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Thread: OT: Miami Florida pedestrian bridge collapse

  1. #171
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    I don't think they looked at those cracks and said to themselves "Yep! Exactly what we hoped would happen."

  2. #172
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    Quote Originally Posted by J Tiers View Post
    You are acting like good jury fodder.....

    The crack could easily be somewhere that is not involved in the basic structure. We, or at least "I", do not know where it was located.

    You assume that whatever caused the crack will continue to operate through the life of the bridge. That does not have to be true.

    Because we do not know, it could be a crack in an essential area, or one in a far corner that occurred in curing. Of course nobody WANTS cracks, but they can vary from cosmetic, fixed by filling, or structurally important, which might mean the correct action is scrapping the entire structure.

    If concrete cracks like that, it likely means that the tensile limit was reached in that area. Since bridge engineers are not generally complete idiots, they do not leave concrete to carry significant or critical tensile loads by itself. The obvious question is then "why is there a section of concrete that is in tension?".

    The answer to that question may be the key to the entire collapse. Or it may be a side issue.
    They attempted a concrete truss,that makes them complete idiots.
    I just need one more tool,just one!

  3. #173
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    Quote Originally Posted by wierdscience View Post
    They attempted a concrete truss,that makes them complete idiots.
    Not so.

    There are thousands of bridges in place with concrete used as the main material of the span. A truss is just a slab span with non-essential elements removed. The tension elements in a concrete span are steel, possibly post-tensioned steel rods, as seems to be the case in that bridge. The steel rods are tensioned to the point that the concrete will still be in compression (maybe just barely) at the maximum design loading, before any derating down to a working load.

    So, since a truss has tension and compression elements, concrete is usable directly for compression, and usable for tension if if the steel elements keep the concrete in compression past the max loading point.
    1601

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  4. #174
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    According to the article, the crack was in truss #11, which is the element that was being tightened, and failed. The prevailing opinion is that it was a design error caused by trying to conform to the aesthetics of the concept. The analysis of the materials and workmanship seemed to find no fault, but perhaps there was too much stress on that element and the tension rod may have stretched or snapped, causing a chain reaction on the others.

  5. #175
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    Note: After more reading I see this was not correct. But I leave it so that other comments below will be understandable. Even Einstein was wrong about some things.

    J,

    The "truss" of the bridge is not the whole thing. There was to be a tower on one side that had cables that helped to hold that "truss" up. And there was to be another, shorter "truss" on the curb side of that tower which would have been, at least, a partial counterweight. In short, it was, at least partially, a suspension bridge. What I find odd is that they put up the deck section, the "truss" FIRST and were going to erect that tower and add the suspension cables LATER. The normal way to build a suspension bridge is to erect the towers FIRST. Then string the cables. And FINALLY hang the roadway/deck from them, starting at the towers and working toward the center. The shorter, shore sections are also hung, more or less in balance with the sections of the center span. This keeps the towers more or less in compression and lowers any bending or torsional loads on them.

    It also places the sections of deck or roadway in compression as they are being drawn toward the towers while the construction is in progress. At no point is there any serious tension in the roadway or deck.

    This Florida bridge was assembled BACKWARDS. The deck or roadway section was acting like a classic truss and therefore, the bottom section of it WAS in TENSION. Now, it should have had steel reinforcement bars in it to handle that tension, and I believe I saw photos of it showing them. But they may not have been enough to hold it up.

    I would have erected the tower FIRST. And had the cables on it before mounting either of the trusses. Then, assuming that the trusses were to be prefabricated on the side, I would have installed the shorter truss next, ATTACHING ALL THE CABLES BEFORE THE TEMPORARY SUPPORTS WERE REMOVED. This would have provided the counterweight and would have placed less stress on the tower than the larger truss would have. Then, finally the longer truss would be installed, again attaching ALL THE CABLES BEFORE REMOVING THE TEMPORARY SUPPORTS. If this took too long, then traffic would just have to have been diverted. PERIOD!



    Quote Originally Posted by J Tiers View Post
    Not so.

    There are thousands of bridges in place with concrete used as the main material of the span. A truss is just a slab span with non-essential elements removed. The tension elements in a concrete span are steel, possibly post-tensioned steel rods, as seems to be the case in that bridge. The steel rods are tensioned to the point that the concrete will still be in compression (maybe just barely) at the maximum design loading, before any derating down to a working load.

    So, since a truss has tension and compression elements, concrete is usable directly for compression, and usable for tension if if the steel elements keep the concrete in compression past the max loading point.
    Last edited by Paul Alciatore; 08-10-2018 at 02:37 AM.
    Paul A.

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  6. #176
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    As I understand it, the "cables" were just translucent PVC pipe or similar material that was only for visual appeal (possibly with internal LED illumination), and not a structural component.

