I've seen in many OpenSees geotechnical examples that mass matrix-proportional coefficient (a) of the Rayleigh damping expression is is taken equal to zero.
I was wondering if (a) is rational to be zero in geotechnical problems.
Typically, the stiffness proportional coeff. represents the loss of energy due to the deformation, what does (a) represents in the damping of soil that should be taken account for?
Rayleigh damping in soil
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nitadorakis
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Re: Rayleigh damping in soil
not sure what examples you are looking at .. just google "rayleigh damping soil models" or something similar .. lots of interesting articles should pop up .. some
provide eaxmples of the rayleigh factors used .. just remember in OpenSees you can provide different factors to different parts of the model, e.g. soil and structure .. this very feature was requested by our geotech experts.
provide eaxmples of the rayleigh factors used .. just remember in OpenSees you can provide different factors to different parts of the model, e.g. soil and structure .. this very feature was requested by our geotech experts.
Re: Rayleigh damping in soil
In the Lysmer-Kuhlemeyer BC the damping coefficient stands for radiation damping. I am not sure how to assign material damping to the soil.
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With Regards,
sb1966
With Regards,
sb1966
Re: Rayleigh damping in soil
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nitadorakis
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Re: Rayleigh damping in soil
In all the examples at http://cyclic.ucsd.edu/opensees/ the mass proportional damping coef. in set to zero. They are about single quadrilateral elements or columns of quadrilateral elements.
On the other hand, in geotechnical examples at http://opensees.berkeley.edu/wiki/index.php/Examples, mass proportional damping is considered.
I am still sceptical whether it is right to exclude mass prop. damping or not and if the Rayleigh damping is appropriate for geotechnical problems since the source of the soil damping unlike the structural damping is radiation and material.
On the other hand, in geotechnical examples at http://opensees.berkeley.edu/wiki/index.php/Examples, mass proportional damping is considered.
I am still sceptical whether it is right to exclude mass prop. damping or not and if the Rayleigh damping is appropriate for geotechnical problems since the source of the soil damping unlike the structural damping is radiation and material.
Re: Rayleigh damping in soil
There are two principal sources of damping for soils: material and radiation damping. The first can be accounted by using a non-linear (plasticity) constitutive model which will dissipate energy. The second one is a geometric effect of the soil being an infinite medium. To consider it, special boundary conditions should be used so propagating waves will not rebound at the soil edges. Another way is to make the model sufficiently large so the effect of the stress waves at the edges are almost null.
Rayleigh damping is normally used to account for one or both of the above mentioned damping types in an approximate form. I think at least some Rayleigh damping should be added to models even if not to account for the above reasons, but to remove some numerical problems. Normally, stiffness proportional damping alone is a good idea because it will make the damping ratio increase for higher frequencies. So typically, you use a small damping ratio (maybe 1-5%) and determine the stiffness proportional factor by using the highest frequency of interest in the following equation:
n = 2*w*z; where n = stiffness damping factor, w = highest frequency of interest, z = damping ratio.
This will make the damping ratio at the selected w equal to the selected z. For lower frequencies the damping ratio will be lower (not having much influence on their contribution) and for higher frequencies the damping will increase (damping out their contribution).
Rayleigh damping is normally used to account for one or both of the above mentioned damping types in an approximate form. I think at least some Rayleigh damping should be added to models even if not to account for the above reasons, but to remove some numerical problems. Normally, stiffness proportional damping alone is a good idea because it will make the damping ratio increase for higher frequencies. So typically, you use a small damping ratio (maybe 1-5%) and determine the stiffness proportional factor by using the highest frequency of interest in the following equation:
n = 2*w*z; where n = stiffness damping factor, w = highest frequency of interest, z = damping ratio.
This will make the damping ratio at the selected w equal to the selected z. For lower frequencies the damping ratio will be lower (not having much influence on their contribution) and for higher frequencies the damping will increase (damping out their contribution).
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