Page 19 of 22 FirstFirst ... 91718192021 ... LastLast
Results 181 to 190 of 216

Thread: OT: Miami Florida pedestrian bridge collapse

  1. #181
    Join Date
    Jan 2004
    Location
    Missouri
    Posts
    29,067

    Default

    That IS interesting.....

    ALL the concrete in the area of the cracks shown in the deck SHOULD have been in compression, due to the tension elements in the deck. But, referencing sheet B-8 from FIGG, (in the proposal document) there is NO central tension element, and therefore the rebar etc seems to have been relied upon to transfer the full outward thrust of "11" to the tension elements on each side.

    Now that I re-look at the pics and the notes in the interim report, and then look at the pics of the fallen bridge, it is clear that the rebar was not doing the job. The cracks ARE definitely in a critical location, right where the load is transferred to the tension elements. (or was supposed to do that). YES, they do seem to show that the transfer connection was failing at the time of the pictures.

    Looking at the picture of the fallen bridge, 11 and 12 are still together, and resting on the pylon. They pulled right out of the end of the deck, indicating that the joint finally failed.

    Anyone looking at the pictures fig 1 and fig 4 and understanding the bridge design at all would have to conclude that it was already failing at that time.

    I actually do not think it was rebar failing that is the issue.... although it did fail. From the look of it the real issue was failure of the design to transfer the thrust from the #11 diagonal to ALL of the deck tension elements. Instead, it appears that the innermost "tendons" were carrying most of the stress, and that THEY stretched enough to allow the concrete to be in tension and permit the cracks. Then the concrete plus reinforcement in the crack areas failed entirely, releasing #11 and #12 so that they tore out of the deck, and allowed the collapse.

    That concrete should NEVER have been in tension. But given the design, it appears that there was nearly no way to avoid it.

    Your natural response when looking at the thing is to think that there should have been a big steel "strongback" to resist the outward thrust of #11 and #12, and distribute it to the "tendons" T1 through T6 on each side of the deck.
    Last edited by J Tiers; 08-10-2018 at 02:45 AM.
    1601

    Keep eye on ball.
    Hashim Khan

  2. #182
    Join Date
    Jan 2016
    Posts
    116

    Default

    Quote Originally Posted by Paul Alciatore View Post
    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.
    correct, the tension members were being de tentioned at the instant the bridge failed.* this has led many to wonder if the tension member was somehow helping hold the deck to the compression member. anyhow.

    you are also correct that the diagonal did not fail during the bridge failure. but photo "figure 3" from the pdf i linked shows there is MAJOR trouble with that diagonal. its only 22 by 28 inches and the photos shows a folding ruler sticking 6 inches into it?!

    that diagonal strut only had something like 10, 3/4" rebars if i recall correctly, with rebar wrapped around the perimeter. the fact that it split like a piece of wood should have shut the whole project down in my opinion.

    Quote Originally Posted by J Tiers View Post
    ALL the concrete in the area of the cracks shown in the deck SHOULD have been in compression, due to the tension elements in the deck. But, referencing sheet B-8 from FIGG, (in the proposal document) there is NO central tension element, and therefore the rebar etc seems to have been relied upon to transfer the full outward thrust of "11" to the tension elements on each side.

    Now that I re-look at the pics and the notes in the interim report, and then look at the pics of the fallen bridge, it is clear that the rebar was not doing the job. The cracks ARE definitely in a critical location, right where the load is transferred to the tension elements. (or was supposed to do that). YES, they do seem to show that the transfer connection was failing at the time of the pictures.
    yes, which is why i asked if anyone did the math on how far the rebar would stretch as it takes up the load to transfer the compression to tension contained by the deck cables.

    you are slightly off though on suggesting there needed to be a strong back to transfer that load to all of the deck tension cables.

    only ~two of the 12 cables are needed for the tension at the edges of the bridge. its in the middle of the bridge where all of them are required. each segment of the bridge adds to the compression in the roof and tension in the deck.

    someone on the eng-tips forum calculated that a single group of those cables (ie, one of the 12 elements) could handle the tension at the ends of the bridge. in the middle of the deck.. was a drain pipe, rather than a 13th set of cables.


    *edit: this may not have been the case. there were initial reports that the tension elements were being tightened to see if they could close up the cracks.


    there is a very interesting post here https://www.eng-tips.com/viewthread.cfm?qid=436699
    by structuralengr89 19 Mar 18 13:56

    I agree with OSUCivlEng's comments...

    The stressing of #11 was due to cracking that was observed and was an attempt to fix an existing issue. And I do not believe they were destressing it, but were stressing the tendon...Though #11 may have caused the collapse...the collapse was due to a design failure of inadequate shear capacity.

    Lets's assume Hokie's numbers are correct and that there is 1650K of compression in #11...Though we want to consider #11 with pinned ends..it is not..and is acting like a beam. The vertical shear at the bottom end of #11 would be about 950k...therefore Vu would have been about 1330k!...If f'c=6ksi...PhiVc would only be about 60k...there is not enough room in #11 (24"x21") to provide enough stirrups to handle the vertical shear in that member.

