Miami Pedestrian Bridge, Part XIII 81

[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

Part VIII
thread815-440072

Part IX
thread815-451175

Part X
thread815-454618

Part XI
thread815-454998

Part XII
thread815-455746


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[Earth314159]

There is a pedestrian bridge over the airport parkway in Ottawa, ON that was originally designed as a cable-stayed bridge, but the designers called for pipes instead. The use of pipes was heavily criticized, the designers were fired, and the new design used cables. This happened about 6 years ago.

I would be interested in hearing what the criticisms were.

 

[Earth314159]

However they are doing it, member 11 gets a Pcr = 18375 kips. Yes - it says 18,375 KIPS.

It looks like the spread sheet is looking at Euler buckling (I suspect. It also says "Pcr"). That is why the "resistances" are so high. It is not looking at the axial compression capacity of the elements which is what is actually important. I found that #11 was over stressed in a previous post. They are essentially saying these are squat columns. The other thing that few people have pointed out is the tie dimensions/requirements don't appear to have been met. I don't use the US code, but I can't image that it would so different than other first world nations.

 

[human909]

I have watched some of the board meeting (about halfway through) and the one thing I can't understand is how did the FHWA's post collapse analysis have such a higher shear demand than FIGG's? Also fig had a model (2-span condition) which had higher shear demands, did they decide that model was no good? I mean if you had just used statics to determine the force at the node, I'm hoping that it would atleast be closer to the model that FIGG used (for FIGG's sake).

The question is more about how FIGG had such a low shear demand.

I believe the dead weight of the bridge was about 1,900 kips. Angle of the strut was 32 degrees. You do the math....

 

[Ingenuity]

A question with regards to peer-review of the NTSB report/findings:

The NTSB board meeting of 10/22/19 stated that that the FHWA did several analysis and capacity checks of the bridge. nodes, components etc, and I believe that NTSB engineers reviewed such, but was there a formal peer-review of the NTSB (and/or FHWA) report/s?
​

 

[hokie66]

Good question, Ingenuity. That should have happened, but their budget probably got in the way. And WJE was already working for the other side. But I don't think the final word is in. Sounds like a good research project for academia.

 

[bob33]

Earth314159
Google "The Saga of a Bridge - 2013 - John Sankey" and check Oct 15 for Buckland and Taylor report and Dec 3 for Delcan report.

 

[Earth314159]

Thanks Bob 33.

It is actually a fairly nice looking bridge. I will read in more detail. It sounds like it should be thread of its own.

 

[3DDave]

A peer review of a peer review? Weren't these reports essentially a peer review of FIGG's work?

If there is a technical problem with them then FIGG can certainly do the work to find them.

 

[Earth314159]

It is amazing that we talk so much about peer reviews but very little about how to minimize errors in the first place. There are no recommendations of other aspects to minimize errors or how errors may occur. What is it about human nature in engineering and the technology that we use that actually leads to mistakes?

 

[RFreund]

The question is more about how FIGG had such a low shear demand.
I believe the dead weight of the bridge was about 1,900 kips. Angle of the strut was 32 degrees. You do the math....

Yeah, I mean. I just need to know why! I don't get it. I'm trying to put myself in their shoes during design. I'm sure there was pressure to keep some preliminary dimensions and make things work and they had a couple different computer programs running the same analysis, but this is just too far off for that.
In the board meeting one of the members actually asked, why did FIGG use the model that had the lowest shear demand. The response was "it's unexplainable".
me - Nooo!!! there has to be a reason.

I might need to read through the interviews now. Maybe there is something there.

The most cringe worthy part of it all though is how they asked if the massive cracks were ok and FIGG said yes. They even supposedly checked the calcs. I mean at that point, don't you think, oh shit maybe the other model was right? I mean there has to be more to this...

EIT

http://www.HowToEngineer.com

 

[Tomfh]

The most cringe worthy part of it all though is how they asked if the massive cracks were ok and FIGG said yes. They even supposedly checked the calcs. I mean at that point, don't you think, oh shit maybe the other model was right? I mean there has to be more to this...

They would have known the cracks were not ok, otherwise why FIGG's sudden request to install big steel channels to “capture the node”. You don't throw a spanner in the works like that if you think it's ok.

Presumably they figured if the bridge hadn’t fallen yet that it would be ok for another day or two, during which time they could bluff their way by saying calculations show it's ok.

So they were walking a fine line, fingers crossed they’d make it across, but unfortunately not. If they hadn't decided to rentension the rods they just might have made it...

