Miami Pedestrian Bridge, Part X 50

[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



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[TheGreenLama]

P107 of OSHA report determines shear capacity of joint as only 22% deficient. Further down, when comparing 11 with 2, they mention that they have included Member 1 & 12 reinforcement when calculating shear capacity. No where, however, do they mention specifically which bars they've included and excluded (for lack of development length). They could be a lot clearer here. I also think that it's debatable whether Member 12 steel should be included, and certainly not all Member 12 steel, as a good portion of the member lies outside of deck.

 

[TheGreenLama]

P110. I realize they are here focused on Member 11 when pointing out the deficiencies w.r.t. shear lag between deck PT and horizontal force from diagonal. The way it's worded, however, they make it sound like shear lag concerns are gone once pylon is cast. This is obviously not the case, as Member 2 will still have these concerns.

 

[JStephen]

Just finished scanning/ reading the OSHA report. One thing I notice is that OSHA seems to confuse design review at 30%, 60% and 90% of design with design review at 30%, 60%, and 90% of construction, which is not the same thing at all.

 

[Vance Wiley]

re OSHA Pg 110

OSHA states the 45 degree influence of PT each side of 11/12 in the deck leaves 11/12 outside the shear lag zone so the horizontal force is not restrained by the PT.
As I recall, "40YearsExperience" was addressing exactly that in his posts, and suggested reinforcing wrapping behind 11/12 and extending into the PT compressed zones.
Member 2 at the South End has a better chance of having some more capacity - the location of PT strands and the 45 degree lines are the same as at 11, but the column member 1 is 36" and flush with the end of the deck, so we gain a foot there (12 extends past the end of the deck), then member 2 is 36" deep and at a flatter angle so we pick up maybe 2 feet there. And the drawings call for 5-#7 hoops with the chance for proper embedment above the shear plane. But a dedicated calc is the only way to know. And how much development can we count on when clipping the compressed zone of the PT ?
However, the OSHA photos of the North end diaphragm show cracking like I think Pate was addressing in his PowerPoint show, so he was right to be concerned about that. It just was not the most critical thing to address at that point in time.

 

[Vance Wiley]

Transfer of Tensile Forces to the Deck, OSHA pg 110.
In members 2 and 11 the first PT is centered 1'-8" (+1/2") from C/L of structure. OSHA addressed the influence of this on page 110, as previously discussed.
The North span of 96 feet has the first PT at 4'-8" from C/L, meaning the end diagonals 14 and 23 are far from any PT zones. Looks like the same problem, just less load.

 

[jrs_87]

Force of collapse was energetic enough to cause minor cracks on face of thick north pier.

Chipping on pier edge appears to be directly related to punch through.

 

[Brian Malone]

I have been following this thread, highly intrigued by some of the comments by those that post and in seeing how the investigation unfolded. From the first reports I saw on the bridge collapse I could tell some of the critics of the bridge design and construction (YouTube, comments sections on news sites, etc.) would point to many non-important items, such as the principal owner of the design firm, FIGG, being a woman, construction was conducted by a minority-owned business, possibly there was some corruption in the design award process, etc., etc. I was hoping as a whole the engineering community could rise above some of this but hey we are human and humans are emotional - it is the way it is.

Well, enough said on that but what I am so amazed with is the OSHA report, in my opinion, has finally pulled the gloves off and said what needed to be said:
1. The design was lacking in structural strength.
2. The design did not provide redundant load paths.
3. The EOR had plenty of information and notification the bridge section erected was showing signs of structural failure.
4. The peer review system failed not only with the CEI (Bolton, Perez and Associates) but also with the "independent" review hired by FIGG - Louis Berger. Even, FDOT and FIU should have put more engineering horsepower into their review of the plans and ongoing construction.
5. The very experienced construction team, Munilla Construction Management, on site did not strongly speak up and call an alarm.
6. Many firms, entities, professional people (with very storied and prestigious degrees and licenses) failed to step up and be "Captain Obvious"; their years of experience and expertise were not put to use when it was most important.

So, how did all this go down? Did this relatively "simple" pedestrian bridge get a quick glance over for the calculations and analysis? Hey, it is "only" a pedestrian overcrossing. Did the hope and interests of FIU to promote their ABC engineering program cause their peer review to take a back seat to getting a landmark iconic bridge - even if the design is a risky concrete truss? Isn't there a term or theory for no one willing to challenge a perceived expert or group of experts? I see this not as a lesson for pointing a finger at a villian, but as a lesson for learning to not to get lulled into complacency.

