Slab on Grade Stability FS

[jtjc23eng]

Hello all,

I have a concrete slab on grade for heavy equipment and I am running the stability calculations with ASD combinations. According to "Principles of Foundation Engineering" by Das, the factor of safety for bearing capacity checks should be 3.0. That seems high for temporary extreme loads like combinations with seismic. I have heard from a geotech that FS 1.5 is typical for temporary extreme load cases. And unfortunately I do not have a geotechnical report....

So my question is, what foundation bearing factor of safety should be used for an ASD seismic load case and if there is a reference I can tie back to?

Thanks as always!

 

[EireChch]

When you say you are running stability calculations, what does that mean? Software or a bearing capacity calculations ?

In IBC, for certain load combinations, you can assume a 33% increase on your static allowable bearing capacity.

 

[Ron]

When a bearing capacity is given by a Geotech, it already has a 3 to 4 FS.

 

[JoshPlumSE]

What load combinations are you using? Do they include a 0.6DL factor? If so, there is an inherent factor of safety for overturning implied in that factor.

 

[jtjc23eng]

I should have clarified. These are for foundation bearing hand calculations. I am using ASCE7 ASD load combinations with seismic like 0.6D-0.7Ev+0.7Eh and 1.0D +0,7DEv+0.7Eh. I pulled the soil bearing capacities from CBC Table 1806.2 2000 psf. I do not have a geotechnical report or any site specific information.



Thanks again.

 

[JoshPlumSE]

I believe the IBC / CBC has a set of "alternate" load combinations for Allowable stress that replaces the 0.6 DL factor with a 0.9 DL factor.... So, long as you have a 1.5 overturning safety factor.

I don't have the current IBC in front of me, so you'll have to look at it yourself to be sure. My memory my be skewed by an older version of the code.