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Strength of 4x4 3/16' Tubing over span of 20ft with weight spread

  1. Dec 5, 2013 #1
    Hey guys I found this forum and very informative i must say.

    My question is i found a similar thread but i am wondering.

    I am building a punching bag frame for 14 punch bags between the weight of 100-120lbs.

    The span is 23' by 20'. The span of 23' will be broken up with a support in the center.

    My question is that having 20' 4x4 tube steel with a wall thickness of 3/16" will it be able to support 300-360lbs of this span. My other question is if 2 people were to jump on this say 500lbs would this support the weight.

    Inclosed is some drawings. Apprechiate the help.


    This is the side brackets witch will be used on the corners towards the centre post side. I have not done the 20' side yet.

    The centre post brace will be on both sides and the bolt holes will be sleeved.
    Last edited: Dec 5, 2013
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  3. Dec 5, 2013 #2


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    The drawings didn't make it.
  4. Dec 5, 2013 #3
    Sorry drawings are up thanks
  5. Dec 5, 2013 #4


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    Half of 23'-0" is 11'-6", not 11'-5".
  6. Dec 5, 2013 #5
    Sorry the main drawing was not done by me . Thanks tho
  7. Dec 7, 2013 #6


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    JDMCRX: If two people jump on the beam, each applying a dynamic force of 1112 newtons (N), plus with your punching bags installed, then the global midspans of your steel square tubes currently appear adequate to support the load (not including joint details, which are considered separately).

    This assumes you did not drill large holes in the top or bottom face of the tube midspans.

    Now moving on to your joints, what will your corner joint look like on the inside of the corner? You only showed the end plate on the outside of the corner, so far, right? Also, what bolt size, thread size, bolt length, and bolt material property class do you currently plan to use? (Examples, not including the bolt length, are M14 x 2, ISO property class 10.9; or 0.5000-13 UNC-2A, SAE grade 8.) What is the thickness of your joint plates? And what do you mean by "and the bolt holes will be sleeved"?
    Last edited: Dec 7, 2013
  8. Dec 7, 2013 #7


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    How are you planning to avoid sway in the structure when the guys begin punching the bags?
  9. Jan 22, 2014 #8
    Hey guys sorry for the delay . New size and design .

    Basicly its going to use the wall for support. We will bolt on 3/4" plywood to the walls and use long bolts with steel plates on each side of the wall to sandwich the plates. The bolts going thru the wall will have sleeves on them to limit compression on the wall.

    When i reffered to the bolt holes being sleeved i mean. The holes drilled on the side of the main 4x4 beams will have a sleave welded in to prevent the bolt from crushing the 4x4 beam. Also the ends of the 4x4 beam will be capped.

    The end plates will be made of 3/16" flat plate and will be 12" high and 20" wide. The main vertical support beam will be welded to the end plates. Then the long horizontal cross beams will be bolted with 3 bolts on each end where it connects to the end plates

    Here is another drawing.

  10. Jan 24, 2014 #9
    The standard for all of these types of materials used in the weightlifting/boxing world is 1/4" thickness for wall material and for reinforcement plates and gussets. I only know because I recently built some corner mounting brackets for an MMA cage and some reinforcement plates to support them as well.

    The individual wanted to create a cage but only had a traditional boxing ring, that required mounts to fit onto the old poles and the new cage parts.

    from the past experiences Ive had from erecting steel frames and mounts, the best thing to do is create the entire frame out of the same material....... frame pieces, reinforcement plates and gussets.....all 1/4" thk.

    the 3/16" is rigid, yet the spans that you are planning to carry the weight over, will take some serious beatings....LOL

    the leverage and forces of the weight of the bags alone will produce a flex on the 3/16" wall over the long spans of 11'-6"......this tension affects the vibration dampening qualities of the steel and the 1/4" will deaden the vibrations, better stabilizing the loads.......

    to reinforce this argument......lol........the 1/4" will better stabilize the multiple attachment points for the bags will be moving in different directions.......as many different forces will be exuded on the frame by heavy hitters so you want to have a strong frame to resist this force......

    The presence of this denser material will also have a stronger presence in the gym, matching the other benches and weight racks density, so it will feel more in place in a gym atmosphere.

    Other than that I would say that you could easily get away with smaller corner plates, or even just welding 2 large triangular gusset plates under each axis of reinforcement.....

    ......floor plates for columns can be made octagonal with 8 gusseted plates to reinforce the frames' vertical columns.........

    let me know if you want a quick CAD drawing......
    Last edited by a moderator: Jan 24, 2014
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