Mr. Pate, is a licensed professional engineer and the FIGG team is composed of multiple PEs; they are very skilled and their record and past designs speak to that. They are very educated and experienced.
Ms. Figg, may not be a licensed engineer, but she has an engineering degree(s) and whether she is licensed as a professional engineer is not the point. She hires very capable people to do the actual engineering, probably without micro-managing them - give 'em the project and turn 'em loose. She continues to run the company her father started; more than likely with a great sense of pride and honor.
Bolton, Perez and Associates and Louis Berger were supposed to have used their engineering expertise to check the work of Mr. Pate and his team. Did they get lulled into giving the analysis a quick glance because of the reputation of FIGG and Denney Pate? The OSHA report calls out the Louis Berger review very pointedly as being insufficient and improper for ensuring load conditions for all phases of construction were reviewed.
FIU with their academic team should have looked more closely at the plans and the ongoing situation. But somehow they coasted and only asked light questions and did not show the engineering expertise that would be expected of a center of bridge engineering. Did they let the FIGG reputation blind them? Did FDOT engineering figure this is all covered by all the aforementioned experts and they wouldn't need to look too closely?
Even the construction company with the more often "boots on the ground" view didn't step back from the project and call for a halt until the cracks and failing structure were addressed. They totally flew by the numbers given by the EOR and since none of the other reviewers objected strongly maybe MCM chose to stay mum, also.

I'm taking this as a lesson for me - though I only design desktop type analyzers. For now on, when I present my calculations and designs for review, I will strenuously ask for thorough review and comment/criticism. This bridge failure has emphasized to me, as a designer/engineer there are times you can be so invested in your design you may miss the obvious and unintentionally precipitate a disaster!

 

[Vance Wiley]

Thoughts about a Prestressed Concrete Truss.
My career included some experience with prestressed concrete.
When still in college, the company I worked for designed an auto dealership in 1962 using 10 ft tees, and with parking on the roof. I did some drafting on that project. About 1966 I designed the additional floor to create a second parking level. In 1995 the School District wanted to use the building for an Adult Education Center, and I established procedures for injecting cracks, full scale load testing, and then the design to current codes for approval by The California State Division of Architect as a school. The remodel required taking down the PT in the topping slabs, creating a two story atrium with skylight, and restoring PT in the remaining sections.
We also did PT design for a small precast fabricator, and developed the necessary stressing frame and procedure for casting bonded prestressed box girders 4 ft by 84 foot length, on a riverbank.
Along the way I learned that prestressed structures are alive, and simple is the best. Strain compatibility, shear lag, walls that steal the PT forces and then crack themselves -all are problems to be addressed.
A concrete truss? I can imagine bigtime problems.
First concern is post-tensioning the deck, causing about 5/8 inch shortening in this bridge. From center out that is 5/16" at the base of 2 and 11. Neither of which really want to shorten, and so cracks develop at their connection to the deck. To minimize this, one could consider stressing the canopy concurrently, keeping the top and the bottom shortening balanced. Reducing the distance between nodes on the deck and the canopy would have caused the thing to "grow" in height, of course. And that would decrease the angles between the diagonals, with strains and possible cracking. Then there is the stressing of only the tension diagonals - how do we suppose the compression diagonals are going to react to the shortening of the stressed tension in the adjacent diagonals? Rotation at the canopy nodes, perhaps? Then there are the joints, with high compression forces under the anchor plates, sometimes crossing of PT, and the need for confinement reinforcing. Perhaps 40YearsExperience could look at this and make a recommendation, using Mohr's Circle. I am asking in earnest. There is a lot going on in these joints.
Faced with these obstacles, I recommend refusing the project.
In it's final stage, the structure will (and for a brief period did) appear simple - but that is deceptive, as has been proven.

 

[Vance Wiley]

The chipping could possibly be due to the impact of the overhanging portion of 12 as it was dragged over the edge.
I can't see any evidence of extra shims at the center, so I guess MCM did not get to that before the collapse. It is good that they were not working on that Friday afternoon.

 

[jrs_87]

Brian Malone, this is how it's done:

https://denver.cbslocal.com/2014/10/23/defective-commuter-rail-bridge-demolished-will-be-rebuilt/

 

[3DDave]

My experience is that reviewers are lazy/without support and only spot check the math as presented, not clean-sheet the assumptions, do their own work, and then look for discrepancies. The problem is that most of the reviewers aren't given the time and budget to do that clean-sheet review, so they shortcut. Most are little better than spelling checkers, complaining about small things.

 

[jrs_87]

Excerpt, page 4, NASA Systems Engineering Handbook (PDF). NASA. 2007. SP-6105

In summary, the systems engineer is skilled in the art and science of balancing organizational and technical in-teractions in complex systems. However, since the entire team is involved in the systems engineering approach, in some ways everyone is a systems engineer. Systems engineering is about tradeoffs and compromises, about generalists rather than specialists. Systems engineering is about looking at the “big picture” and not only ensuring that they get the design right (meet requirements) but that they [highlight #FCE94F]get the right design[/highlight].​

Emphasis mine.

This excerpt from NASA's handbook sums up my entire motivation for visiting this thread.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080008301_2008008500.pdf

 

[3DDave]

That's SP-6105, not 610s

 

[Pete_K]

Bolton, Perez and Associates and Louis Berger were supposed to have used their engineering expertise to check the work of Mr. Pate and his team. Did they get lulled into giving the analysis a quick glance because of the reputation of FIGG and Denney Pate?

