Foundation (Slab-on-grade) Load Interference?

[Boozie]

Hello,

There is an existing warehouse structure, say 30' tall PEMB, bearing on a slab-on-grade with a 40" deep thickened edge.

Our task is to design and build a new warehouse structure, same height, same build, bearing on a slab on grade. The new will be directly adjacent to the existing...separated by an expansion joint. There is no easement between the two buildings. We have conservatively decided to match the thickened edge depth and bearing elevation to existing.

However, we now found out that we are in a flood zone, and need to raise our FFE by 1'-0" to meet FEMA requirements.

Can we raise our entire SOG and thickened edge up by 1'-0"? If we do, the new slab will be bearing 1'-0" higher than the existing. Is there any load interference issues in this scenario? I know load interference is a thing for isolated pad footings - is the same true for SOGs? If not, when does load interference become an issue?

See the simple picture I provided to graphically show what I mean.

 

[Greenalleycat]

You have to raise your slab FFL but why don't you just take the perimeter footing down to the existing SOG footing depth anyway?
That's what I always do in these situations

 

[Boozie]

Thank you for your response. So from your experience, is there a concern with the two different bearing levels as per the image?

The only concern with your suggestion is the thickened edge is already considerably massive at 40". Taking it down would mean adding 12", making it 52" thick. I don't think we need 52", let alone even 40". We really just did it to match existing. Taking it down further will drive up the cost, though I'm not sure how much as of now. The owner/PM is cost-conscious and not a fan...but if there is a need to keep the bearing levels the same, what you're suggesting is the easiest approach.

 

[JAE]

Whether you keep it at the same elevation or not you will still have overlapping bulbs of pressure from the two building column loads.
Having the new slab footing base higher will also impose some lateral loads on the existing footing, which can cause problems so Greenalleycat's idea to lower your new footing to match the existing makes sense in that regard.

But either way the new loading will cause initial soil displacement downward over the existing footing area of influence in the soil. Whether this is a problem depends on the soil capacity and the column loads applied together like that.

 

[SE2607]

I second (third?) Greenalleycat's recommendation.