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Re: L in deflection criteria

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Greg,

You are absolutely correct!  As part of the design, you *do* have to check 
for serviceability, however, I believe that the original question had to do 
with the use of "L" in the L/360, L/240 limitations which have to do with 
cracking of brittle finishes and visual perception of sagging.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

. > 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
. >