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We are building a pergola-swing with two 6x6 posts as the main supports. The posts are to be set in a 12" diameter sonotube footing, 36" deep. I am unsure as to how much concrete to buy being that there is only about a 3" gap around after the post is in. I have calculated the volume of the cylinder (4071.5 cubic in.) as well as that of the post(1089cubic in.) I took the volume of the cylinder and subtracted the volume of the post to get a volume of 2982.5 cubic in Than I determined that a 50lb bag of concrete is 648 cubic inches. (.375x1728=648) 2982.5cu in/648cu in=4.6--> 5 Based off my calculations shown above, I figure I would need 5 bags per post x 2 posts for a total of 10. Just looking for someone to look over my work and verify it for me. Thanks, -Nick

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  • What torque will the swing apply to the soil and how much shear stress/strain will that yield? You can always take back extra concrete bags. What did you assume? Sep 23 '20 at 10:01
  • A much taller swing 10’, made of 4”D steel plumbing as an inverted U set in a 6”D x 3’ deep concrete makes an excellent sturdy swing with minimal effort. Sep 23 '20 at 13:26
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Your math looks right on. I couldn't verify the concrete but it looked pretty similar to other stuff I've used. The 12" Sonotube seems small for a 6x6 post. Their site recommends three time the post width so that would mean an 18" Sonotube. you might want to look into that. that means you have to do your calculations for concrete over again.. lol

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