Florida International Bridge Collapse

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

  1. hardalee Registered Senior Member

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    382
    [QUOTE="Peter Dow, post: 3510536, member: Quotes from Ingenuity on eng-tips.com
    "Here is a photo of a closer view of the north-end failure, from NTSB video:"

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    Damage

    "And another showing spalling to underside of member #11 - the same side that the PT bar that was being stressed/de-stressed was located:"

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    If a P.T. Bar fails under high stress the two ends fly apart like the whip from hell. All that energy has to go somewhere, hence the spalling that you see on the underside of member #11.

    The question now is - was the failure of P.T. Bar the first event that caused the failure of the bottom joint of member #11, "the critical joint", or conceivably a secondary event that happened subsequent to a prior failure of another component?[/QUOTE]
     
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  3. hardalee Registered Senior Member

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    I had not seen the damage shown in the close up of the north end diagonal before.

    It is a significant point. Calculations show that witn two 1 3/4” bars stressed and the dead load, the diagonal would have to he very heavily reinforced with mild steel reinforcing bars to be safe. This would include confinement ties at a small spacing with cross ties.

    Add a little cracking as reported and maybe a small void or two and we are there.

    Good call.
     
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  5. Peter Dow Registered Senior Member

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    That NTSB video helped a lot.
    Like I said, the smoking gun.

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    You mean each stressed within the specified range?

    Sheet B-17, page 115, recommends a P.T FORCE/BAR between 200 KIPS and 320 KIPS depending on the truss member concerned.

    Remember that if one of the P.T.bars failed - snapped in tension - as the evidence indicates - that could be because the operator of the jack exceeded the recommended force for a 1 3/4" diameter bar, beyond the rated, say, 150 KSI and it snapped at somewhere about 2.4 square inches x 150 KSI = 360 KIPS.

    So at failure you might have had a modest 260 KIPS on one P.T. bar that survived fine and 360 KIPS on the one that was being over-jacked and failed, snapped in tension.
    So we might estimate a total force of 620 KIPS from both bars.

    With regard to the dead load and my previous calculation for the compressive stress on member #11.

    Actually, the figure quoted everywhere on the internet now for the weight of the mainspan is 950 tons which is an increase of 35 tons from the "915 tons" stated in the proposal pdf.

    So because
    and now assuming the 950-tons figure for the dead load, the updated calculation is

    950 tons = 950 x 2000 = 1,900,000 lbs = 1,900 KIPS

    Half the weight of the mainspan =
    0.5 x 1,900 KIPS = 950 KIPS

    The compressive bridge dead load in member #11, the force transmitted from the other truss members, was
    1.7 x 950 = 0.85 x 2 x 950 = 0.85 x 1,900 = 1615 KIPS

    So 1615 KIPS rather than the "1555 KIPS" I had previously calculated for the effect from the dead load.

    Now add in the compressive load to resist the tension in the bars - 620 KIPS

    The total compressive load on member #11 - bridge + bars = 1615 + 620 = 2235 KIPS

    I think you may be drawing the wrong conclusions.

    You see if the bar snapped - failed in tension - it was because the concrete had survived the compressive load up until the point of snapping - the damage to the concrete seen was due not to an initial failure of the concrete in compression but from the initial explosive release of elastic energy from the snapping bar.

    If the concrete had initially failed in compression then the bar would likely not have snapped.

    There was a fight between the bar and the concrete - and initially the good strong concrete won - initially the bar snapped and in snapping it exploded part of the concrete as a secondary effect - the part of the concrete that was next to the failing bar.

    I might suggest that to be "safe", the construction industry might be better discontinuing the use of thick P.T. bars such as 1 3/4" - since we can see what happens when they fail - disaster.

    With 0.6" diameter strands or tendons if one of those snaps it is not so much energy released explosively - at the same stress, it is only about 12% of energy released explosively compared to the energy released from a 1 3/4" diameter bar.

    Of course the stress rating of a 0.6" tendon could be 270 KSI which is 180% of the rating of a the 150 KSI rating of a 1.75" bar, so the energy stored at the higher stress would be more.

    So constructing with 0.6" diameter tendons is much safer than constructing with thick P.T. bars - the thinner strands / tendons are more idiot proof when they are over-jacked and snap. The thinner tendons would snap more gracefully with less of a risk to destroying so much of the concrete.

    Well I had some help.

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    Last edited: Mar 23, 2018
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  7. hardalee Registered Senior Member

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  8. hardalee Registered Senior Member

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    No, I 'm saying that if the bars, we can now call them bars for sure based on the picture of the all thread bar seen in your recent post, were stressed it could cause the concrete to fail as it broke, the results of which also are shown in your recent post.

    In general, the machinery used for stressing is not capable of over stressing to the point of failure unless something is wrong with the bar (unlikely) or the anchor. In the case of an anchor failure, a "puff" at the bottom, like that seen in the recently published photos would likely be seen. It would be the dust from the crushed concrete behind the anchor.