When I read this it reminded me of the Boeing/FAA relationship as it relates to the MCAS problem.

 

[Sym P. le]

In response to Vance Wiley, it is not a small issue that the FIU bridge was a real time, real life experiment carried out on Main Street with thousands of unwitting participants invited to defy the odds by passing beneath the awesome structure. In any other circumstance, exhaustive, even destructive testing is demanded to ensure product safety before it is unleashed upon the masses.

 

[Settingsun]

Why does OSHA refer to the 2014 FDOT manual setting out independent review requirements instead of the 2016 version?

Does FDOT require 30% and 60% independent review as OSHA says? Looks like 90% and 100% only to me. The 30 & 60 are FDOT reviews, on a normal project where FDOT is the client?

The FDOT requirements place a lower standard on the 'constructability' review - not intermediate construction stages - than OSHA seems to think is the FDOT intention.

In summary, I read FDOT's requirements differently than OSHA. Louis Berger probably did its job of reviewing the completed structure and we'll never know if the completed bridge would have collapsed.

 

[LittleInch]

Following the OSHA report I decided to go back and look at a post I made on 31 Mar 18 when trying to summarise the whole thing with information at that time. Here is an updated version with additions in Red

Things we know we know
The Bridge fell down and killed 6 people and injured many more. Let's not forget this has affected lots of people who were in a 1 in 10 million chance of being in the wrong place at the wrong time.
The collapse happened very quickly and hence finding a root cause at this time with the limited public data available is next to impossible - Now rectified after multiple release of information
The collapse happened at the Northern end within the last bay containing members 10, 11 & 12(the end vertical member).
There was a crew working on the bridge doing something with the PT rods in member 11 - We now know they were tensioning the PT cables in an apparent attempt to close the cracks and prevent further movement.

There was a crack somewhere in the North end of the bridge
That crack wasn't deemed serious enough by the construction company to close the road underneath it. The main cracks were in the base of member 11 where it met the deck and in the diaphragm at the base of member 12. No one thought the cracks serious enough to override the opinion of the EOR that there was no safety issue.

The bridge was in structural terms a concrete span, not a cable stayed bridge (the "cables" were going to be mainly cosmetic and non supporting, but could have limited any vibration or bouncing)
The artistic design led to an asymmetrical supporting member design
The bridge was in essence a rigid concrete beam with some interesting features making analysis more complex
The second span on the North side wasn't yet installed
At some point the bridge span was lengthened and the north pier moved to allow for a future road widening
Moving the pier meant that the two transporters moved inboard from the ends ( the concept design) requiring special re-inforcement of members 2 and 11 to take account of loads seen during movement of the bridge only.

Things we know we don't know for certain
Well virtually everything else:
What the crew were actually doing at the time of collapse. See above - Now Known.
Whether the initial crack had anything to do with the collapse. Not only were the cracks growing, but it had been there since they removed all the construction supports BEFORE the bridge was moved into position.
Whether member 11 failed at the top or bottom or was just caught in collateral damage. Now very clear that the base of 11/12 punched a hole out of the base deck.
Whether the upper or lower bridge flanges failed first or broke as a result of failure of something else maybe a few milliseconds earlier. See above
What the final design was or what the design analysis showed. Now issued
Whether there was any failure in any tendons or PT rods. No failure of the rods.
Whether planned reinforcement details were altered on site and other ducts installed. This to me is still not clear. The 4" ducts adjacent to member 12 seem to be ducts for tensioning rods intended for the vertical tower structure. But when they appeared in the design is unclear.
Whether the span had been adequately analysed for its temporary condition before the second span and tower / "cables" were installed. This is still not clear. It is stated in OSHA report that the final full build would have reduced forces on the blown out joint, but it is not clear to me whether this temporary stage was properly analysed.

Things we don't yet know we don't know
Everything that has been handed to the NTSB or disclosed in testimony The OSHA report fills a lot of those gaps. In particular I didn't know that the original cracking and noise happened when they removed all the supports prior to moving the bridge.

We can speculate on these items above until the final report is written and given the probable lack of real time data, even that may never pin down 100% a single cause only one or maybe several probable causes. I think we now have a very likely cause - failure of the 11/12 connection to the base deck.
Many of the potential causes have been identified in the many many posts in parts I to V. People coming late to this post are clearly not reading through 900+ posts and so the debate has started to get very circular and mainly just rehashing old ground.

It will start again on the release of the data from the NTSB.


People are encouraged to read the earlier posts which should help answer most questions.

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

 

[TheGreenLama]

I hope OSHA and the NTSB get their acts together so that there are no conflicts between their respective reports.

 

[MikeW7]

I hope OSHA and the NTSB get their acts together so that there are no conflicts between their respective reports.

It would also help if they could get basic facts correct, like the order that diagonals 11 and 2 were detensioned. In today's toxic fake-news culture, a single mistake like that is enough to discredit an entire report in some people's minds.