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  1. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Waross, A number of the other joints were critical (overstressed by code). The south was theoretical even worse. If the construction information is on the drawings and specs, there is no need for the contractor to know how stressed the joints were. However, in similar situations, I think it is...
  2. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Link Here is a link to an interesting POD cast to watch. I personally think too many people are making too much of the bridge being a truss when the real issues were a design flaw in the joint (which could have been designed properly) and more importantly an ethical issue. When the design...
  3. Earth314159

    Miami Pedestrian Bridge, Part XIV

    There are hinge failures in the decks but those are secondary. You can't get a hinge failure in the decks until the much stiffer "truss" starts fail in some manner (unless there a localized hinge due to the spanning of the deck between joints which is not the case here). You also need a high...
  4. Earth314159

    Miami Pedestrian Bridge, Part XIV

    I just did a FBD with a rigid horizontal stick supported one end. Acceleration varies linearly from zero at one to max at the free end. I get an acceleration of 1.5g at the free end, 0.75g in the middle. The load at the support is 0.25wl. Hopefully I didn't make any mistakes.[glasses]
  5. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Vance W., The north end of the bridge falls faster than 9.81m/s^2. It is not a free falling object. There is a rotational force added by the south pier which makes the north end accelerate faster. That is why the falling person is "slower" than the north end of the south segment. This is not...
  6. Earth314159

    Miami Pedestrian Bridge, Part XIV

    3DDave, This is more of a truss. In reality there is no such thing as a pure truss but this is close enough that you can make a reasonable assumption for calculating loads. It is analyzed on a computer so it is almost irrelevant that we call it a truss or a frame because the computer accounts...
  7. Earth314159

    Miami Pedestrian Bridge, Part XIV

    waross, The technical concept and ideas you are expressing are fundamentally correct. I would just say that it is better to say that the PT increased the concrete compressive stresses in member #11 or increased the shear stresses at the pour joint. I am being picky but the compressive force in...
  8. Earth314159

    Miami Pedestrian Bridge, Part XIV

    There are other concrete trusses (although they are rare) and they sometimes make sense. It is just that they have to be designed properly. Any material used can fail.
  9. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Is there a copy of the WJE presentation? i only got to see part of it.
  10. Earth314159

    Miami Pedestrian Bridge, Part XIV

    What does Figg and WJE hope to gain? The failure was a fundamental engineering design flaw and it is an understatement to say that the response from the design engineers was unacceptable even if the surface was roughened and the "the collapse would not have occurred". It is just not that the...
  11. Earth314159

    Miami Pedestrian Bridge, Part XIV

    It is hard to see Figg surviving. Who would hire them? They have to keep 100 employees busy.
  12. Earth314159

    Miami Pedestrian Bridge, Part XIV

    I haven't checked in a long time but I read a few of the latest posts. Cracks usually go straight through a member. In the NTSB board meeting, they mentioned a standard that considers cracks to be an issue if they are more than 0.5" deep. 99.9% of cracks are more than 0.5" deep. So I don't...
  13. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Hi 40 years, No, that is not what I am suggesting. There is still shear in the structure due to gravity loads. The PT in a "true" determinant truss does not add to any of the member forces (including shear). It only adds minimally to the horizontal shear at #11 and #2 in this truss which is not...
  14. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Vance, No worries, it is just Canadian money. I am a Stuct. Eng., so I have to get CPDs hours done anyways.
  15. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Hokie, I am back on the computer. The total shortening of the bottom deck from the initial bottom deck PT is 0.56" but that is taken up mostly by camber of the truss. I made a slight error. The DL deflection is closer to 1.5" rather than 1.3". I work more on this later. I have to go back to work.
  16. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Vance, The upward camber is short term with all the PT in the deck (no other PT applied yet). I am trying to set the initial temperatures for the PT strands. The temperature will be kept the same through out the analysis. Shrinkage is models as a negative temperature on the concrete. How much...
  17. Earth314159

    Miami Pedestrian Bridge, Part XIV

    I am slowly working away on the PT model when I can. The forces in the diagonals due to the initial PT of the deck are 14Kips and 7Kips respectively for #2 and #11. That will go down with creep and shrinkage. Keep in mind that this is only looking at the initial PT of the bottom deck in...
  18. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Yes it will help. Thank you.
  19. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Hokie, The stiffness of the diagonals is not relevant. Take them as being absolutely rigid. No mater how rigid they are, they will only bow the top chord (and truss). It is the flexibility of the chords relative to the truss that maters if differential shortening is going to cause shear stress...
  20. Earth314159

    Miami Pedestrian Bridge, Part XIV

    Hokie, The top chord and diagonals shrinks/creeps as well. So you have to look at the relative movement/shortening. Also, there is no significant restraint from the joints or top chord. If the bottom chord shrinks relative to the rest of the truss, it will just camber up. The creep and...

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