    This cracking of the column on one side exposed a lightly reinforced member, which much less steel that I would have expected in terms of both vertical and horizontal steel. I would have expected tightly placed number 11 bars (1 3/8 " in diameters) tightly placed #4 ties (.5 " in diameter). I look forward to finding out how the diagonal was reinforced as constructed and whether it was in accordance with the design.

    I will guess the size of the bars and their pattern based on the picture and run some calculations this weekend. It may give some interesting results.

    Congratulations on your certificates. They are very impressive.

    I am a Licensed Professional Engineer (Structural) struggling with the problem from the outside with insufficient data. But but because of that, I'm not free to make detailed judgement on insufficient data. That's why I an not yet weighing in on design vs construction causes till I have the designs and know how the diagonal was constructed.
     
  9. hardalee Registered Senior Member

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    382
    I made a mistake in the above. All thread rod was incorrect, what I saw was a normal bar with different deformations that the one to the reight. See: https://www.nbcmiami.com/news/local...ion-on-North-End-of-FIU-Bridge-477571043.html for the video I was looking at.

    Post tensioned rod can be seen in plastic, not metal duct.

    Sorry about that.
     
  10. Peter Dow Registered Senior Member

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    189
    This "all thread bar"?

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    As the bar broke, it became a "whip from hell" and the concrete took one hell of a whipping.

    Hold on a minute. Bars come in many different diameters with many different strengths.

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    So a hydraulic jack capable of tensioning a 2-1/4" diameter P.T. bar to 490 KIPS is easily able to break a 1-3/4" P.T. bar with a minimum ultimate strength of 390 KIPS.

    It may be the safest practice certainly only ever to issue workers with jacks which could not exceed the minimum ultimate strength of the bar the worker working on, but some jobs will use different bars - in this job we know at least 2 sizes were used - 1-3/4" and 0.6" - and unless you have different jacks and are sure to issue the correct one to each worker for each job then it can easily be that a worker finds himself with a jack which is capable of breaking the bar he is tensioning.

    The crushed concrete behind the anchor seen in the "puff"

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    could have been crushed by the whip from the snapped bar. The anchor may be not have failed initially but once the bar had snapped the anchor was not able to stay in place to keep the crushed concrete from puffing out.

    Which "cracking"? In the photograph I posted, "cracking" that I call "spalling"? That's a bit more than a "crack". The concrete at that side has been disintegrated from a high energy impact from the snapping and whipping bar. The better term for that is spalling. A "crack" in concrete is what you might get from a failure of concrete in tension. This spalling is damage more like what you would get from shooting a machine gun at the side of a concrete column!

    Well the proposal pdf specifies 1-3/4" diameter bars throughout but didn't specify any bars whatsoever in member #11, which were add in a revision to the plan when they released they needed tensile strength for the move.

    So it could be that in the revision they selected 1-3/8", 1-1/4" or 1" PT bar - in which case all the easier to snap such a thinner bar using a worker with a tool he had been using before on 1-3/4" bar, right?

    Well do post your results here.
    Thank you!

    I think we make a great team!

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  11. hardalee Registered Senior Member

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    No need to get exact as we don't know so much. When I use your numbers of about 1615 kips dead load in the diagonal, which is about what I had, plus say 2 x 200 kips for the post tensioned load, and then multiply by a factor of say 1.5 to get the "ultimate load", its about 3000 kips that the diagonal should be designed for. Assuming #11 bars (they appear to be smaller in the photos I looked at) and an assumed distribution of bars equally around the column as you would for purely comprehensive load my guess would be 14 #11 bars at most. That would come to about 4.3% of steel.

    I cannot make the column work under those assumptions which I consider to be liberal in favor of the design, based on observations. It can safely carry about 2,500 kips, which is close to the ultimate load it had before stressing. The caveat it that I am not sure exactly how the column was reinforced.

    On the TV news program I quoted the link too, you can see if you stop it that the pt bar coming out cut the column ties. This pt bar is in a white plastic duct.

    I do not know how to embed this in a link or put it on. Maybe you can. It contains up close shots of the broken diagonal face showing vertical steel, horizontal steel and the duct and pt that blew out, cutting the ties that I am basing calculation on.
     
  12. Peter Dow Registered Senior Member

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    189
    So this is not "all-thread-bar" then?

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    Well it is not a very clear picture so it is hard to see what it is.

    Ah, thanks. I've taken some stills from that video too.

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    What do you reckon to that bar labelled "B"? So do you think that looks more like ribs on a rebar than an all-thread-bar now or what?

    No problem I have got a picture of the post-tensioning bar - or "rod" as you and NBC 6 are calling it.

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    Well there doesn't seem to be such an obvious thread on that bar "R" but there might be a thread on it but the video doesn't have the resolution to show the thread clearly. What we'd need is good quality photograph.

    If the duct of diameter "T" has a 3" diameter that would make the bar / rod of diameter "R" to be 1.75" as expected.

