Florida International Bridge Collapse

Discussion in 'Architecture & Engineering' started by hardalee, Mar 18, 2018.

  1. Peter Dow Registered Senior Member

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    189
    What is your source? A link please.
    A link with more information was posted on Eng-Tips.

    Florida Department of TRANSPORTATION - Denney Pate signed and sealed FIU bridge construction plans - 2016 & 2017
    https://cdn2.fdot.gov/fiu/13-Denney-Pate-signed-and-sealed-FIU-bridge-construction-plans.pdf

    From this new information ...

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    - which is completely different from the PT bar tensioning requirements suggested in the proposal pdf, but there still doesn't seem to be any obvious method employed in calculating the PT bar tension from the maximum tension force in the truss members that I calculated.

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    My method was somewhat arbitrary - recommend a PT bar force equivalent to about 130% of the maximum tension force that member experiences.

    I don't know what the recommended practice for setting PT bar tensions is - or if indeed there is a recommended method - how much in excess of the tension required is the bar tension set to - is but my guess is that neither does W. Denny Pate. Or perhaps he is using a method but he starting from a radically different truss calculation?
     
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  3. hardalee Registered Senior Member

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    Peter,

    Over here it is considered unethical to comment negatively about another engineers work without full information so I cannot join you in your evaluation.

    The information about the NTSB’s finding that the rod was being tightened came from the Miami Hearld newspaper locally.
     
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  5. Peter Dow Registered Senior Member

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    Really? Well I had assumed that "over there" it was "unethical" to circle the wagons around an incompetent engineer who had just dropped a bridge on citizens killing them and who should be drummed out of the engineering profession before he brings his whole profession into disrepute, but perhaps we should discuss that one in the other topic in the World News forums -

    http://www.sciforums.com/threads/florida-international-bridge-collapse-world-event.160676/

    I asked for a link so if you have one to the Miami Herald story then please share it. Or if you at least can quote the title and the date of the story and the journalist then I can search for it myself.
     
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  7. DaveC426913 Valued Senior Member

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    I suspect that, until the official report is published, and possible charges laid, it is not only unethical, but possibly libelous.
     
  8. Peter Dow Registered Senior Member

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    Yours is another point which I suggest would be more "on topic" in the other thread which I started in the World Event forum, so I'd invite you to make your point there, where I, as the OP of that thread, would definitely welcome your input about the legal issues surrounding this event and where I would feel more comfortable replying to your point substantively.
     
  9. Peter Dow Registered Senior Member

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    189
    The newly revealed plans, state on page 63, sheet B-38

    NOTES.
    ...
    8. P.T. BARS IN MEMBERS 2 & 11 WILL NOT BE GROUTED AND WILL BE DESTRESSED AFTER MAIN SPAN CONSTRUCTION IS COMPLETE. DO NOT REMOVE BARS.


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  10. DaveC426913 Valued Senior Member

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    No, that's all right. Your quoted post, here, was phrased as a question, and mine was a direct response to it.
     
  11. hardalee Registered Senior Member

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  12. hardalee Registered Senior Member

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  13. Peter Dow Registered Senior Member

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    189
    Thanks. Quoting from that article

    "On the day of the collapse, the main NTSB investigator for the said that there are 10 “diagonal members” that connect the walkway portion of the bridge to the canopy portion. He said on Thursday, construction workers were tightening cables within those diagonal supports. "

    The NTSB investigator was speaking on the day of the collapse before he had time to invest which members they were tensioning andwhich they were destressing, so he just assumed they were tightening all the cables - a wrong assumption in the case of members 2 and 11 because the plans call for destressing members 2 and 11, as I posted earlier.

     
  14. Peter Dow Registered Senior Member

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    189
    Maybe another day. I have an important post to make tonight before I go to bed.
     
  15. Peter Dow Registered Senior Member

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    189
    Analysis of the newly released FIU bridge plans has revealed that my earlier suspicion -
    that member 11 was dangerously under-reinforced has been confirmed, to such a degree that the collapse of member 11 under the compression load after the bridge was placed on the piers but before destressing was to be expected.

    The first point to note from the plans is that the plan's P.T. bar tensioning begins once the concrete reaches a strength of 6,000 psi or more, as this quote shows -

    "CONSTRUCTION SEQUENCE - STAGE 2 - SUPERSTRUCTURE PRE-CASTING
    2. AFTER CONCRETE COMPRESSIVE STRENGTH HAS REACHED 6000 PSI, STRESS POST-TENSIONING OF THE MAIN SPAN IN THE FOLLOWING SEQUENCE ..."​

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    Next, I have shown in my post on Saturday -
    - that the plans recommend a P.T. bar setting for the 2 P.T. bars in member 11 which together total a P.T. bar tension of 560 KIP.
    Together with the results of my truss calculations, also reported again in my post of Saturday, the dead weight of the bridge exerts a tensile force of 304 KIP while the bridge is being transported and a compressive force of 1367 KIP when the bridge is placed on the piers.

    Now let us consider what all those forces together in the sequence they were applied mean for the compressive force on the reinforced concrete of member 11.

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    I have considered 5 different stages, A, B, C, D and E. The bridge collapsed either in stage D or as a result of damage sustained in stage D, so the bridge never got to stage E in good order, sadly, but inevitably given the plan followed.

    Stage A
    The concrete has hardened to at least 6,000 PSI and so post tensioning is about to begin but at this stage the mainspan is still resting on the ground, so there are no troublesome forces on member 11, no PT bar tension, no bridge dead weight and so the reinforced concrete is not being compressed very much at all except under its own weight and that of the canopy immediately above it, but we will ignore that for now.

