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Question for the masses? Does a compression post have to be plumb?

Discussion in 'Commercial Seismic' started by Builder Bob, Aug 13, 2019 at 3:46 PM.

  1. Builder Bob

    Builder Bob Sawhorse

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    After researching the google master wealth of knowledge, I cannot find an answer to this riddle. If I have an non-structural roof deck and the compression post must go to structure- what is the maximum deviation from plumb that a compression post can be? The guideline only indicate that the ceiling grid cannot deflect more than 1/4", the spray wires cannot exceed a certain degree angle but remains silent on the amount of plumbness required for he compression post.


    Case and point, the ceiling grid is 6 feet from the steel deck. Bar joist run approx 5 feet on center. We need to angel the compression post os it is about 1 in 3. The spray wires at the bottom are all at 45 degrees as required, the ceiling does not deflect more than 1/4 inch........ but the compression post are not plumb.

    Is this a problem? if it is - please direct me to the section that indicates so.

    I can't find it in E580 or in the Cisca 3-4 pamphlet.
     
  2. Mark K

    Mark K Platinum Member

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    I do not know how you can have a non-structural roof deck.

    It appears that you are dealing with bracing for a ceiling grid.

    The correct answer is that it depends.

    Ask the Architect or engineer on the project.
     
    tmurray likes this.
  3. mtlogcabin

    mtlogcabin Sawhorse

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    Vertical wires have to be within 1 to 6 of plumb but I could not find anything on a compression strut where it would be permitted to be out of plumb.

    Or tighten up on the pattern where the compression post are plumb between the 5ft center of the bar joist
     
  4. HForester

    HForester Member

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    A compression (or tension) member has to be aligned with the direction of the load. How close that alignment needs to be, depends on the type of end connections of the member and the stiffness of the member. If the end connections are pin only (full rotation allowed), the load direction and member axis have to be perfectly aligned....an impossible task. If one end connection resists rotation (i.e.,not a pin connection), then the load direction and member axis can be (somewhat) misaligned. But in doing so, moment is introduced into the fixed end of the compression member and that contributes to more of a tendency of the compression member to buckle (under compression load). Using a stiffer member can help offset that problem but how much stiffer is an analysis question. How much resistance to moment you can achieve at the end connection is also an analysis question.

    Looking at the diagram in the Cisca 3-4 pamphlet
    [​IMG]

    Figure 2 applies. This Compression Post is actually a lateral force brace (per the text in pamphlet.) What lateral force? The grid (with the weight of the ceiling panels) is a membrane that will move up and down during a seismic event. Why does the membrane move up and down? Because the seismic motion is mostly side-to-side (in the plane of the grid) which puts the grid structure in compression. Compression of a slender member causes buckling (membrane attempts to move out of plane: up-and-down). Hence, the term "lateral force brace".

    As was said earlier: "Ask the Architect or engineer on the project." Maybe he can recommend a suitable "beam" to span that joist space to allow for the Compression Post to be perpendicular to the plane of the ceiling grid, In this case, more or less, "plumb" as you can read on a spirit level. Given that the Compression Post is most likely "clamped" in some way where it connects to the bar joist, a slight amount of "out if plumb" doesn't have much impact. A 1:3 slope is definitely way out-of-plumb.
     
    ADAguy likes this.
  5. ADAguy

    ADAguy Sawhorse

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  6. e hilton

    e hilton Bronze Member

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    I don’t understand what the purpose of the compression post is. I looked at the Cisca document, it mentions a compression post but none is included in the illustrations, including figure 2.
    Like HF said, I am guessing that a compression post is actually a rigid member on an angle, to replace or supplement a splay wire. The wire, like a piece of string in the classic Statics and Dynamics class, can only be in tension: cut the string, the beam falls. A lateral brace can resist both compression and tension.
    But why would it be vertical? To keep the ceiling plane from lifting up? To add support to the roof deck? (Not likely). And if the compression post was connected to the roof deck and the ceiling plane, every time a maintenance person walked on the roof the ceiling plane would deflect in response to his weight moving across the roof.
     
  7. mtlogcabin

    mtlogcabin Sawhorse

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    Exactly
    It is a requirement in certain seismic zones
     
  8. Mark K

    Mark K Platinum Member

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    The system of 4 splayed wires and a "vertical" post is supposed to limit the horizontal displacement of the ceiling assembly with respect to the floor or roof above. When there are lateral forces on the ceiling at least one of the wires will be in tension and the post will resist the upward component of the forces in the wires.
     
    ADAguy and jar546 like this.
  9. HForester

    HForester Member

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    Well stated, Mark.
     
  10. Phil

    Phil Sawhorse

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    Builder Bob,
    If you are looking for a reliable reference rather than a code requirement. California DSA has an Interpretation of Regulation, IR 25-2 for lay-in ceilings. In section 2.3.e, it specifies the same 1H to 6V for compression posts. OSHPD's pre approved ceiling pre approved details for suspended ceilings, OPD-0002-13, has the same limit in note 13.d on page CL0.02

    See
    https://www.dgs.ca.gov/DSA/Publications#gypsum
    https://oshpd.ca.gov/construction-finance/preapproval-programs/oshpd-preapproved-details-opd/
     

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