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Web Stiffeners (& Thanks for the Slab Help!)

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[Sorry if this is a repeat, it appears the server was down and missed my first try.]

First off, I feel I have to thank everyone for the great references and suggestions I had in reply to my question about a payloader slab design.  (Especially to Mr. Roger Turk who sent me an extensive off-list follow-up.)  I went to the library and looked up all of the references everyone gave and it was _very_ interesting.  I never would have found so much so fast any other way!  Of particular note is that an old reference to Westergaard's formula for extreme maximum moment, 0.315P was within 5% of the moment I finally calculated by other methods.  There is a place for rules of thumb. 

Now my question is about the requirement for web stiffeners.  Salmon & Johnson state, "The important requirement is not that yielding be prohibited everywhere but that it not occur over a significant length of critical section such that crippling failure can occur."  (They also show illustration that seems to represent a base plate bearing on the top beam flange.)

For example, where a 6" x 1/2" thick cap plate rests on a 3/8" wall square tube column and calculations show 3.5" bearing is required {fc=R/[(tw(N+k)]}, my understanding is that I am OK.  Someone suggested that stress concentrations in the flange over the column walls could be an issue.  But isn't that what the N+k term is supposed to consider, basically an average distribution of stress - even though there *are* stress concentrations?

I'd like to have a rational procedure...short of FEA how would one determine the proper cap plate stiffness for the 'N' calculation to be appropriate?

Regards,

Ed Fasula E.I.T.