Damping in a model.
Damping in a model.
(OP)
I have a situation where I have a concrete foundation [supported on concrete caissons] where I am trying to validate the damping I am using in my model. I am using STAAD Pro which lets you put in a overall damping percentage [for all modes; or you can input one for each specific mode] but I am trying to think of how to figure a composite damping factor that takes into account the material damping and the damping from the supports. (Or at least be sure the 5% I am using is not overestimating the damping.) I'm not sure how exactly to do that.
One school of thought I've heard is: for modes that mostly deform one type of material, that material's damping constant should be specified. So I guess if you specified the lowest damping constant for all modes, you should be covered. But I am a bit curious as to how to consider damping from a support (if possible). I suppose, thinking of it as a Single Degree of Freedom system you could combine the "dashpots" [working in parallel] into an equivalent single constant.
Thoughts?
One school of thought I've heard is: for modes that mostly deform one type of material, that material's damping constant should be specified. So I guess if you specified the lowest damping constant for all modes, you should be covered. But I am a bit curious as to how to consider damping from a support (if possible). I suppose, thinking of it as a Single Degree of Freedom system you could combine the "dashpots" [working in parallel] into an equivalent single constant.
Thoughts?





RE: Damping in a model.
5% damping seems an awful lot, I think you'd be unwise to use that for modes that are largely in the structure rather than ones that excite the foundation.
Sorry I have no simple solution, when I need a damping value I take my trusty hammer and hit the thing.
Cheers
Greg Locock
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RE: Damping in a model.
But recognizing that the concrete could deteriorate over time, and the fact I have seen that data that suggests the damping ratio is lower for materials in higher modes of vibration: today I ran my model with a low damping ratio (i.e. 2%) for all modes and was satisfied with the results.