then so be it; but assuming they were in the loop I'm very curious to hear what the other officials involved in this project have to say.

I wonder who was pushing for this bridge to go in more; the owner, the governmental organizations, or the EOR/contractor. Sounds like everyone was more than willing to turn a blind eye to this.

Regarding the cracks shown; those are not simple hairline cracks from a bad move of the bridge. That is a failure of the bridge. This is not looking favorable to any involved. I agree with these above who are wondering who gave the "okay" for this and who else was in the background screaming for this to be addressed properly. Could this be the challenger disaster all over again?

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[MrHershey]

Haven't commented thus far for similar reasons to TME. Don't know enough and didn't want to speculate. And still won't speculate.

But those are some big ****ing cracks. Those are so big I really don't want to even call them cracks anymore. All concrete cracks, it's not necessarily a bad thing. We expect most of what we do to crack for one reason or another, that's what the rebar is for. These are in a whole different realm than 'cracking' to me.

 

[epoxybot]

I thought one of the outcomes of the Loma Prieta Earthquake was the determination that if cracks are .25 inches/6.35mm or greater, the engineer had to be concerned about rebar dis-bonding and possible rebar deformation.

I went back to look at BDI, the sub-contracting firm who instrumented the bridge for moving. They were hired by Barnhart Crane and Rigging; the SPMT mover. Seems hard to believe they didn't pick up some unusual readings.

I would have thought an engineer would have wanted Impulse Echo testing done ASAP, to get a grasp of the extent of discontinuity resulting from cracking. Maybe Impulse Echo isn't appropriate to prestressed concrete, it that is the case then they should have ordered up Ground Penetrating Radar to map the cracking. It sure seems like they were making decisions in the blind.

As RickyTickyTavi noted, zooming in on Figures 1, 3 & 4 in the NTSB update, one can see faulting & suspect faulting, Vertically, Horizontally & perhaps in Rotation. Finally in Figure 2 we see the diagonal crack in #12 passing through the diaphragm into the #12 buttress column just above the drain pipe where it appears the column ultimately separated from the diaphragm during collapse. It is no small crack, consistently about 20 mils across the section.

A lot of the cracking looks to have taken place when #11 was in tension. I suppose the faulting could have been a result of a change in loading from tension on #11 to compression. If so, that too should have been a huge red flag. Who beside Barnhart Crane and Rigging was privy to the data collected by BDI?

 

[MatthewDB]

I wonder about the difficulties of anchorage to support the asymmetrical weight of a pure cable-stayed bridge and the concentration of the weight on the one central pier that would happen, given the close proximity to the canal.

One option would be to make the two halves equal in weight. Make the short span really thick to act as a counterweight to the other half.

 

[hokie66]

Reports about materials, all fine. Reports about cracks/breaks, terrible. Now for the report about the design, which should be terrible as well.

 

[JoshPlumSE]

I thought one of the outcomes of the Loma Prieta Earthquake was the determination that if cracks are .25 inches/6.35mm or greater, the engineer had to be concerned about rebar dis-bonding and possible rebar deformation.

I thought it was Northridge, not Loma Prieta. Regardless, the papers I've read which tried to tie crack width to RC performance were not very rigorous. Crack width means something very different for concrete structures designed with proper confinement and detailing to be ductile than it does to structures that were not designed to be ductile. Even then I don't think those papers would apply to PT structures like this bridge.