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09142009, 01:13 AM  #1 
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I read that i need to dig the holes 1012" wider then the posts, is this good enough or over kill.
im using 4x6" posts 8 feet tall 4ft under & 4ft above ground im trying to figure out how much cement im going to need. I cant figure out the formula to find the volume in qubic ft/yd for the cylinder hole that is 12"diameter by 4ft deep but minus the 4x6"x4' post thats in it and to top it off multiply is by 24 holes... can someone please try to help, im stil working on it too in ust not that good at math Advertisement 
09142009, 04:47 PM  #2  
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There are a couple of different ways to approach this problem – basically, you can compute everything as cubic inches (in³) or you can do the math using cubic feet (ft³). Either way will work and neither is particularly more difficult (or easier) than the other. I am going to do the computations in in³, but then convert to ft³ for the final answer. For a 12” diameter tube at 48” deep and using the formula πR² (formula for finding the area of a circle) = 3.14 x 6² (radius) = 3.14 x 36 = 113.04 (this is the area of the tube) Multiply 113.04 (area) x 48 (depth) = 5425.92 in³ (volume of the tube) which then divided by 1728 (conversion from cubic inches to cubic feet or 12 x 12 x 12) = 3.14 ft³ of concrete to fill the tube Drop a 4” x 6” x 48” post in the middle and you have to subtract the volume of the post from the volume of the tube. Multiply 4 x 6 x 48 = 1152 in³ (volume in cubic inches of the post). Divide 1152 by 1728 (same conversion factor) = .6667 ft³. Then subtract the volume of the post from the volume of the tube or 3.14 ft³  .6667 ft³ = 2.47 ft³. So, you need 2.47 ft³ of concrete per post hole. Multiply by 24 holes: 2.47 x 24 = 59.58 ft³ of concrete for all 24 holes. Since one cubic yard of concrete equals 27 cubic feet, you can divide 59.58 by 27 to get the total concrete in cubic yards or 59.28/27 = 2.195 yds³ of concrete needed for all 24 holes. AdvertisementLast edited by oberon; 09142009 at 05:04 PM. 

09142009, 04:55 PM  #3 
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Previous post tells you everything you need to know about the math. However, it is a bad idea to install a wooden post directly into concrete. Better to use a Simpson or equivalent post support bracket embedded in the concrete. Check out the Simpson Strong Tie website for details on what type of support you are going to need, and how to install it in the concrete. The wooden post is attached to the support bracket.

09142009, 05:03 PM  #4 
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Or..................... a little over 3 and a half 80# bags of Sakrete per hole (or just under 5 and a half 60# bags)
But like Dan said, you are just asking for serious rotting trouble by plopping posts down in concrete.
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09142009, 05:22 PM  #5 
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If you fill a hand dug hole, you will use more concrete than if you use a Sonotube that is much better when it comes to being raised by the surrounding frozen soil.
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09142009, 05:24 PM  #6 
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Here's a volume calculation cheater for you guys Formula Calculator
It will even figure a truncated paraboloid for you. (That's the shaped hole most of us dig.)
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"True eloquence consists in saying all that is necessary, and only that which is." François Duc de La Rochefoucauld Willie T Last edited by Willie T; 09142009 at 05:31 PM. 
09152009, 01:25 PM  #7 
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Well im starting to second guess my intial foundation ideas...
I bought 10 acres in west Texas near sierra blanca, and after looking at satellite views, its all dry river and stream beds. So can i assume that when it rains it'll run off through there? well thats why im going with the house on stilts foundation, is it still not okay to put the post in the concrete even if im using pressure and chemical treated post and wood (protects against rot and bugs) Otherwise whats the cheapest easiest foundation 1 or 2 man team can do? 1000 sq ft foundation 
09162009, 12:16 PM  #8 
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You could always look at using the Bigfoot System. www.bigfootsystems.com. I used them for a deck and they were very easy to work with. I think they make bigger sizes for foundations.
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