Seismic base shear for equipment foundation

[structural-eng]

I am designed a small mat foundation to support some electrical equipment in an area that has moderate seismic loads. The lateral seismic load applied to the center of gravity of the equipment is determined per IEEE 693 and is similar to the load resulting from ASCE 7. Do I need to include the weight of the foundation in the lateral seismic load? The foundation is 2 ft thick and the top of it is 6" above finished grade. I started by conservatively including the weight but because the lateral seismic load is 0.33*W and the coefficient of friction that I was given by the soils engineer is 0.32 the safety factor for sliding does not even equal 1.0. If the foundation weight does have to be included, it seems I would have to use passive pressure to resist the force. Is it common to use passive pressure instead of friction for foundations like this or is the weight of the foundation neglected from the foundation stability analysis. Thanks.

 

[canwesteng]

Do you need to stop sliding or is stopping it from overturning enough?

 

[MotorCity]

You do not include the foundation weight when calculating the seismic load. The foundation is a means of resisting the seismic base shear, it does not contribute to seismic base shear. Your foundation does need to resist sliding and overturning and may be resisted by passive pressure

 

[structural-eng]

I need to resist the sliding and overturning. I have typically resisted sliding from wind with friction because these foundations are too shallow to develop enough passive resistance. I couldn't find anything that stated clearly that the weight of the foundation wasn't included in the seismic base shear.

 

[MotorCity]

The foundation is below grade and moves with the earth during a seismic event (i.e. no relative acceleration, so no force). It cannot sway back and forth, so the period is zero. Finally, look at the seismic coefficient "C" in F= CV (see ASCE 7). "C" depends on "h", the height above base. If h = 0, then C = 0, and F = 0.

 

[structural-eng]

Thanks for the explanation. Is it still ok to neglect if the top of the foundation is 6" above grade?

 

[canwesteng]

Well, what is engaging the slab to move with the ground if not lateral earth pressure and friction on the base of the slab? I would argue that because the magnitude of displacements is relatively small that sliding isn't a concern, but maybe that's just me.