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Fw: Dynamic design of machine foundation - Shear Modulus

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The Arya book gives the following typical values for low strain amplitude shear modulus for clays:

Soft Clay        3,000 - 5,000 psi
Stiff Clay        10,000 - 20,000 psi
Very Stiff to        
Hard Clay        > 20,000 psi

This is another reason that the analysis is very tedious because you typically have to run the analysis at least twice with the upper and lower shear modulus values in order to bracket the solution.  Or, it you get a value from the geotech you will want to use +/- 25% since soil properties are never exact.

Thomas Hunt, S.E.
ABS Consulting


12/13/2004 01:30 PM

Re: Dynamic design of machine foundation


The best guide book for rotating equipment at grade is "Design of Structures and Foundations for Vibrating Machines", by Arya/O'Neill/Pincus however it is out of print and hard to find.  It is a very tedious procedure with lots of small numbers but in essence you develop springs and damping ratios for six degrees of freedom then calculate the deflection from the out-of-balance forces generated by the rotor shaft.  This is an ideal situation for a program or spread sheet but if you are doing this for the first time note you will spend a lot of time debugging and trying to get it right.

Thomas Hunt, S.E.

ABS Consulting

"Rich Lewis" <sea(--nospam--at)>

12/13/2004 12:49 PM

Please respond to

Dynamic design of machine foundation

I am designing a foundation for a 130,000 lb compressor with dynamic
loads.  The foot print of the equipment is over a 30X40 ft. area.  It
will have a separate mat foundation below it.

I have done some research and it appears to me a dynamic load such as
this requires a special analysis to calculate the deflections based on
the dynamics.  I found several references on this, one in particular by
Bowles in the 5th ed., chapter 20.  He has a program to calculate the
vertical, horizontal and rocking deflections due to the machine
dynamics.  Is the method he describes in his book what is commonly used
in practice today?  Is there an easier way to do this?

Bowles method requires the dynamic shear modulus.  Are there some tables
with typical values for clay soil other than what he has in his text?
Bowles uses primarily metric in this chapter and doesn't have an
equation for English dynamic modulus.

I would appreciate any suggestion I can get for foundation design of
dynamic equipment loads.

Richard Lewis
Lewis Engineering

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