# Dynamic load limits of a horizontal 4x4 or 6x6 beam?



## livingegg

I need help determining the minimum beam size and material needed to support the following scenario:

An ~8ft long beam sits horizontally. Please just assume it is sufficiently supported and stabilized at each end. From the center of the beam hangs a ~200lb object such as a punching bag or a (living) human body. The object will not just hang statically, it will also swing. This rig is indoors only.

I do not know whether the beam is a 4x4 or a 6x6. I do not know what type of wood, but I would like it to be Douglas Fir if possible because that is what they seem to have in cheap abundance at my local Home Depot. I do not know the grade. I do not know if it is pressure treated or not.

All these unknowns are what I need help with. What is the minimum size and wood type necessary to support the given dynamic weight at the given span? For example: would a typical low grade 4x4 Douglas Fir beam from Home Depot easily surpass what is needed to support the dynamic object? Thanks! :thumbsup:


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## Daniel Holzman

I am not in the over the internet design business, but this fact should help your design. A swinging object exerts a maximum vertical load of twice the weight of the object. The proof of this requires some relatively advanced mathematics, which you can find on many web sites, Google pendulum forces for more information. 

So in your case, if this is the only load, you need to design for the weight of the beam (static load) plus a 400 lb point load at the center of the beam. Add in an appropriate factor of safety, in the U.S. typically this is 1.4 for dead load, 1.7 for live load (the swinging object). There are also horizontal loads that must be accounted for, due to the swinging of the object, so these must be resisted by the supports.


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## livingegg

Daniel thanks for that info, that is definitely helpful, knowing how to convert the swinging load to the static load, so that the beam can be assessed for static load.

So.. I guess what I really need to find out is, what is the working static load, hanging straight from center, of an 8' 4x4 (douglas fir, whatever grade is the cheap grade home depot sells, sorry thats non-specific but lets just go with lowest common denominator). I have googled pretty extensively and cannot seem to find this information anywhere, although, admittedly those span tables confuse me because there is all kinds of roof engineering factors incorporated into them and what I need to know is much more simple. If I find out that the 8' 4x4 static limit does not reasonably exceed ~500lb, then I'll need to know whether I can just step it up to a 6x6, or a higher grade.


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## danpik

I think a couple of the terms here have been confused. A static load is a stationary load. IE, the 200 lb bag just hanging there plus the weight of the beam with no energy input other than gravity pulling down on it. A dynamic load is a moving load such as the bag swinging. Dan H explained that part pretty well. I'm guessing what you are looking for is the failure load for the given beam you are hanging this bag from. I am not sure there are charts out there that will tell you that a given beam over a certain span will fail at "X" weight.


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## hand drive

if it is free standing I would run the top horizontal member long past both vertical posts a couple feet. then tie back down onto vertical post with angle braces from outside structure. that covers sway for side to side but does not account for front to back movement so the posts would need to be sunk in the ground.also a 2x6 or 2x8 beam would work better on top than a 4x4 or 4x6.


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## concretemasonry

Dynamic designs are beyond the common man. They are not in just two dimensions, but three dimensions and change directions directions depending on movement frequency. Even absolutely rigid footing can be a problem.

The cheapest "bull in the wood" approach is to go heavy in all directions if you just want hang something up and punch it to death.

Dick


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