ACI 351.3R-04 - Effective Bearing Stiffness? 1

[ARS97]

I'm designing a foundation for a fan. I'm planning on using ACI 351.3R-04 as the main guidleline for this design.

ACI discussed five (5) common procedures for estimating dynamic forces for rotating machinery. My question deals with the last method, shown in section 3.2.2.1e. This method utilizes the maximum allowable vibration and the effective bearing stiffness.

For a fan manufacturer, specifying the maximum allowable vibration is ideal since most fans will have a vibration trip level. However, it appears to me that calculating the effective bearing stiffness is quite complex.

ACI 351.3R-04 doesn't seem to discuss this specific term (effective bearing stiffness) any further, but it does dive into "dynamic soil properties" and "impedence provided by the supporting media". To be honest, at first glance, this appears to be very complex and I'm not certainly not familar with it.

Does anybody have any experience calculating the effective bearing stiffness for something like this?

 

[ARS97]

UPDATE

Equation 4-13a provides a way of calculating the vertical stiffness using the Richart-Whitman model. However, it introduces additional unknowns.

v = Poisson's ratio for the soil (can be estimated by various references)

R = equivalent foundation radius (use equation 4-9a)

G = dynamic shear modulus of soil. This is the tough one. Various empirical relationships to the void ratio and probable confining pressure is shown (based on soil type), but it's noted that it's not a very reliable estimate. You're better off with either lab results or field testing. (it's better than nothing though.)

Does anybody have any good references to the dynamic shear modulus of various soils?

 

[ARS97]

anybody?