Interior Footing Embedment

[sgs114]

Hello,

I have a question regarding minimum embedment for interior footings. Per IBC 1809.4 it states you need minimum 12" undisturbed for all footings. In the field it seems as though there are frequent cases where interior pad footings bear directly on the soil with minimal embedment. Anyone know a code reference for interior footings either strip or pad footings for residential applications? I have been unable to find anything with regards to this, so I am guessing it is min 12" per 1809.4? Thanks for any input.

SGS

 

[dcarr82775]

IRC Section R403.1.4 has almost the same language.

 

[nac521]

If by embedment, you mean footing thickness, then IRC403.1.1 states 6" thickness minimum. Chapter 7 of ACI 332-10 also covers residential applications for concrete footings and states similar requirements.

 

[jayrod12]

and if you're talking about being keyed into the native soil then I don't know of a minimum embedment. If you are requiring the pad to take lateral load then there's more issues. Especially in residential.

 

[AELLC]

It is more of the Geotech's call because you can get a bearing failure if the surrounding soil can heave the slab upward.

12" would be absolute minimum. I would not ever allow 6" thickness in any residential ftg, 12" is minimum for pad ftgs, and 8" is min for wall ftgs.

Here in Phoenix, despite no frost, it is actually 18" minimum embedment req'd

 

[AELLC]

ps

I meant 8" thick wall ftg with 4" ext grade cover, but here in PHX we need that wall ftg to be embedded at least 18" BELOW top of ext grade.

 

[jayrod12]

Maybe I read the question wrong, but I believe he was asking about interior footing/pads, therefore zero frost concerns. you can place the footing directly below the main floor/basement slab and it shouldn't matter.

 

[AELLC]

If you disregard frost, then you need at least 12" embedment (read bearing depth) anyway.

Or 18" here in PHX.

 

[jayrod12]

I come from an area where basements, or at least crawlspaces are the norm because we already have to go to at least 6 feet for frost on the exterior anyway so I've never been concerned about interior pads.

 

[msquared48]

I high seismic areas such as where I am, we are concerned with the interior footings literally walking laterally under seismic action, so we always embed the footings at least 7" to prevent lateral movement and loss of support of the column/beam structure above. 12" would be good.

Mike McCann
MMC Engineering

 

[IsaacStructural]

Here we drop exterior footings 42" for frost, and commonly place the top of interior footings directly below the slab on grade and provide a diamond control pattern at the column itself. Depth to bottom (bearing) of the footing depends on the footing thickness, but won't ever be less than 12".

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)