    I believe that prior to failure #11 had some upward bending (concave up) and had cracking on the bottom side of #11. To address this, the engineer attempted to tighted the lower strand. The rupture first occurred at the bottom of #11..and with #11 removed..hinges were formed in the bottom of the slab near #10 and the top of the slab adjacent to #11.

    When I step back and look at #11...It just doesn't not look big enough to handle the shear loads that would be imposed.



    where is the rebar?



    looking at this a second time.. its weird to me that the rebar from 11.. is straight. why? seems to me if the rebar pulled out of the deck it should be bent in a hair pin turn.


    --imagine for a moment there was no hair pin turn of the rebar in #11.. it just goes straight into the deck, wire tied off to whatever. the real drawings or photos of the rebar before the cement was poured would be really interesting..
    Last edited by johansen; 08-10-2018 at 04:03 AM.

  3. #183
    Join Date
    Jan 2004
    Location
    Missouri
    Posts
    29,067

    Default

    Quote Originally Posted by johansen View Post

    you are slightly off though on suggesting there needed to be a strong back to transfer that load to all of the deck tension cables.

    only ~two of the 12 cables are needed for the tension at the edges of the bridge. its in the middle of the bridge where all of them are required. each segment of the bridge adds to the compression in the roof and tension in the deck.

    someone on the eng-tips forum calculated that a single group of those cables (ie, one of the 12 elements) could handle the tension at the ends of the bridge. in the middle of the deck.. was a drain pipe, rather than a 13th set of cables.......
    The strongback suggestion was not as you say.

    The bridge was designed as an I beam. The ONLY tension members forming the lower part of the beam were the "tendons". Those had to take the entire tension load of the "beam" at the ends, just as an I beam flange would. Therefore that load had to be transferred into those "tendons".

    However, there was obviously very little acting to transfer the outward push of 11 and 12 to enough (if any) "tendons". The cracks show that at best, the inner two, or possibly four, were carrying most of the load, and had apparently stretched, allowing the cracking.

    The full set of "tendons" were not carrying the load, because the concrete and rebar was not carrying the load out to them. The 11 & 12 elements outward force had to be resisted by more of them, but that was not happening, that force was not being carried to them.

    If one assumes that the middle part of the span did not fail, which seems to be true, then looking at the end shows that the "system" of 11, 12, the roof and the deck had to support about half he total load, dead load plus whatever other loads were in place due to equipment, and test loads.

    The end "triangle" of roof, 11, and 12 wanted to rotate around the end of the diagonal where it connects with the roof. The tension elements had to support that and prevent the lower ends of 11 and 12 from pushing outward away from the end of the deck. The maximum tansion would have been mid-span, but the end gets a decent loading, since half of the total span dead load plus any live load is being supported.

    The strongback comment was purely because that would have been an obvious method of distributing the load among all, or more of, the "tendons".

    It seems amply obvious that at best, the middle four were the only ones getting the load effectively, or the crack in fig 2 would not have been possible.

    A "strongback" on the end of the deck, with all "tendons" passing through it, and made to "collect" the outward force from 11 & 12 would have been one way to avoid the shear failure of the concrete and rebar.

    There was apparently no feature in the design which performed that function of collecting the rorce and successfully distributing it to the "tendons". Apparently the outer ones, no matter how "sufficient" they were, did not have the load distributed to them at the end by any structural element that was capable of carrying the shear load.

    Since the failure was at the end, obviously the tension elements were both sufficient, and also had the forces distributed to them from the middle, where the tension is greatest.

    The design problem was the connection of 11 and 12 to the "tendons".
    Last edited by J Tiers; 08-10-2018 at 02:02 PM. Reason: Add description of end load.

  4. #184
    Join Date
    Sep 2006
    Location
    Southwestern Ontario, Canada
    Posts
    5,040

    Default

    Quote Originally Posted by Lew Hartswick View Post
    "Outlaw Bridges" they kill people .
    ...lew...
    Wrong!

    Bridges don't kill people, the people who build them do.
    The shortest distance between two points is a circle of infinite diameter.

    Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

  5. #185
    Join Date
    Jan 2003
    Location
    On the Oil Coast,USA
    Posts
    19,070

    Default

    A very good rundown of the failure by a Polish engineer,it's in Polish,but Youtube's closed caption feature can translate to English-

    https://www.youtube.com/watch?v=znzC...ature=youtu.be
    I just need one more tool,just one!

  6. #186
    Join Date
    Jan 2003
    Location
    On the Oil Coast,USA
    Posts
    19,070

    Default

    Quote Originally Posted by loose nut View Post
    Wrong!

    Bridges don't kill people, the people who build them do.
    In this case people who design them do.
    I just need one more tool,just one!

