Residential Foundations

[xfmrexpert]

I am buying some new construction up in Saratoga, NY. I'm an electric power engineer, so this is a bit out of my depth. The foundation is 8' with 6' backfill, 8" thick. I'm told the vertical reinforcement is #4 at 48"oc (although this isn't on the drawings). The IRC (and New York State Residential Code) seems conflicted on this. R404.1.1(1) allows 8" plain concrete walls for GW, GP, SW, SP, GM, GC, SM, SM-SC and ML, but requires 10" for SC, MH, ML-CL and inorg. CL. However, table R404.1.1(2) for 8" reinforced concrete walls specificies a minimum of #4 at 48"oc for GW, GP, SW, and SP, etc. What I'm driving at is how can an 8" wall with no rebar be OK for some soils in R404.1.1(1), but the next table specify a minimum rebar requirement for the same size wall and type of soils? Which is right? Is #4 @ 48" OK, or is it under-reinforced?

 

[dik]

If granular soil, the vert rfg may be adequate... if clay, then might be deficient...

Dik

 

[xfmrexpert]

Without a geotech report, how do I know if the soil has any significant amounts of clays or silts? Or more importantly, how did the builder know... Is it typical to design for the worst case condition of clay?

 

[jen4950]

#4's at 48" isn't even minimum Temperature and Shrinkage steel in a 8" wall/slab.

0.0018 x 8" x 12" = 0.1728 in^2/ft

0.20in^2 / 4ft = 0.050 in^2/ft

#4's at 4'O.C. is not worth anything structurally is my punchline.

 

[dik]

In Ontario, the code allows for 8" CMU (unreinforced) walls and in Manitoba, the norm for residential is to use 3 - 20M
horiz?... I don't know what local codes permit and how much they rely on the flexural strength of plain concrete...

Dik

 

[ctcray]

Another point to remember about the IRC is that it's prescriptive. You can spec your wall based on the unreinforced table OR you can spec it based on the reinforced table, OR you can design by ACI. An inspector is only going to make sure that it meets one of these requirements and then move on.

 

[BSE05]

For years here in New England the concrete walls were 8" with no reinforcing or footing. The large majority are still crack free. Our soil is mostly sands and gravels.

The passive soil loads without water are low, its the weight of water and ground surcharge that will fail an unreinforced wall. (#4@48" I consider unreinforced)

 

[HouseBoy]

Most of the time 8 ft tall 8 inch thick with 6 feet of backfill will be OK. Exceptions are; sloped site (toward the house) very bad clayey soil or otherwise problematic soils.
You could consider the wall as plain concrete in bending (allowable stress is something like 5 x.65 x sqr. rt. f'c or 178psi for 3000psi concrete.
Can't answer why the tables don't jibe other than the prescriptive comment by ctcray.