building on posts

[rsqdivr]

I'm not sure where to ask this question. I'm wanting to build a workshop/shed (unfinished inside) off of a dropoff on my property. There is about a 10' steep elevation drop. I want the shed to be at ground level but the foundation will start about 10 below. Similar to a 2nd story building or deck. I can't find any info on how you figure the amount weight the support posts can handle or even how much an unfinished (shingle roof) shed would weigh (not decided on size but probably 10'x20' or less).
What should support posts spacing be, etc.? 4x6 or 6x6?

My thought was to pour cement footers to support the posts, then frame the shop floor at ground level above. How deep/wide do the footers need to be?

Location is near Mobile, AL. Any recommendations? Thanks for any help.

[RTF]

Soil type and your proximity to the coast will play a major factor in your structural design. You may want to get the advice of a Structural Engineer before going much further. Building on posts is fine, but they may need to be pile driven or encased in concrete as part of the footing.

[stevethepirate]

@ RTF - is there a depth that you should go down for the footings? Generally how big are the footings?

[MtnDon]

A 10 foot drop in how mant horizontal feet?


A footing must be below frost level. Where there is no frost the minimum depth is 12 inches.

Size can depend on load expected and load carrying ability of the soil.

Slopes introduce special concerns. Some slopes slowly creep downhill all the time; sometimes the trees if any can give a clue to that. ("pistol butts") IRC section R403.1.7 covers building on or adjacent to slopes. In many cases a slope means an engineer may be required, maybe two, a GeoTech and a Structural.

http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_4_sec003.htm

Note that the IRC does not cover post or pier foundations as far as providing a prescriptive method for building on them. That is because posts/piers do not offer the stability of more conventional poured concrete strip footings that go around the full perimeter. At least the way many DIY make pier foundations is not as stable. Correctly done, very deep and with a large cross section thay can be very good. Once again a reason an engineer may be required.