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[SEAOC] RE: [SEAOC] Single jolts like an EQ, but not EQ!!!!!

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Obtain some more information.  I would tend to discount any residual 
"locked-up stresses".  They would have manifested into perceptible 
vibrations some time ago.

Check the mechanical equipment.  If there are isolators, are they still in 
proper working order?  Can the timing of the perceived vibrations be linked 
to any of the mechanical equipment, like the compressors for the HVAC 
equipment?  Have the HVAC compressors switched on in various sequences in an 
effort to simulate the perceived vibrations.

What were the environmental conditions immediately prior to and during the 
vibrations.  Did the perceived vibrations occur at night, sunny conditions, 
etc.?  Elements heated by solar gain during the day will shrink again when 
the sun goes down.  This cycling can produce stresses in the internal 
structure of the building.

Check to see if any heavy equipment is operating close to the building.  A 
large refuse bin dropped a few inches can send a shock wave through a 

Try to determine a pattern of the vibrations, and a pattern to potential 
vibration sources, and see if you can find a match.

Good luck.
Harold Sprague, spragueho(--nospam--at)
From: seaoc
To: seaoc
Cc: seaoc; asl-cse
Subject: [SEAOC] Single jolts like an EQ, but not EQ!!!!!
Date: Wednesday, November 06, 1996 3:32PM

I am talking about a 5-story office building in the S.F. Valley, designed 
constructed in the early/mid sixties. Steel frames in both directions (with
welded cover plates), grade beams at the base, pile foundation. Floor 
is lt.wt. concrete fill over plywood on wood joists spanning between steel
A few  tenants (at one side of the building) are complaining about random
"jolts" that started about a year ago and are occuring increasingly.
Naturally the owner would like to find out what is going on (and so do I !)
By the way, the one and only side of the building where the complaints are
coming from has continuous brick spandrels which are supported top, btm. &
mid-height at every column (about 21 ft o/c).
One of my guesses is that there might be some "locked-up stresses" which are
being "released". (Spandrel connections and/or welded connections).
There appear to be no visual deformations and/or cracks. The building was
"inspected" after the EQ by a structural engineer, but no beam/col or
spandrel/col connections were exposed or inspected.
I thank your for the upcoming comments and suggestions.
Antonio S. Luisoni
Consulting S. E.