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Thread: Buckling in Structures

  1. #21
    Technical Fellow
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    Feb 2011
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    Dale,

    The lighting structure may have a rated load of 30K-lbs, but that is a distributed and downwards load. It may only have 200-lbs lateral or even less.

    Have you calculated how much your entire wall weighs?
    Is the floor a concrete slab on compacted soil?
    Is the floor wood with a crawl space, basement or car-park underneath?
    Is the floor thick enough to support that weight? It may need a cast in beam.
    How is it attached to the floor?
    Is the Humidity controlled so that the OSB cannot absorb moisture and increase in weight?

    Those are all rhetorical question Dale, this project bothers me and I am not commenting further. I suggest you get a Structural Engineer in there to look at it pronto.

  2. #22
    Project Engineer
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    Feb 2011
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    Rhetorical though they may be, I will answer them for the sake of others who are reading this thread.

    Have you calculated how much your entire wall weighs?
    Yes. A little less than 500# per 20' section; the 12' section is about half of that,
    for a total of a little less than 1300#. (This assumes full, un-cut sheets of OSB)

    Is the floor a concrete slab on compacted soil?
    Yes, with 241 (1.5 inch plywood) bonded to the concrete.

    Is the floor wood with a crawl space, basement or car-park underneath?
    No.
    Is the floor thick enough to support that weight?
    Yes.
    How is it attached to the floor?
    Lag screws in the center of each 4' wide 'sub-section'.

    Is the Humidity controlled so that the OSB cannot absorb moisture and increase in weight?
    Yes.

    Those are all rhetorical question Dale, this project bothers me and I am not commenting further. I suggest you get a Structural Engineer in there to look at it pronto.
    Thank you for your comments.
    If it bothers you, it bothers me.
    I will get a Structural Engineer in there today.

  3. #23
    Project Engineer
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    It seems this thread has had a large number of on-lookers / bystanders.
    At Dave's suggestion, I engaged a Professional Structural Engineer
    to review the design and installation.

    So, for the curious, or perhaps morbidly curious, I will detail the results.
    Please see the attached diagram.

    Structures Example 4.jpg

    Diagram A is a side view of an impossibly slender wall,
    which is made up of 3 sections, one stacked upon the other.
    Each section is 8 feet tall.
    The wall sits on the floor. The wall is in compression.
    The wall is bolted to the floor.

    It is not a "simple" wall, but rather, a very complex structure.
    This is due to the fact that, what would normally be considered the sheating,
    is hung several inches off of one side, via stand-offs.
    Besides that, the stand-offs are of varying lengths, creating a wall of varying thickness.

    Some have expressed the concern that the wall could fail similiar to Diagram B.

    That failure mode is not likely because section 1, the top section,
    is firmly attached to a lighting grid at the top of section one.
    (The grid is shown as a black horizontal line at the top of Section 1 in Diagrams C and D.)

    The weight of the wall is well below the load rating of the lighting grid in any likely failure mode.
    (Likely failure mode? F5 tornados, 7.0 earthquakes, and fully loaded freight trains running into the wall are excluded.)
    (The weight of the wall is 1/5 of the WLL of the grid, which itself has a safety factor of (about) 2:1.

    The entire wall can easily be supported (suspended) by the lighting grid.
    It was not, due to some complexities of the install.

    The Professional Structural Engineer (PSE) identified the possible failure mode shown in Diagram C.
    Althought unlikely, it would be theoretically possible for the bottom section, section 3,
    to shear ALL of its bolts attaching section 3 to section 2, and then tip over.

    That section of the wall is 20 feet long and weighs less than 150 pounds.
    The risk (likelyhood x consequences) is low and it was decided that no further treatment was necessary.
    Even so, if the risk was higher, the Section 3-Section2 joint could be reenforced by welding a plate over that joint,
    or by attaching Section 3 to Section 2 with a safety cable.

    The PSE also identified the failure mode shown in Diagram D.
    Again, theoretically possible, but not likely.
    (Possible? Think of a speaker stack falling against the wall, hitting at that exact joint.
    It is possible that ALL of the bolts attaching Section 2 to Section 1 would shear, creating that failure mode.)

    This failure mode is possible only because the wall is in compression.
    If the wall were in tension (suspended), it would not be possible.
    (Well, or at least, not very likely at all)

    The risk (likelyhood (not very) x consequences (pretty high) is medium.
    To mitigate that risk, a safety cable (actually several) was run from the grid to the top of section 2.
    (By law, a safety cable must be able to withstand 6 times the static load.)

    Should the wall start to fail as in Diagram D,
    the safety would immediately "catch" Section 2 (and the attached Section 3) preventing failure.

    That work was completed immediately.

    As others have pointed out,
    engineering is not only about designing structures to resist / support the intended loads,
    but to also resist / support the unintended loads too.
    The "trick" is to do both, while paying attention to cost, installation details, fabrication details and risk.

    Questions?

  4. #24
    Principle Engineer
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    Mar 2011
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    No, as drawn the skinny weakly attached column does virtually nothing.
    If it were continuously attached it would help.

    Are you a student trying to "stump the experts" ?

  5. #25
    Project Engineer
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    A student?
    Not in the way you are thinking.

    More of a "hobby engineer".
    (Now there is a scary thought!)

    The latest drawing was merely to clearly show the fact that the wall was built in three sections.
    In real life they are securely bolted together with Grade 5 Bolts/Washers/Nuts.

  6. #26

  7. #27
    New Member
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    Jan 2024
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    Trying to find a one way concrete slab calculator, that uses imperial, rather than metric units. Are there any within the Engineers Edge group of calculators? I can hand calculate one-way concrete slabs, and the required reinforcement, etc. But I would love a calculator, allowing me to play with variables, choose options. Can't seem to find any using EE's search

    Jack A

  8. #28
    New Member
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    Nov 2023
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    Buckling in structures is like when a tall tower or column gets wobbly and bends under pressure, kinda like a spaghetti noodle that can't handle too many meatballs. Just like a straw might collapse if you press on it from the sides too hard, structures can buckle if they're not strong enough. Engineers work to make sure buildings and things don't buckle by designing them strong and sturdy, like a superhero holding up a heavy bookshelf. So, buckling is like a warning sign for structures they need to stand tall and strong to stay safe and sound!

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