Foundation Engineering

Started by paullad, June 01, 2016, 04:22:31 PM

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I'm trying to figure out a post and beam foundation for a story and a half cabin with a crawlspace. The posts (6x6) and beams (4x12) are straightforward for the 9'6" span. I'm just not sure how big of a gravel or concrete pad I need in my sandy soil. Does anyone have any good resources? Thanks!


Paullad, your profile does not show your location.  What and how you can build is usually determined at the county office that issues permits and does code inspections.  That would be a good place to start.  Depending on jurisdiction and how the building will be used, you may or may not have to satisfy an inspector that your foundation design is adequate.

Beyond that, you will want to ensure to your own satisfaction that the foundation is sufficient for your needs.  This question just came up in another thread and the discussion may prove helpful:

In simple terms, your pad needs to be big enough, and then some.  For basic footing size determination, divide the load that the footing supports by the bearing capacity of the soil.  If you are supporting a 10,000 lb structure with 4 posts, each pad will be supporting something in the ballpark of 2,500 lbs.  The uncertainty is due to the fact that loads don't get distributed evenly.  Different soils have different bearing capacities ( ).  If you are in clayey soil, your footing needs to bigger than 1.25 sq ft to keep the pressure below 2,000 lbs per sq ft.  For your building, plug in actuals for total weight, number of posts, and soil strength.  Then double (at least) the square footage per pad.

Your county probably has adopted the International Residential Code (IRC) which establishes prescriptive construction methods suitable in most areas.  What that means is if you build to code then you won't need to hire an engineer.  For designs and methods that aren't in the code, you'll typically need an engineer to review and stamp the plans in order to get a building permit.  Please note that post and pad foundations are not in the IRC.
My cabin build thread: Alaskan remote 16x28 1.5 story