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RE: mass vs vibration

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FYI, in searching for the basis for the rule of thumb to make a
block-type equipment foundation at least three times the weight of a
rotating machine, I found the following brief explanation in ACI 351.3R,
Foundations for Dynamic Equipment: 

"The concept of this method is to provide sufficient mass in the
foundation block so that the vibration waves are attenuated and absorbed
by the block and soil system." 

Bill Sherman

-----Original Message-----
From: Daryl Richardson [mailto:h.d.richardson(--nospam--at)] 
Sent: Sunday, July 16, 2006 10:41 AM
To: seaint(--nospam--at)
Subject: Re: mass vs vibration


        Changing the mass changes the natural frequency of the system. 
Changing the mass in any way whether increasing it or decreasing it
could, in theory, move the natural frequency closer to the forced
frequency of the loading.  This would be moving towards resonance rather
than away from it.


H. Daryl Richardson

----- Original Message -----
From: <William.Sherman(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Sunday, July 16, 2006 10:15 AM
Subject: RE: mass vs vibration

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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