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Re: L in deflection criteria

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Speaking from experience ("experience is the comb life gives one after one's
hair has fallen out") as someone how has been burned by excessive deflections
in cantilever beams (especially wood), I would use the length of the
cantilever as the "L" value in L/240 and L/360. I realize that using the
principals of mechanics renders my opinion very conservative, but my actual
experience has proven otherwise. I believe that the divergence of reality to
theory is based in the fact that the typical load calculation performed is for
a static condition of either a blanket load or a point load whereas the actual
behavior of the cantilever is a dynamic phenomenon. 

I therefore use the more conservative approach mentioned above. Another
approach, developed by TrusJoist MacMillian (a leader in long span technology)
is to change the allowable live load deflection criteria from L/360 to L/480
when a beam or joist spans exceed 15 feet.

In any case, conservative cantilever deflection criteria is worthwhile since a
"bouncy" cantilever is very difficult ( and expensive and embarassing) to
stiffen. Even more difficult is to convince the building owner that the
defelection causing the cracking of a drywall ceiling is within the normal
limits of tolerance for cantilever deflections. If the homeowners turn out to
be trial lawyers, my advice is move to Mexico.

Greg Riley PE