Critical damping equation of rotational vibration
Critical damping equation of rotational vibration
(OP)
When calculating the damping ratio of rotational vibration of a concrete block foundation I realized that I am not sure the critical damping equation is correct.
If I use the quation: Cc=2*sqrt(K*m) (where K is the stiffness, m is the mass, Cc is the critical damping constant) and compute the damping ration using D = C/Cc, the units cannot be cancelled out! The ratio ends up having a unit (ft) left over. Apparently something wrong here.
For rotational vibration, K is in k.ft/rad, C is in k.ft.sec/rad, m is in k.sec^2/ft.
Could somebody point out the problem here? Thanks
j1d





RE: Critical damping equation of rotational vibration
In that case, the spring constant k is in kips/ft and m is in kips-sec^2/ft. Multiplying that through will yield kips-sec/ft, which are certainly the units for damping. I think the problem lies in the units you have for the spring constant.
A more thorough treatment of this subject can be found in MIL-HDBK-1007/3, which is at
http://w
As far as rotational vibrations are concerned, unless the entire system rotates, if you have rotating machinery with eccentric loads, the load will manifest itself as a axial load which continuously changes direction. This in turn can be broken down into z- and x-components, which can usually be analysed in a linear fashion. This is similar to a vibratory pile driver, which I deal with at length at
http:
http://www.pz27.net
RE: Critical damping equation of rotational vibration
But still I don't know how to calculate the critical damping for rocking. In a specification it is required that the geometric damping ratio of pile group for rocking mode is less than 0.10.
Regards,
j1d
RE: Critical damping equation of rotational vibration
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There's some other info (albeit theoretical) in Verruijt's book on Soil Dynamics, found at the same page as the MIL-HDBK.
http://www.pz27.net
RE: Critical damping equation of rotational vibration
Regards,
j1d