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Structural response to mechanical systems is one of the reasons this
mechanical engineer has gotten interested in structural engineering. 
Yes, in most cases vibration isolators do a good job at protecting the
structures.  If the isolator manufacturer can get good data from the
structural engineer, the mechanical/process engineer laying out the
equipment, and the equipment OEMs, life tends to be good.  Problem can
arise when what goes on the floor changes, either after the specs are
made and not adjusted for or down the road, if equipment is
added/removed/deleted (and if the specs were off, too.)

If there is just one driving equipment item, it is not difficult to
check for natural resonance for the structural steel.  However, have
several equipment items on a floor, or even several major items within
the building off the ground floor, can create a dynamic situation that
is not easily calculated.  Variables will include if one or some of the
equipment is off/idling while others are at full speed.  While this
situation is often addressed by "overdesigning" the structure, it
doesn't always work.

Your isolator mfr. will not promise it will always work due to liability
and the many unknowns that occur.  However, these isolators are pretty
reliable if they have all the data they request and its accurate at the
time the equipment is in use.  I would do a "gut check" on the floor's
natural frequency to be sure you're not near the equipment's, though.

Bart Kemper, P.E.

Vinod Sahni wrote:
> Hi,
> I have a question about the mezzanine floor (30ft x 24ft, 4" NWC /3"
> Composite Metal Deck/W18's at 8' OC) I am designing, for  mechanical
> equipment (air handlers 250, 500 and 1000CFM).  For floor vibrations
> induced due to human activity, AISC Steel Design Guide No. 11 can be
> used.  I am wondering if there is a similar source of info for vibrations
> induced due to mechanical equipment.
> Talking with the manufacturers of the air handlers and the vibration
> isolators to be used, I am told that the vibration isolator mfr will design the
> isolators so that the floor vibration will not be a problem.  A relatively stiff
> floor (deflections < typical code limitations) should be provided.  In fact if
> the floor appears to be too flexible, the vibration isolator mfr will inform
> appropriate parties involved as per their standard practice.  In the
> absence of vibration isolators I would try to keep the frequencies of the
> equipment far apart from the floor natural frequency as possible to avoid
> resonance.  I am wondering if I do not need to be concerned with floor
> vibration if vibration isolators are used.  Do they completely filter out
> vibration transmittal to the floor.
> AISC Guide No. 11 does talk about isolators.  My reading is that they do
> work but there is no assurance.
> In the meantime I am going to rely on following criteria to minimize floor
> vibration:  Deep stiff beams (Depth = L/20) for deflection control.
> Seismic Spring Vibration Isolators with 2" vertical spring deflection.
> Concrete floor with reinforcement (I am not counting on deck composite
> action because it may be lost in floor vibration).  I will review shop
> drawings along with calculations performed by the isolator mfr to see if it
> tells me something important.
> I would like to know how you would approach this.  Thanks a lot in
> advance for sharing your experience.
> Vinod Sahni, SE

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