IRC Foundation Design?

[SteelPE]

I am in the process of designing a residential garage that will have vehicles on two levels (suspended floor and slab on grade). Code is IBC/IRC.

My current wall height is approximately 9’-6”. I am having a crisis in terms of trying to design the foundation wall. The client is OK with waiting to backfill until the floor is in place. So, we now have a propped wall case in terms of wall design.

I have always been taught to use 70*(the height of the wall) for the pressures acting in this instance. When I calculate the wall pressures this way, I end up with quite the lateral force at the base of the wall. My crisis comes from the pressure. I need quite a substantial footing to resist these loads. However, when I sift through IRC, all it appears I need is a 2’-0” wide footing (table R403.1) at the base of the wall with little to no reinforcing in the wall (table R404.1.2(8)) assuming a 12” thick wall.

What gives?

I am now torn between the two and will more than likely settle for something in-between. I am guessing my pressure of 70*h is just a bit too much.

 

[dauwerda]

I seem to recall (though I could be wrong) that the IRC also requires a minimum 3.5" thick concrete slab to be poured between the walls at the base - this is what resists the sliding force. The reinforcement in the wall (or lack of) is a whole other issue.

 

[r13]

I think your soil pressure is too high. I assume you are going to have clean backfill, for which the friction angle Ø = 30° to 35°, and Ko = 1 - sinØ. For a backfill with Ø = 33°, ϒ = 120 pcf, p = (1-sin(33°))*120 = 55 psf - 12.5% less than 70 psf. Also, do you assume uniform pressure over the entire height of the wall? It is another unnecessary conservativism, should use triangle distribution.

 

[SteelPE]

retired13,

I tank a triangular distribution on the wall however, I do add a surcharge at grade equal to 1 foot of additional soil. This is to account for any LL that may be placed on grade.

 

[phamENG]

The IRC may allow minimal reinforcing, but how often do you see foundation walls that are failing after 20 years from frost heave, water infiltration, etc.?

Your 70*h does feel a little arbitrary. I'm with retired on this one - sharpen your pencil on the lateral load calculations and I think you'll find a more reasonable wall design works.

 

[WARose]

Your pressure may be off.....but then again: so may be your loads. The IRC is a prescriptive design based on certain situations. Without seeing your details, your "suspended floor" with a vehicle (on top of another level with a vehicle and other loads) is likely more load than the IRC anticipates. (At least to the best of my recollection.)