 

[Vance Wiley]

Consider this post - - - - - -

 

[Hiway.1]

Consider this post - - - - - -

Someone needs to investigate whether Figg withdrew their protest to cover up their design error on the FIU pedestrian bridge.

 

[Vance Wiley]

I am more focused on the thought that a failure of this bridge to perform as intended would negatively effect the lawsuit and possibility of an $800 million project and how much did that influence the decisions of March 15.
The decisions of March 15 are hard to accept as being engineering decisions.

 

[human909]

Presumably they figured if the bridge hadn’t fallen yet that it would be ok for another day or two, during which time they could bluff their way by saying calculations show it's ok.

Pretty much this I suspect.

Really it question of whether they were totally incompetant engineers, or less that perfect when it comes to the ethics of admiting a mistake. If nothing happens the latter is far less costly in terms of reputation and financial cost.

It is easy for us to judge now. Better to recognise the lessons and not repeat the mistakes of others.

 

[Vance Wiley]

NTSB Meeting Miami 1:19 into recording re Cold Joints.
Vice Chair asked about a % difference ( loss) in AASHTO preparation and the joints as constructed. Well, not that clearly asked, but that was the gist of the query.
Staff could not respond off the cuff - and missed a perfect opportunity to go into the difference in coefficient of friction and factor of 1.0 vs 0.6, which clearly is a 40% loss of capacity.
The discussion did not cover the fact that FIGG design was based on AASHTO "roughened to 1/4" amplitude" but instructions to the job were to follow FDOT specification, which is quite different. They did discuss whether the drawings required AASHTO 1/4" and the fact that the details did not show that requirement.
I would like to have seen a discussion that covered the fact that if the design were based on no intentional roughening of the joints, the joints would then require more steel reinforcing across the joint and could be made adequate to resist the demand.
I got the feeling the discussion was slanted to the benefit of MCM.

I have wondered if FDOT intentionally left off the requirement for the 1/4" amplitude, and in doing so are requiring a design with more steel reinforcing across the shear plane. I think that could provide a more predictable capacity.
Does FDOT accept the AASHTO 1/4" prepped joint with a mu of 1.0? Is the FDOT spec joint prep assigned a mu of 0.6?
I have assumed the FDOT spec was to provide weather resistant joints which would bleed less and be more durable.
Interesting that Bolton Perez basically recommended special attention to the joints but did not push a recommendation in that regard. Then FIGG back tracked from their design basis by referencing FDOT. Apparently FTGG did not know the difference.
Then later in the meeting, NTSB presses the point that the structure would likely fail regardless of the joint preparation, because the design was so deficient. Again letting MCM off the hook.

I am going out on a limb here but with the joint as intended by FIGG it would have 66% more capacity and maybe not have failed when it did. Had it lasted 6 months and the back span been completed, this joint might not have been the critical point. But there were other issues, and this may have been the most fortunate time. That offers no comfort to the families of those lost and to those injured.

 

[Tomfh]

[pre]If nothing happens the latter is far less costly in terms of reputation and financial cost.[/pre]

Yeah. It’s quite similar in many ways to lemessurier and Citicorp. The building was in trouble, they knew, and they chose not to evacuate the building and surrounds. They said a bit of steel strapping is in order but the building is fundamentally sound. Secretly he’d calculated it could fall in moderate winds. The wind stayed calmed, it didn’t fall, and they got the straps in in time, and he’s now considered Mr Ethics in action, because he saw a problem, got his straps in and fixed the problem. Had the tower fallen over before his straps went in then he’d be arch Villain.

 

[PA PE]

Cables vs. pipes: Since these members were not to be load bearing, maybe there was a concern that cables would sag and ruin the aesthetics.

A question: would daily thermal expansion/contraction in the pipes be cancelled by the corresponding vertical thermal movements in the pylon, to prevent cyclical loads on the top of the truss?

 

[Vance Wiley]

Good question. I should think the thermal mass of the concrete pylon far exceeds that of each faux stay, and the stays would heat and cool faster than the concrete.
The pylon dimensions are 5 feet X 6 feet at the bridge and taper along the height.
The computer programs used addressed thermal loads on the structure - I do not know if they modeled the stays also and if they considered thermal lag.
The same would be true of a cable stayed structure - does anyone know if cable stayed structures experience daily cycling of deck elevations? Due to the size of the pipes they have more exposed surface to weight ratio than cables, and they would likely also heat and cool faster than cables, so the effect could be more pronounced in this structure than the authentic cable stayed structure.
Thanks,

 

[fel3]

Vance....You design your structures to be half full???

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill