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Thread: load on a 4x4

  1. #1
    Join Date
    Nov 2006
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    Gulfport MS
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    load on a 4x4

    I won a wooden porch swing in a contest, not one I'd have bought but still I figured I'd put it in the backyard so I went out & picked up 3 4x4's & some cement. I'd set the 2 posts about 6' apart (it's a 4' swing) & was going to lag bolt the last 4x4 across the top of the 2 posts. My wife comes out & says "Is that going to be strong enough?" Well I say "Of course it is!" & she goes back in the house. Now though I'm wondering & I hate that. So what kind of load can you put on a treated pine 4x4?

    g
    We are here on Earth to do good to others. What the others are here for, I don't know.

    W. H. Auden

  2. #2
    Join Date
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    Floor joist tables typically start at 2x6, so it's gonna be hard to give a specific answer to your wife. Because your load will be at the end of the 4x4 (basically 1' in from each end) instead of evenly loaded across the whole 4x4, you will get less deflection. It sounds strong enough, although that's assuming the swing hangs over the top of the 4x4 and on something drilled through the 4x4.

    You will likely want a pair of braces in front of and in back of the vertical 4x4s (running diagonally down to the ground, at a 30 or so degree angle from the plane of the swing). Those are to keep the vertical 4x4s from rocking as you swing back and forth.

  3. #3
    I built this swing about 15 years ago and used a 4 x 4 cross beam and 2 x 4 legs. The swing is 48" long. We've had three adults on it with no problem.
    Lee Schierer - McKean, PA

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  4. #4
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    Lee,

    This is one of those cases where a picture is worth 1000 words!

    Rob

  5. #5
    Join Date
    Jun 2007
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    Quote Originally Posted by Greg Stanford View Post
    .... So what kind of load can you put on a treated pine 4x4?
    Assumptions:
    1) 6' 4x4 clear Pine beam loaded with equal vertical loads 1' from each end and simply supported at each end.
    2) Actual cross section dimensions of the beam is 3.5"x3.5"
    3) 1/2" hole drilled vertically through the beam at each load application point.

    From "Standard Handbook For Mechanical Engineers", Seventh Edition (AKA "Marks Handbook"), the fiber stress at the proportional limit for clear shortleaf pine is 7700 psi. Applying a safety factor of 4 gives an allowable stress of 1925 psi.

    These values and assumptions used in a very simple (f=Mc/I) bending analysis of the beam indicates the allowable stresses in the beam will be reached with a total static load on the swing of 1965 lbs (983 lbs at each attachment point).

    This does not consider crushing strength of the beam at the joint with the vertical columns or at the swing attachment points, nor does it consider any dynamic loading on the beam or the columns. It does not address the column strength of the vertical supports. It only addresses the bending strength of the beam supporting a static load in the swing. Beam deflections are not addressed.

    Knots, cracks, splits or other deviations from a clear, straight grained shortleaf pine beam will reduce the load bearing ability of the beam. The applied safety factor of 4 is an attempt to account for those deviations.

    All in all, since it's doubtful the swing itself will support almost a ton of dead load, I'd say that you'll break the swing before you break the beam.
    Last edited by Tom Veatch; 07-22-2008 at 2:15 PM. Reason: added deflection disclaimer
    Tom Veatch
    Wichita, KS
    USA

  6. #6
    Join Date
    Jan 2008
    Location
    Eastern MA Burbs
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    Quote Originally Posted by Tom Veatch View Post
    Assumptions:
    1) 6' 4x4 clear Pine beam loaded with equal vertical loads 1' from each end and simply supported at each end.
    2) Actual cross section dimensions of the beam is 3.5"x3.5"
    3) 1/2" hole drilled vertically through the beam at each load application point.

    From "Standard Handbook For Mechanical Engineers", Seventh Edition (AKA "Marks Handbook"), the fiber stress at the proportional limit for clear shortleaf pine is 7700 psi. Applying a safety factor of 4 gives an allowable stress of 1925 psi.

    These values and assumptions used in a very simple (f=Mc/I) bending analysis of the beam indicates the allowable stresses in the beam will be reached with a total static load on the swing of 1965 lbs (983 lbs at each attachment point).

    This does not consider crushing strength of the beam at the joint with the vertical columns or at the swing attachment points, nor does it consider any dynamic loading on the beam or the columns. It does not address the column strength of the vertical supports. It only addresses the bending strength of the beam supporting a static load in the swing. Beam deflections are not addressed.

    Knots, cracks, splits or other deviations from a clear, straight grained shortleaf pine beam will reduce the load bearing ability of the beam. The applied safety factor of 4 is an attempt to account for those deviations.

    All in all, since it's doubtful the swing itself will support almost a ton of dead load, I'd say that you'll break the swing before you break the beam.
    Tom,
    I agree with your final conclusion, but 1925psi allowable on pine is a bit high, NDS standards have many other factors to determine allowable stress other than simply applying a safety factor of 4, temperature, wet service, load duration ect... I don't think anyone would ever actually try to apply this to construciton of a swing as member sizes are probably driven by serviceablity (deflections) and connection sizes. This is one of those situation when rules of thumb come into play. Also, shear would be a concern if this was to actually be analyzed, it might control.

  7. #7
    Join Date
    Sep 2006
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    West o Atlanta
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    The swing in my back yard is exactly the set-up you describe. Never had a problem. It's even on its second swing.
    MMMMMMMMMMMBEEEEEERRRRRRRGOOOOOOOOOOOODDDDDDDDD!!! !!

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