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RE: mass vs vibration

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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. 

Bill Sherman

-----Original Message-----
From: Christopher Wright [mailto:chrisw(--nospam--at)] 
Sent: Saturday, July 15, 2006 12:20 PM
To: seaint(--nospam--at)
Subject: Re: big dig structural failure - epoxy anchors overhead
supporting gravity

>> (it was written somewhere that the ceiling was that heavy to prevent
>> vibrations)
Whoever wrote that may have been looking for an excuse. Additional mass
does not prevent vibrations. Vibration occurs when you have a mass
supported elastically. More mass reduces the system frequency, but tends
to make the amplitude higher for a given excitation and increases the
dynamic loading. Additional mass makes the situation worse, not better.

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

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