    That would make the threaded / ribbed bar "B" from the previous picture above of diameter about 0.875" if anyone is interested.

    I got another picture of the jack from that video too.

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    Last edited: Mar 23, 2018
  13. hardalee Registered Senior Member

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    382
    I don't know how to upload this file. It is a pdf document of the colmn calculation. It is called a P-M diagram. May cause more confusion that helping.
     

    Attached Files:

  14. hardalee Registered Senior Member

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    382
    Your measurement of the vertical confirms my suspicion that the bars were number 8"s or 1" diameter bars. I used #11's as the largest they could be but will now recalculated with #8s. I will be slightly worse as the concrete carries the majority of the load.

    Ok, use of 14-#8 bars drops the ultimate load to about 2200 kips or service load of about 1500 kips. Does not look good at this point. Design load remains 3000 kips ultimate. Twice design load is a for sure failure.

    I look forward to more information but the chances of things changing are getting smaller.
     
    Last edited: Mar 23, 2018
  15. Peter Dow Registered Senior Member

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    189
    Not number 7s or 0.875" diameter?

    I measure the bar B to be half the diameter of the rod R, so if R is 1.75" then B=0.875" and so B is more likely to be #7 rebar.
     
  16. Peter Dow Registered Senior Member

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    189
    A "service load of about 1500 kips" which is LOWER

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    than than the 1615 KIPS that I calculated for the bridge load on member #11, WITHOUT the additional compression from the P.T. bars.

    I dread to ask but how low does it go if you use 14 #7 rebars, because I think it is #7 rebars they have used?

    No it doesn't. If your calculations are correct, member #11 was in danger of an initial failure under compression.

    Which is another worry but the evidence of the snapped P.T. bar and spalling of member #11 along its length does point to an initial failure of the PT bar in tension which may have done enough spalling damage all along member #11 to then cause a secondary failure in compression, at the critical bottom joint as described in the NCE article and giving rise to the puff and perhaps also in the middle of the column where it has been totally crushed into 2 parts by the roof of the bridge falling onto it.

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    A "sure failure" is what we got.
    Thanks. Interesting. Can you do me one of those assuming #7 rebars? Also don't you have to subtract a bit of strength for the two PT bar duct holes in the concrete?
     
    Last edited: Mar 23, 2018
  17. Peter Dow Registered Senior Member

    Messages:
    189
    This video shows a pure compression failure at the top of a concrete column.


    Whereas what member #11 looks to have suffered is quite different - an initial spalling failure all along member #11 that is consistent with an initial snapping of the P.T. bar inside and an explosive release of elastic energy causing spalling destruction of concrete along along one side of member #11 and down to its bottom critical joint.
     
  18. hardalee Registered Senior Member

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    382
    Once the first diagonal in compression gave up its load or most of it, the load transferred to to the adjacent tension diagonal which become a compression member on what was no longer a truss and the bridge failed.

    Fining up calculation now without more knowledge is just putting lipstick on a pig. Its still a pig.
     
  19. Peter Dow Registered Senior Member

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    189
    So about that P-M diagram - can you do me another one assuming #7 rebar (0.875" diameter) and add a couple of holes for the 2 x 3" ducts?

    Please.

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  20. hardalee Registered Senior Member

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    Fyi: http://www.ce.memphis.edu/6136/PDF_notes/i1__axial.pdf if you really want to know how concrete columns work, this is it.
     
  21. Peter Dow Registered Senior Member

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    189
    Thanks but don't you have a program that you can type in some dimensions and it works it all out for you, draws you a nice graph and everything?
     
  22. Peter Dow Registered Senior Member

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    This kind of thing - an online calculator, saves so much time. A great wee tool. I assume you are using something like this yourself?

    Design and analysis of reinforced concrete column calculator
    by Jonathan Ochshorn
    https://courses.cit.cornell.edu/arch264/calculators/example7.5/index.html

    Using method A
    I chose middle values for
    f'c(ksi) = 4,
    f'y(ksi) =60
    shape - rectangular (tied)
    I've typed in W = 21 inches. L=24 inches
    14 x #7 rebar

    The result the calculator gives is
    Maximum allowable design (factored) load or capacity, Pu: 1138 kips

    So the maximum design load for the column they actually built (never mind that it has 2 more holes for the 3" ducts and therefore must be a bit weaker still) is only 1138 kips, but the bridge load is 1615 kips!
    So the actual load was 141% of the allowable load for member #11.

    It was a wonder that the mainspan didn't fall apart when they made it at the side of the road. It was living on borrowed time.
     
  23. hardalee Registered Senior Member

    Messages:
    382
    Sorry, I'm now away from my office and programs. Will not likely return until next Saturday before Easter. That's why I sent you the info so you can play with it.

    I don't have much doubt that by the time I return, the problem will have been solved by many. I will be out of internet contact from Tuesday until late Saturday, deep in the dark woods of Georgia. Time to get away for a while.
     

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