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Question on footing width and soils testing

Rio

Silver Member
Joined
Mar 31, 2010
Messages
224
Hello:

We're doing a 2nd story addition on an existing 1 story slab on grade house and in the past have made the exterior footings larger by underpinning them to a new width of 15" and a depth of 18". From reading the 2010 CRC (California's version of the 2009 IRC but basically the same); Per TABLE R403.1 it appears that if we can show the load-bearing value of the soil to be 2,000 psf or more then we can keep the width of the footing at 12".

Also, per R403.1.4 "R403.1.4 Minimum depth. All exterior footings shall be

placed at least 12 inches (305 mm) below the undisturbed

ground surface. Where applicable, the depth of footings shall

also conform to Sections R403.1.4.1 through R403.1.4.2."

So from these two pieces of data it appears that if we do a soils test and it comes back at the soil bearing value to be at least 2,000 psf then we can forego having to upsize and underpin the existing slab on grade footing.

Any feedback and opinions would be appreciated,

Rio
 
your existing foundation is adequate without any modifications usually. The foundation will be supporting a floor and a roof when the second story is done,same as a foundation for a single story building not on a slab. table 1809.7
 
Rio,

Also check out IRC section R401 for load bearing soil types. Table R403.1 does allow a 2-story on 2,000LB soil to be on a minimum 12" wide footing, can the footing width be verified?

pc1
 
Thanks for the feedback. I think I'll talk with the AHJ to make sure they'll be okay with using a 12" x 12" footing if the soils test allows it and then have the GC do some excavation to verify that the footings are at least that size.
 
Rio-

Did you understand what pwood had to say? Thickened edge, slab on grade footings are only supporting the walls and roof of a single story building. Making that existing into a two story building only adds a floor load to the load supported by the footing and the footing would still only need to be sized as supporting one floor and one roof. No different than a one story footing.

We see this application all the time here with two story garage applications. I agree with running it by the AHJ, I was just wanting to be sure you got what was a very good point by pwood.

ZIG
 
zigmark said:
Rio-Did you understand what pwood had to say? Thickened edge, slab on grade footings are only supporting the walls and roof of a single story building. Making that existing into a two story building only adds a floor load to the load supported by the footing and the footing would still only need to be sized as supporting one floor and one roof. No different than a one story footing.

We see this application all the time here with two story garage applications. I agree with running it by the AHJ, I was just wanting to be sure you got what was a very good point by pwood.

ZIG
thanks zig!

You can lead the thirsty horses to water but you can't always get them to drink :mrgreen:
 
zigmark said:
Rio-Did you understand what pwood had to say? Thickened edge, slab on grade footings are only supporting the walls and roof of a single story building. Making that existing into a two story building only adds a floor load to the load supported by the footing and the footing would still only need to be sized as supporting one floor and one roof. No different than a one story footing.

We see this application all the time here with two story garage applications. I agree with running it by the AHJ, I was just wanting to be sure you got what was a very good point by pwood.

ZIG
Thanks for pointing that out. I did understand the point that pwood made which, if I'm not mistaken, is that a 12" wide by 12" deep footing under a stem wall that is supporting a raised floor with walls and a roof is essentially the same load wise as a slab on grade footing supporting a second floor as the second floor is taking the place of the raised floor. It is a good point and one that I will keep in mind when discussing what we are thinking of doing when talking with the AHJ.

However, with that said, the normal way that we have detailed out 2nd story structures when using a slab on grade 1st floor has been to have a footing that is 15" wide by 18" deep so that leads me to question why, if the situation is identical to a 1 story raised foundation with subfloor, has that situation not been the normal way to design slab on grade 2 story footings? Any ideas on why that would be?
 
Architects usually detail a wider footing for a block foundation. Aside from force of habit, this allows more tolerance in case the footing is laid out wrong. Engineers often double what the code requires for starters.
 
I'd say all of the above comments are accurate about the "why" and add that at some point people in general felt one way was better and it became the norm. At some point in time xx contractor started building all of his homes with certain sized footings and certain sized wall studs and only used plywood and that was perceived to be higher quality so more and more contractors went that way to sell their product and it stuck. It also points out just one of many code minimums that are overlooked. I had someone in yesterday that told me I didn't know what I was talking about when I told them they could use 2x4 framing @ 16" o.c. throughout their 2-story home if they wanted to come up with a way to attain the required insulation value in a 3-1/2" wall cavity (not that difficult). These things are done for all the same reasons we inspectors hear about why something was constructed incorrectly.. aparently.

ZIG
 
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