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Re: mass vs vibration

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On Jul 16, 2006, at 11:15 AM, <William.Sherman(--nospam--at)> wrote:

Your statement that "mass makes the situation worse, not better" seems
contradictory to the rule of thumb to make the mass of a foundation at
grade several times the mass of vibrating equipment.  Please clarify.
I should have said that sprung mass makes the situation worse. If you'll remember, the subject was the supported concrete panels, not vibration in general. The effect of additional sprung mass is to decrease the system natural frequency so for a given response acceleration the displacement and stress increase. Again, if you'll remember we discussed a couple of situations where even additional sprung mass can help. If you have a structure that stiffens under stress like a membrane, the additional mass loads the structure, makes it stiffer and may, up to a point increase the natural frequency. Or the additional mass may make the structure softer and move the natural frequency of the structure away from some excitation frequency. Either way you're trading off additional static loading and the expense of added mass against reduced dynamic loading.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)

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