Response to a reader question on the damping estimation formulas in NSCP 2010 Section 207

A reader asked,
Equations 207-38 & 207-39 equate damping to 0.016/h and 0.23/h, respectively. However, Equation 207-40 equates damping to 0.007/n, in which h = height of structure, and n = natural frequency of the structure. Are Equations 207-38 & 207-39 correct, or should h be replaced by n?
Just because there are some errata in a document, that doesn't mean everything else is in error. But sure, it might seem that this is confusing, though let me put it this way, too: is Equation 207-40 meant for the same thing as Equations 207-38 and 207-39? The answer is NO. Equations 207-38 and 207-39 are for estimating structural damping. Equation 207-40 is for estimating aerodynamic damping. Two different bananas. I think it's not confusing at all, and it's very clear in the NSCP text. Sure, the subscripts are small but again, there are texts that accompany the equations, anyway. Well, it's good to ask and now you know. :)

Also, many people are studying damping because while it is just one parameter in wind load estimates, it is associated with high variability. I hope we learn at least one new thing now: there is such a thing as aerodynamic damping and it is very different from structural damping.

May I just add that the damping and natural frequencies for wind loading purposes are different from those we should use for earthquake loading purposes, because under wind loads, we assume linear elastic behaviour, but we are already in the nonlinear/inelastic phase under earthquake loads. More to discuss on damping in future posts.

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