Basic Foundation Overturning Question

[HZStruct]

Hello,
I am checking the overturning stability of a slab-on-grade supporting a pipe support. I am checking the overturning about the bottom edge of the slab, which is 14" below finished grade. My resisting force is the sum of the vertical load (weight of pipe and concrete slab). My question is related to the overturning force. Do you consider the overturning moment of the slab in addition to the overturning moment from the weight above the slab? Is it too conservative to include the overturning affects from the slab?

Thank you for the help in advance!

 

[Blackstar123]

Do you consider the overturning moment of the slab in addition to the overturning moment from the weight above the slab?

By overturning moment of the slab, do you mean the eccentric moment produced due to eccentricity between the resultant force and centroid of the base? if yes, then the answer is no. Because considering this eccentric moment for stability calculation signifies that the point of rotation of the resultant force is at the centroid of the base.

As I see it, the simplest way to determine the overturning stability of a system is by assuming that all forces are acting on a rigid body, which is pinned at a logical point. Summing up the moments due to external forces based on their respective positions from this point of rotation will consider any inherent eccentricity present in the system.

 

[JoshPlumSE]

What I generally do is take the "De-stabilizing load" and make an overturning moment from it. Then I take the stabilizing load and make a stabilizing moment from it.

That sound obvious, but it's a little more complicated than you might think. I created a write up about this some years ago (when I used to work for RISA). See link below where I call this method the "True Safety Factor Method":

https://risa.com/risahelp/risafound...Stability and Overturning.htm?TocPath=_____11

 

[HZStruct]

Does the "De-stabilizing load" account for the de-stabilizing load of the slab and the structure it is supporting or just the structure above the slab? I have attached a simplified diagram for reference.

 

[JoshPlumSE]

I see what you're saying now. You're talking about the lateral seismic load from the slab weight.

Yes, that would be included in the lateral, destabilizing load. Though, there is always some debate over how far below grade you have to be before that load is not included in the structures seismic shear. I think there is a pretty good argument that anything below grade need not be included in the seismic mass of the structure. We're talking about Inertial effects after all.