Dynamic Analysis of Foundation 1

[sharathkumar]

How do we calculate Dynamic Spring and Damping Constants for bedrock and soil, for the case given below.

Type of Structure : Steam Turbine Generator
Operation Speed : 3600 rpm
Foundation : Pedestal Type, 46'x106'x42'high
Existing Ground Elev. : 790.0'
Top of Rock Elev. : 786.0'
Footing bottom elev. : 778.0'
Shear wave velocity (soil): 211 m/sec
Shear wave velocity (rock): 1275 m/sec

VALUABLE GUIDANCE IS HIGHLY APPRECIATED. THANKS IN ADVANCE.

 

[Qshake]

I recommend that you get a copy of Federal Highway Administration booklet on Sesmic Analysis and design of Bridge Foundations authored by Earthtech.

Also Foundation Analysis and Design by Bowles probably has the equations your looking for spring constants.

references by Gazetas is also good.

I don't know about damping though.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BigH]

Why not Richart Hall and Woods? Also check out any works by Novak of University of Western Ontario.

 

[sharathkumar]

First of all, Thanks for the replies.

I have drilled every equation from Bowles and came up with some numbers. The numbers were unacceptable by our client and also the Ao factor in this case was 12.5, but bowles procedure was good for Ao<1.5

I tried Novak, but the procedure in his 1974 paper is applicable to concrete and wooden piles and can be stretched to H-piles in some given conditions.

Dobry and Gazetas procedure is the same thing given in Bowles.

I personally did not check Richart,Hall and Woods,but my co-worker did, only to be unsuccessfull.

Thank you.

 

[mrMikee]

sharathkumar,

Richart, Hall, and Woods is an old text (1969) but I thought it did give enough info to set up a dynamic model. I could be wrong however.

Another idea is some software I saw that looked interesting at cadreanalytic that is described as a "utility for determining the stiffness values for foundations and footings in order to consider ground/structural interactions." I have no experience with this program.

The program is Rescol seismic version 2.0

http://cadreanalytic.com/products.htm

Regards,
-Mike

 

[BigH]

There is another book by Arya (you can do a search of the various threads here. Someone did give the correct name and the like). It is pretty detailed. How did the client know the numbers were unacceptable?? Is that really 42ft high? or 42 inches high? That'd be a pretty thick pedestal. If you are founding deep into rock, unless it was weathered, it should be able to give you a very high damping - same with the embedment. These would significantly reduce any amplitudes - but I haven't worked out any numbers.

 

[SlideRuleEra]

I will not butt into the solution of this important problem (it is beyond my skills). However I can offer an (electric utility focused) explanation of what is going on.

A steam turbine/generator pedestal is "tuned" to cancel out vibrations from operations. The typical pedestal is a very large, isolated, reinforced concrete "table", probably with about 8 stout legs that sits on a thick reinforced concrete inertia block. An inertia block 12 feet (or so) thick is not unusual. The top of the pedestal (table top) may be a 4 to 6 feet thick. The legs may be 8 to 10 feet square (or rectangular) in cross-section. A total height of 42 ft. is typical.

During both start-up and shut-down of the turbine/generator the rotating speed goes thru several resonant vibration frequencies. There is even some vibration at synchronous speed (3600 rpm for a 2-pole, 60 Hz machine). The dampening effect of this entire, integrated mass (the inertial bock, pedestal itself & the turbine/generator) is critical to keep vibration of the accelerating/decelerating/operating unit within acceptable limits.

I have had to explain this principle on several occasions (as an electric utility engineer) for legal purposes - in a "nutshell" have referenced to this whole structure as a kind of "anti-tuning" fork.

Now, I'll get out of the way and try to learn something from you.

http://www.SlideRuleEra.net

 

[fndn]

"Dynamics of Bases and Foundations" Translated by Barkan might be useful. It is about 30 years old. Try to search it at

http://www.abebooks.com

or similar.

 

[mrMikee]

SlideRuleEra,

I for one always like to hear your two cents. Based on what you say, this one is beyond me too.

-Mike

 

[sharathkumar]

Thank you all for your timely responses. The numbers by Bowles were deemed unacceptable because of the Ao factor.
Right now we are trying to create a model in Dynamat, Ensoft Inc. software. Hopefully this will work.

I have come up with some numbers based on the "FHWA, Geotechnical Earthquake Engineering for Highways, Circular No. 3" as suggested by Qshake. The numbers were in the same range of that of Bowles, but there were no bottlenecks in the procedure.

Next step would be comparing those numbers with Dynamat results and go from there. All your suggestions are really helping and I will keep you posted as I tackle this Dynamic Analysis.

MORE SUGGESTIONS ARE WELCOMED AND HIGHLY APPRECIATED.