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Re: L in deflection criteria

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Bill Sherman wrote:

. > Roger Turk wrote: 
. >  
. > >..."L" should be the distance between points of inflection.   
. > >With regard to a cantilever..."L" equal to twice the cantilever length
. > would  
. > >be a conservative assumption ...  
. >  
. > I do not agree with this.  I think that "L" should simply be the length of
. > the 
. > member relative to its support points, i.e. for a cantilever, the length
. > from 
. > support point to the free end of the member; or for non-cantilevered
. > members, 
. > the length between support points.  The concern is maximum total 
. > deflection relative to the support point where "zero" relative deflection 
. > is assumed - why should the type of beam curvature matter? 
. > 

I think that there would be no disagreement in considering a true cantilever, 
i.e., one that is rigidly connected to some rigid object such that there is 
zero slope at the support, that the shape of the elastic curve (damn! I had 
to use a term from academia) under uniform load would be the same as the 
deflected shape of half of a uniformly loaded supported beam, similarly 
loaded.  If the deflection limitations are based on the criteria that I 
stated, and the maximum allowable deflection of the simply supported beam is 
based on the length of the span between supports, why should the maximum 
allowable deflection of the cantilever be based on half that length?  (True, 
that would be **really** conservative, so I couldn't really argue against 
that in a *real* situation.)

Isn't this what we do when we consider lateral stability in columns?

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