Damping in a model. 1

[WARose]

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?

 

[GregLocock]

In a 'proper' normal modes analysis each mode has an associated /global/ damping (and frequency). For each mode there is effectively only one dashpot.

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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[WARose]

Great post Gregg. The structure I am modeling is basically a pile cap [2’ thick] sitting on some caissons (represented as springs). I calculated that the concrete wouldn’t crack [its overdesigned], so 5% didn’t seem out of line compared to recommended values. [Some texts put the range higher than that (i.e. 7-10%); and 5% falls on the high end of cracked concrete recommendations (i.e. 3-5%).]

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.