  7. #187
    Join Date
    Dec 2004
    Location
    East Coast, USA
    Posts
    6,280

    Default

    I always thought the weakest force killed them.
    Work hard play hard

  8. #188
    Join Date
    Jan 2004
    Location
    Missouri
    Posts
    29,067

    Default

    Quote Originally Posted by wierdscience View Post
    A very good rundown of the failure by a Polish engineer,it's in Polish,but Youtube's closed caption feature can translate to English-
    Seems clear and probably on-target. The interesting thing is the difference in the two ends. The end that did not collapse had a different and longer joint into the deck, one that may have been considerably stronger. There has been no comment about cracks that may or may not have been seen at that other end. Perhaps that is because there were no such cracks (called "scratches" in the translations).
    1601

    Keep eye on ball.
    Hashim Khan

  9. #189
    Join Date
    Apr 2006
    Location
    citrus heights, ca
    Posts
    2,246

    Default

    Interesting information recently released by the NTSB: https://www.ntsb.gov/investigations/...ive-update.pdf

    I would bet that there will be at least a few people going to jail after it is all said and done.
    Below is a transcript of a message the PE left addressing the cracks shown in the photos that are in the above link.

    “Hey Tom, this is Denney Pate with FIGG bridge engineers. Calling to, uh, share with you some information about the FIU pedestrian bridge and some cracking that’s been observed on the north end of the span, the pylon end of that span we moved this weekend,” Pate said."

    “Um, so, uh, we’ve taken a look at it and, uh, obviously some repairs or whatever will have to be done but from a safety perspective we don’t see that there’s any issue there so we’re not concerned about it from that perspective although obviously the cracking is not good and something’s going to have to be, ya know, done to repair that. At any rate, I wanted to chat with you about that because I suspect at some point that’s gonna get to your desk. So, uh, at any rate, call me back when you can. Thank you. Bye.”

    Steve

  10. #190
    Join Date
    Aug 2012
    Location
    Warwickshire, UK
    Posts
    743

    Default

    Quote Originally Posted by J Tiers View Post
    That IS interesting.....

    ALL the concrete in the area of the cracks shown in the deck SHOULD have been in compression, due to the tension elements in the deck. But, referencing sheet B-8 from FIGG, (in the proposal document) there is NO central tension element, and therefore the rebar etc seems to have been relied upon to transfer the full outward thrust of "11" to the tension elements on each side.

    Now that I re-look at the pics and the notes in the interim report, and then look at the pics of the fallen bridge, it is clear that the rebar was not doing the job. The cracks ARE definitely in a critical location, right where the load is transferred to the tension elements. (or was supposed to do that). YES, they do seem to show that the transfer connection was failing at the time of the pictures.

    Looking at the picture of the fallen bridge, 11 and 12 are still together, and resting on the pylon. They pulled right out of the end of the deck, indicating that the joint finally failed.

    Anyone looking at the pictures fig 1 and fig 4 and understanding the bridge design at all would have to conclude that it was already failing at that time.

    I actually do not think it was rebar failing that is the issue.... although it did fail. From the look of it the real issue was failure of the design to transfer the thrust from the #11 diagonal to ALL of the deck tension elements. Instead, it appears that the innermost "tendons" were carrying most of the stress, and that THEY stretched enough to allow the concrete to be in tension and permit the cracks. Then the concrete plus reinforcement in the crack areas failed entirely, releasing #11 and #12 so that they tore out of the deck, and allowed the collapse.

    That concrete should NEVER have been in tension. But given the design, it appears that there was nearly no way to avoid it.

    Your natural response when looking at the thing is to think that there should have been a big steel "strongback" to resist the outward thrust of #11 and #12, and distribute it to the "tendons" T1 through T6 on each side of the deck.
    You keep saying that concrete shouldn't be in tension. The whole principle of reinforced concrete is that the addition of rebar to a concrete member makes it possible for that member to take tension, wheras an unreinforced concrete member can't. In calculating the load capacity, only the tensile strength of the rebar is taken into account, any tensile strength of the concrete (yes it does have a little, but not much) is ignored. The concrete is such a member, if the rebar is properly calculated and installed, does indeed crack, but the cracks are small, insignificant, uniformly distributed through the length, and generally barely visible to the naked eye. Crack width is dependent on the cover to the rebar (thats the distance from the outside surface to the rebar, and there is a calculation which should be done to determine this. Often, in lightly loaded members (not the case here), it is necessary to add more rebar, over and above that needed to carry the structural load, just to keep the surface cracking under control. Properly done, the cracks are too small to allow the ingress of water which would corrode the rebar.
    All of this only works if the rebar has been properly calculated and installed, so the tensile loading in the rebar is within allowable limits, and is within the elastic tensile zone. Get it wrong, so that the rebar is overloaded, beyond yield, then the crack widths in the concrete expand dramatically, just like the photos, and the member fails.
    Overload a tensioned member, beyond the applied tension plus any capacity of the secondary rebar, and there will be similar results.

    If I was responsible for a bridge which developed that level of cracking during construction, I'd have been s------g myself. Forget casual phone calls, it would have been immediate call for action by the design team, whatever the time of day or night, plus immediate closure of the road under the bridge, followed by temporary propping, probably utilising the transporter vehicles if they were still available.
    'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •