Concrete Pier Load Capacity/Calculations

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Old 09-17-10, 12:21 PM
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Cool Concrete Pier Load Capacity/Calculations

Can anyone recommend a way to calculate load bearing capacity of a concrete pier/post. Say 12" diameter sonotubes sunk 2 feet in the ground and 1 foot above the ground to support a wooden deck. How much weight would it carry?Same question for an 8" sonotube. I'm looking for a rule of thumb calculator so I can evaluate different diameters, depths, and soil conditions. Thank You.
 
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Old 09-17-10, 02:01 PM
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Pier

What type of soil?



The first sentence was too short.
 
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Old 09-17-10, 02:28 PM
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Where are you located? Different locations have different frost lines. Boston= up to 4'......Florida= throw it out dek blocks (not really, but you get the point). 24" may not be deep enough. Why do you want 1' of ugly showing above ground?
 
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Old 09-17-10, 07:28 PM
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I live in South Texas, no frost issues here. Deck is next to pond on my property so it's going to be elevated above highest water level when it rains, fills, and runs over the dam. Soil is clay, one source I've found says presumptive bearing capacity for clays is around 2K lbs psf ...... Thanks
 
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Old 09-17-10, 07:45 PM
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Your soil is reasonable. Now we have the know the area any type of details (area and dimensions, number of posts and whether you go with a free-standing or take a chance on a ledger support that requires proper flashing under your moisture barrier. This will determine the loads on the posts. This entails removing some siding. If the loads are high, they may be too much for the soil bearing area, especially if it saturated, then a 10" or 12" may be required.

You may need a permit for legal reasons or even just for resale value and a permit will probably require a detailed drawing and some calculations.

Dick
 
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Old 09-17-10, 08:08 PM
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Build it to minimum Deck Code, at least--- pp.9, table 4:

http://www.lancova.com/deckinfo.pdf

Gary
 
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Old 09-17-10, 08:31 PM
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Smile

This is what I needed, thanks. I found Decks.com has a couple of calculators, concrete volumn as well as a footings calculator based on beam and joist dimensions. My deck is going to be a free standing 16 X 16 treated lumber deck out in a field, no house involved. Thanks again, I think I can take it from here. Bug
 
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Old 03-04-11, 07:15 PM
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Unhappy Help take it one step further - support piers for sagging center post

I've been looking for similar data to create a support post grid to stabilize the center post in my house. I have a two story 1897 victorian - approximately 32' x 38' with four huge beams intersecting where the eroding central (unused) chimney sits in the middle of my basement. This chimney is where the beams meet to support the first floor structure.

I live in Illinois and I have a SOLID (concrete like) dirt floor in the basement that is two feet below the frost line so heaving is not an issue.

I want to dig out whatever dirt is necessary (fortunately have friends with strong backs and weak minds...) then I want to pour solid footings and piers(?) (- as few as four as many as six - and create a grid with I-beams and screw jacks to support/counter/stop any further settling.

I live in a small town - haven't been able to locate anyone locally who can provide information on how big footing should be to allow me to spread the load for the "square" i want to create to stop the problem. And unfortunately, experience has taught me that I'm usually smarter than the contractors out here. I don't have the money or access to an engineering firm to do that.

Anybody have any good suggestions ideas or thoughts on how to proceed?
 
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Old 03-05-11, 05:00 AM
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If you really want to know how big a footer is needed you need to know how much weight you need to support and the strength of the soil under your house. Without real numbers any footing size is just a guess.

Generally a 2' square by 12" deep is considered the minimum size for a pier/spot footer. Don't forget to figure in if your piers be supporting any weight of the chimney?
 
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