    Stage B
    The P.T. bars of member 11 have been tensioned to the recommended amount - a total of 560 KIP and that tension force on the P.T. bars is being provided by an equal and opposite compressive force of 560 KIP on the reinforced concrete. But the mainspan is still on the ground so not much in the way of dead weight of the bridge to worry about yet.

    Stage C.
    The mainspan has been lifted onto the transporters and now the dead weight of the bridge is exerting an external tension force of 304 KIP on member 11. This has the effect of reducing the compressive force on the reinforced concrete of member 11 by 304 KIP down to 256 KIP.

    Stage D, "D" for danger and for "Doom". This is when things take a turn for the worse. The bridge gets placed on the piers and now the dead weight of the bridge is applying a compressive force of 1367 KIP on to member 11. So now the reinforced concrete has to take the full 560 KIP from the P.T. bars plus the 1367 KIP dead weight to suffer a whopping 1927 KIP of compressive force, which is more than the member is able to cope with, especially so if the concrete has not reached is full strength of at least 8500 PSI, as is shown in this table from my concrete column calculator and this bar chart.

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    The plans only call for 8 number 7 (diameter 7/8" inch, area 0.6 square inches each, axially orientated reinforcing bars, just barely 1% of a reinforcement ratio for that size of concrete member.

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    As I have noted there, W Denney Pate's drawings don't draw a section through member 11 with only 2 P.T. bars, only section B-B "(TYP. FOR ALL MEMBERS WITH P.T. BARS)" which draws 8 reinforcement bars. Bars marked here "7S11" are 8 size 7 (7/8" diameter) bars confirmed by the table on page 98, SHEET B-98, SUPERSTRUCTURE REINFORCEMENT BAR LIST AND NTSB images.
    It's inadequate reinforcement for the compression before destressing the P.T. bars.

    So it turns out that this picture was "the smoking gun" after all, but not for the reason I thought at the time, but because it shows one of the far too small and far too few reinforcement bars, marked "<-B->".

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    Stage E. After destressing is complete. We don't know for sure if this stage was actually reached, but even if it was, by that time the damage was done and the member 11 was failing and bridge had begun the process of collapsing and it was just a matter of time.

    One small post for a scientist, one giant leap for forensic engineering.

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    Last edited: Apr 25, 2018
  16. hardalee Registered Senior Member

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    381
    The Miami Heralds article I can't find was said to be a recent announcement by the NTSB stating that number 11 was being tightened when the bridge collapsed. I am starting to think about an anchor problem at the base of 11. Columns do not fail explosively like this one did if just under designed. They are fairly flexible. I can't find the anchor plate thickness on the drawings.
     
  17. Peter Dow Registered Senior Member

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    189
    There's been no such "recent announcement by the NTSB" and that's why you can't find it. You are remembering that old article about what was said on the day of the collapse.

    The problem the anchor had was that the concrete of member 11 it was anchored to had cracked and crumbled under the 1927 KIP of total compressive load that was way above the strength of the fresh concrete and that even fully set concrete would have been under designed according to code. A thicker anchor could not have un-cracked the concrete.

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    They do if they are carrying the full weight of the structure. There is no redundancy in the FIU bridge design whereby other concrete columns can take the load if truss member 11 fails. It was all on member 11. When member 11 goes, the whole bridge goes.

    The compression strength of member 11 was 99% concrete by areal cross-section - only 1% rebar. Concrete is not "flexible". Concrete is brittle. The P.T. bars were "flexible" sure, but they were not contributing to the long term compression strength of member 11 - in fact the P.T. bars had ruined the long term compression strength in stage D by cracking the concrete.

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    Possibly at Stage D the cracked and crumbled concrete of member 11 was temporarily being precariously held together by static friction increased by the additional compression on the concrete provided by the tense P.T. bars. (Sort of like if you crush a biscuit between your hands, the crumbs don't fall out until you release your grip.)

    Then when destressing began the already shattered concrete of member 11 fell apart.
    The anchor plate thickness is irrelevant. If it had been twice, three times the thickness, it would not have mattered.
     
    Last edited: Apr 26, 2018
  18. hardalee Registered Senior Member

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    381
    Peter,

    Good luck with your analysis.

    I do not appreciate your insinuation that I am referring to an article that does not exist.
     
  19. Peter Dow Registered Senior Member

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    189
    Oh I am sure you believe it exists.
     
  20. Peter Dow Registered Senior Member

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    Now considering member 2, which survived intact,

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    member 2's "SECTION C-C" -

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    - has 12 x size 8 (diameter 1", area 0.785 sq-in) reinforcing bars and member 2's P.T. bar force setting is

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    560 kip (same as member 11).
    My truss calculation results were -
    in transit tension of 458 kip
    in situ compression of 1920 kip

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    Now let us consider what all those forces together in the sequence they were applied mean for the compression force on the reinforced concrete of member 2.

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    At the critical "Stage D", when the bridge is placed on the piers but before the P.T. bars can be destressed, the reinforced concrete of member 2 has to take the full compression force of 560 KIP from the P.T. bars under tension plus the 1920 KIP dead weight of the bridge to suffer a total of 2480 KIP of compression force, but member 2 was able to cope with that load of 2480 kip, so I calculated the maximum allowable design factored load to find out how strong the concrete must have been, in this table from my concrete column calculator and this bar chart.

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    Therefore the survival of member 2 may be expected, during Stage D anyway, if the strength of the concrete was 7,000 psi or more, albeit a reasonably safe load factor (at least 1.2) is not to be enjoyed until the concrete hardens to at least 8,285 psi.

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    Simply applying commonly used engineering design equations for concrete columns suggests clearly why member 2 survived but member 11 failed. Pate's under-design of member 11 was significantly further out of code than was his design of member 2.

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  21. LOWGUYJACK Registered Member

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