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RE: Help with interpreting Vibration Analysis

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Regarding design for the service vibrations:
You might want to refer to Sec. 1.4 (Machine Foundations) of NAVFAC DM
7.03 (Soil Dynamics and Special Design Aspects).  "Vibrations in
Structures-Induced by Man and Machines" by Bachmann and Ammann, 1987
(published by IABSE) provides a brief but useful overview of the
issues.

Regarding your test results:
Data corruption due to electrical noise (A/C power at 60 Hz) is a
notoriously common problem in data acquisition circles.  In most civil
structural applications the frequencies of interest are much lower
than 60 Hz so hardware filtering of the signals (low-pass at say 50
Hz) is used.  If you believe that you have real response that you want
to capture in that frequency range, you obviously can't simply filter
it out of the signal.


- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E.                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.              Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699      Fax:        -1201

-----Original Message-----
From: jwhitty(--nospam--at)itac-net.com [mailto:jwhitty(--nospam--at)itac-net.com]
Sent: Thursday, May 17, 2001 5:11 AM
To: seaint(--nospam--at)seaint.org
Subject: FW: Help with interpreting Vibration Analysis




 -----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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