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Vibration design practice for condominiums

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This is a question about current practices and design economics in
relation to floor vibrations and sound transmission in dwelling units. 
In condominium construction, for example, what steps, if any, are
typically taken in the design of the primary structure to  limit impact
sound transmission through the floors?  More specifically, if the floor
structure is a composite concrete slab on metal deck, are any special
steps ever taken in the design of the floor structure itself, narrowly
defined?  Is it ever economically justified to add concrete thickness
and mass to obtain a better IIC rating?  Aren't there almost always
better (more economical/cost effective) ways?

The UBC and IBC require that floor-ceiling assemblies in dwellings meet
an impact insulation class (IIC) of 50 (1997 UBC Section1206.3.  See
also ASTM E 492).  Floor coverings, carpet pads, sheathing on sleepers,
gypsum board ceilings on resilient channels, fiberglass batts,
perlite-sand toppings, rubber sheeting, proprietary "floating floors,"
and all kinds of other materials can apparently be used to help achieve
a desired IIC rating.  An industry has grown up around this issue, but
it seems to me that the matter is typically dealt in design through
architectural specifications.  Do structural engineers really have a
genuine role in this issue?

As I see it, the code is concerned with **sound** transmission through
the floor-ceiling assembly.  The issue of harmonic footfall-induced
vibrations traveling laterally through the floor and causing discomfort
for occupants seems to be separate.  Investigation of vibrations induced
by rhythmic footfalls does not seem to be addressed by the codes at all.
 Investigation of and potentially design to mitigate these vibrations is
something structural engineers are sometimes called upon to do, but I
don't believe they typically act except on client's specific
instruction.  On the subject of sound transmission, though, I have my
doubts.  Are there any practical steps to be taken?

Well, colleagues, what do you think?


Thomas B. Higgins, P.E., S.E.

Group Mackenzie
0690 S.W. Bancroft Street
Portland, OR 97239-0039
Phone (503) 224-9560
Fax (503) 228-1285
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