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Fwd: floor deflections, was: Re: building code minimums for wood frame

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>General comments to the many opinions raised about the issue of "code
>Although I'm not an attorney, I don't believe that a client can
>successfully sue an engineer for designing above code minimums
>particularly if the design engineer can prove that his design was
>warranted by calculations, etc. For example, the code requirement for
>live load deflection is L/360. Based on my experience, this criteria
>will produce a "soft" floor in a residence with long (>18') spans. My
>preference is to use L/400. I cannot even conceive a qualified expert
>witness being able to successfully debate this issue.

My experience in floor performance is that even for spans greater than 14',
a L/480 deflection criteria is not desirable especially if there is not
sheet rock on the ceiling or any loads on the floor such as furniture to
dampen the vibration. A  while back I specified 9-1/2" TJI/25 at 16" o.c.
for a 14' span which exceeds L/480 and in hindsight I felt it was a bit
bouncy. For custom homes in particular, this is an issue. An owner may
never know the difference unless it is brought to their attention, then it
will be considered unacceptable and they will do the old
"jumping-up-and-down-test" through out the house. Trus Joist will provide
performance crtiteria for their longer spans, because even if the strength
and deflection is well within the code and table limits, the floor will
vibrate. The TJI tables can be misinterpreted to allow an inexperieced
engineer to specify a poor performing, but legal floor for average to very
long spans. AISC has commentary that empirically limits vibration based on
the depth to span ratio for steel members. 

Jeff Smith. S.E.
phone: (415) 543-8651
fax: (415) 543-8679
email: smthengr(--nospam--at)

Smith Engineering
27 South Park
San Francisco, CA 94107

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Richard Lewis, P.E.
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