  7. #177

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    Quote Originally Posted by Paul Alciatore View Post
    J,

    The "truss" of the bridge is not the whole thing. There was to be a tower on one side that had cables that helped to hold that "truss" up.
    No, the "cables" were hollow 16 inch diameter schedule 40 pipe that was bolted to the top of the roof deck with cement anchors. they were intended to dampen vibrations and oscillations, they would not have held up but a few dozen tons in the event of the bridge failing.

    there has already been a bunch of the math done by the folks on the other major engineering forum which has something like thousands of posts about this bridge; while the strain in diagonal #11 is within reasonable limits for 1100 psi compressive stress, there does not appear to be any substantial rebar making the 60 degree acute hairpin turn as the tensile stress in the deck pulls on the diagonal strut. others have noticed a flat chunk of cement which shows: 1, No rebar in it, and 2: shows that the cement was poured in a way that the most critical joint had one or more cement "joints" where the cement was poured into the bridge on separate days.

    anyhow, there are something like 12 sets of 7x19 cables running through the deck. if they had added a 13th cable rather than a drain pipe directly down the middle of the bridge, strut 11 would not have blown out of the bridge. in fact, even a single group of those cables could handle the tensile needed to offset the thrust on that diagonal strut. instead they were relying on rebar and the sheer strength of the cement to transfer the compressive stress to the tension cable groups on either side of the joint, and that wide crack shown in the released photos is simply the rebar ( or lack of it) yielding.

    additionally on both sides of the compression strut, is 2 x 3 or 4 inch pcv electrical conduit pipes. it may very well turn out that someone couldn't' figure out how to get the rebar to fit into the bridge so they left some out.


    there are also some theories that the tension member (only needed for when they moved the bridge) which ran through the compressive strut 11: the plate to which it was connected to was on the other side of the sheer line of the joint, so it may have actually been somehow preventing that joint from failing. hence why the bridge failed after they released the stress on it.
    Last edited by johansen; 08-10-2018 at 01:17 AM.

  8. #178
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    Correct, the "cables" were entirely cosmetic.

    Clearly, the very heavy span had a dead load that was far in excess of the usual live load, and likely in well in excess of the load even if the bridge was packed wall-to-wall with people. So it was not "un-loaded" while they were loosening and tightening things. If I read the excerpt correctly, parts of it had been loose for several days before the collapse.

    The drawing of the tension elements does not show the rebar design, but it does show that the tension elements were clearly relying on the concrete/rebar to hold things together, because there is nothing else to do it, and the tension elements are not connected.

    And, I wonder to what extent the plates that distribute the tension element end pressure into the concrete might have introduced discontinuities that affected the strength. There has to be a balance of reasonable pressure vs still having the plate inside the concrete.

    But, the real point of the matter is not about the design, or the setup procedure in and of itself. Bridges have been poorly made before, and have fallen down before. The main issue here is that traffic was permitted under the bridge during the setup procedure. That seems to me to be the main problem. Cracks, schmacks. It is the idiocy of messing with the structure, which is a known possible cause of failure, while people were allowed to be under the thing, which bothers me.

    If it fell down, but nobody was under it, we would be thinking of the whole matter somewhat differently. It would be an engineering failure, but not a newsworthy tragedy as it is now.
    1601

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  9. #179

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    having read the report now i have to say the photos are very damning.

    compression strut 11 had already failed by sheering through the bulk of it, and had already moved a good half inch perhaps relative to the bridge deck.

    the report says that the rebar in that joint was according to design.

    what i would wonder.. did they account for how much the rebar making that hair pin turn would stretch and pull out of the bridge before it took up the 900 or is it 450 or so tons of tension? because the answer isn't zero.

    its quite possible in order to make that joint as per the bridge drawings take up the stress without cracking, on the ends of those rebars after they make that hair pin turn, they would have to have screw threads and anchor plates to tension the rebar before the bridge is set on the piers.

    https://ntsb.gov/investigations/Acci...ive-update.pdf

    see figures 3 and 4

    figure 2 shows that the entire joint had been pushed out of the bridge deck by maybe 3/4 an inch.

  10. #180
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    I did some more reading and yes, the "cables" were not there to support anything. They were for sway control and decor, I guess. That brings up the question of why the arrangement of the diagonals of the truss in such a manner that they look like the cables are holding it up? That is contrary to any other truss design that I have ever seen. And it must have made the calculations much more complicated. I mean, each and every one of them is at a different angle. This is a unique design. Insane.

    Another point that I seem to hang up on is that first diagonal; the one that they were apparently re-tensioning when it happened. That member would have been in tension while the span was being moved into position because the movers were inboard of it and the ends would have been trying to bend down. But after it was set on the piers, it would have switched to compression and the pretension that had been set in it would only add to that compressive load on the concrete while lessening the tension in the rod(s). Seems to me that they should have backed that tension off WHILE the weight of the span was being transferred from the movers to those piers. And the stresses at it's attachment points would have reversed. If the concrete there had not completely cured ..... It does look like the diagonal itself did not break: it was sticking several feet THROUGH the upper deck after the collapse. So, apparently, it did not fail either in compression nor in tension. It was apparently still in one piece but no longer attached to that upper deck. And that could have been a fatal problem.

    Consider the joint where the top end of that diagonal was attached to the top deck. First, while being transported the diagonal would have been pulled in a diagonally downward direction, stressing that joint one way. Then when it was set on the pier that would have reversed to a diagonally up direction, stressing that joint in the opposite direction. Finally when they released the tension on it, that diagonally up pressure would have actually increased. Could that have caused the joint to fail? Bad joint design? Uncured concrete? But, of course, that may have nothing to do with it.

    I guess I will wait for the official report.
    Paul A.

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    You can't win and there is a penalty for trying!

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