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Re: ST - Allowable stresses for steel tees

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At 15:14 10/9/97 +0500, Bill Sherman wrote:
>
>A couple of people have responded to me to refer to AISC B5 and/or
Appendix B. 
> Please note that B5 only gives allowable width/thickness ratios, not 
>allowable stresses; Appendix B applies if the member exceeds the ratios in
B5. 
> However, when a tee meets section B5, AISC is not clear as to which
allowable 
>stress formula in Chapter F is to be used.  Thru direct e-mail, one
responder 
>has suggested that section F1.2 applies to tees, using Fb=0.60Fy.  This 
>appears to be the most plausible answer - however, the title of section F1 
>does not clearly include tees, resulting in some confusion.  Does anyone
have 
>an AISC document or text which confirms the applicability of section F1.2 to 
>tees? 
> 

Table B5.1 Defines the limiting d/t ratio for stems of tees.  Under this
table, there is a listing under "compact" that says NA so AISC is telling


you it is not compact.  Under the Noncompact column the limit is
127/SQRT(Fy).  Section F1.2, last paragraph defines Fb = 0.6Fy if none of
the above conditions are met (they're not).  

Note the very restrictive bracing requirement of 76bf/SQRT(Fy).  E.g., if
you have a 1/4" stem in compression, bf = 0.25" when you determine the
bracing requirements for the stem (i.e., the element you are trying to keep
from buckling).  The stem would need to be braced at about 3.17" o.c. for
A-36 steel.  Since F1.3 does not apply to tees if the stem is in
compression anywhere along its span, no provisions exist to treat it as a
slender member, i.e, brace it at the spacing required (with Fb = 0.6Fy) or
don't use a tee.

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BILL CAIN, SE
OAKLAND, CA
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