Support location in cantilevered grade beam

[Buleeek]

Hi everyone,

I am designing a PEMB foundation that is going to be near existing building. The new foundation by the existing building will be placed not directly under the columns but offset a couple feet. Because the new PEMB is small the footings will overlap, so one continuous foundation will be installed.
See attached sketch. Is the assumed beam geometry and pin supports accurate for this situation ? If not, what is your recommendation to properly approach this problem.

Thank you,

 

[JoelTXCive]

I'll repost your photo...

My thought is that if the proposed slab and the existing foundation are touching, then you will be sending load to the existing foundation until it fails. Upon failure, you will begin to see cantilever behavior.

If you want cantilever behavior, then you need an air gap of some sort.

What is the magnitude of axial loads on the column?

 

[human909]

If those dimension are even vaguely accurate do you even need the grade beam? Can't you just design it as a raft slab? Or are you doing local thickening at the grade beam and a thin slab everywhere else?

 

[BAretired]

If the column loads are equal, the c.g. of load occurs 6'-0" from the left end. That coincides with the right edge of the kern of the new rectangular foundation. Without relying on the existing foundation, soil pressure on the new foundation varies from 0 at the left end to twice the average pressure at the right end.

If the existing footing can carry additional load, your detail may be okay; otherwise it may be better to adjust the shape of the new footing so that it's c.g. is closer to the c.g. of the new loads, perhaps "Tee" shaped or tapered.