foundation calculations

Started by Bearmech, April 11, 2017, 09:10:27 AM

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Bearmech

Just trying to get this right. Say 20x40 core + 12 foot overhang on each end. Assuming 50 lbs roof load and 100 lbs floor load. Ok, 60*22*50=66000 lbs for the roof and 20*40*100=80000 lbs for the each floor. 80000+66000=146000 lbs. Continuous footer (40+80)*1.5=180 square feet. 146000/180=811.1 lbs per square foot. Do these calculations sound right for soil loading? I'm just looking at the method of calculation not the actual size of the structure. Are the 50 and 100 loading numbers accurate as a general rule?

ChugiakTinkerer

You have the right idea.  I think your assumptions are on the conservative side, but not knowing how or where you are building it's a good way to start.  For a living space floor the design load is a minimum of 40 lbs per square foot and 10 lbs psf for dead load.  If you are building with heavy material the dead load would go up, such as with tile floor with cement board underlayment.  A sleeping space floor has a minimum 30 psf live load and 10 lb dead load calculation.  The roof is based on the snow load, if any, or a minimum 20 lb live load and a 10 lb dead load per square foot.  If the house is one story in snowless country, then your load calc would be for 20x40x50 = 40,000 lbs for the floor and another 22x50x30 = 33,000 lbs, for a total of 73,000 lbs for the structure.

If there is a sleeping loft or second floor you would add that weight using 30 or 40 psf for live load.  Bump up the roof load if you have snow.  You can find it in your area at http://snowload.atcouncil.org/  If there is much snow it can get really interesting in estimating loads.  Snow likes to form drifts and in some climates they can get very heavy.  Definitely time to see a pro in that situation. 

Your footer calc is spot on.  You've got the gist of it.
My cabin build thread: Alaskan remote 16x28 1.5 story


Bearmech

Thanks I'm in NE Texas Not much in the way of snow, I'm more worried about the bearing capacity of the soil. I just want to make sure the footer sizing is right. I would rather over build the foundation, there are better places than the foundation to save on materials and such.

Bearmech

Looking at those calculations, even in poor soil that footer would hold up the 1 1/2 story cabin, I will be building it Single story. I hate stairs. :)