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FW: Help with interpreting Vibration Analysis

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 -----Original Message-----
From: 	jwhitty(--nospam--at)itac-net.com [mailto:jwhitty(--nospam--at)itac-net.com]
Sent:	Wednesday, May 16, 2001 8:54 AM
To:	'seaint(--nospam--at)seaint.org'
Subject:	Help with interpreting Vibration Analysis

Gentlemen,

Could use a bit of help with some results of a recent vibration analysis we
performed.  The issue was this:  our client bought a fairly new warehouse
facility to place a manufacturing process in.  The process was an operation
with several machines that bolted down to an 8" slab on grade.  It had
operating frequencies of 60 to 66 hz during normal operations although
sometimes during maintenance/troubleshooting the operating frequency could
go as low as 33 hz.

I performed a quick calculation of the natural frequency of the floor based
on the 8" slab with cracked/uncracked section modulus on 6" stone subgrade,
proof rolled with a subgrade modulus in the 175 pci range per the geotech's
soil report.  The natural frequency came in at roughly 48 hz (unloaded) and
31 hz (loaded).

For backup I had a testing outfit brought in who used a electromechanical
shaker with an operating range of 2 to 100 hz.  They excited the slab in the
areas where I was going to place the equipment.  There were no other
operations or loads on the slab that would skew the results.   The results
were all over the place.  The testing outfit did not get tight natural
frequency values for any of the slab areas.  There were large variations.
For instance, testing one area gave results of both 1 hz and 60 hz.

I know there can be variations based on void, varying subgrade, location of
joints, etc.  but I was not expecting such large variations.

Obviously, if the natural frequency is 1/2 or twice that of the forcing
function I would be okay.  I'd even consider 1.5 to be okay but I can't
really with the skewed test results.  I'm looking at cutting up the floor
and increasing the mass, a pin pile arrangement or limiting of operational
frequencies to eliminate vibration problems..

I know this is not an exact science but I could use some help here.  Any
thoughts?  I'd really like to do nothing.  It would save them oodles of
money, however, I don't want them to have major operational problems in the
future.

TIA

John Whitty, P.E.


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