Interior Bearing walls (residential)

[Antnyt23]

I am curious on some others thoughts and opinions on foundations below interior bearing walls on residential structures.

I have heard of some engineers utilizing strictly the slab on grade (4 or 3 1/2" slab) as bearing for interior bearing walls not subject to uplift, however, I can't justify this by the code.

To me ACI (2014 being referenced here) states in section 1.4.7 and 13.2.4.1 that if the slabs-on-grade transfer vertical forces from other parts of the structure they are to be designed per this code.

Once they do that I would consider them as either plane concrete footing or reinforced concrete footing both which require 8" thickness (sections 14.3.2.1 and 13.3.1.2).

Now the IRC (2015 being referenced here)sections R403 show 6" thick footings for "slab-on-grade" which would reference their detail of thickened-slab-on-ground footing at bearings walls or braced wall lines. To me this says 6" thick which doesn't meet ACI requirements.

Just looking for any clarifications that can be made on the following:
A - are footings required under bearing walls not subject to uplift (yes in my opinion)
B - What depth is required (8" in my opinion)(IRC may lead us to believe 6" possibly)

 

[atrizzy]

Design it as you would any other footing. Depth (frost) concerns are minimal in an enclosed/heated space. Calculate the width required for the load and the depth required for shear/flexure. I personally would thicken a slab on grade to provide a specific bearing point and avoid any local settlement of the slab, but I believe this practice is debatable.

 

[Antnyt23]

Atrizzy,

My issue is i'm okay with a 4" slab on grade for a certain load "X"/ft, however, code (to me) specifically has requirements to increase depth to 8" regardless of that capacity per ACI (and then IRC throws some confusion in for 6").

I also agree with your points on local settlement at the bearing points/slab. I just want to be able to justify a code minimum requirement for this situation. Often run into customers that would prefer to know the min. required by code if that is their choice given a reduction in quality.

 

[XR250]

I generally thicken the slab but mainly to get down to better soil. Around here, they usually just scrape off the top soil and add 4" of gravel.