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Re: ASD Table B5.1

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     The noncompact width/thickness ratio you site (called lambda-sub-r in 
     the more complete LRFD Table B5.1) is designed to prevent local 
     buckling in the flange before the yield stress is reached in the 
     flange.  Flanges are unstiffened plate elements, meaning that they are 
     loaded in compression on opposite edges, free on one edge, and 
     supported on the remaining edge.  Flanges depend on the support of the 
     web on their one supported edge.  As the web becomes more slender, it 
     offers less external support, requiring an increase in the flange b/t 
     ratio for its own stability.
     
     In classical plate theory term, k-sub-c is the buckling coefficient 
     for the flange compression element, which varies with the degree of 
     fixity of the supporting web.
     
     Rick Drake, SE
     Fluor Daniel, Aliso Viejo, CA


______________________________ Reply Separator _________________________________
Subject: ASD Table B5.1
Author:  Andrew Abbo <AndrewA(--nospam--at)formsys.com> at fdinet
Date:    7/23/99 3:37 PM


     
In table B5.1 of ASD, foot note e defines an expression compression 
element restraint coefficient, kc. This expression includes the ratio 
h/t.  What is the definition of h and t used in this equation? While I 
think that h is well defined, the value of t is open to interpretation 
as either tw (web thickness) or tf (flange thickness).
     
 I have been given different interpretations of both h and t by
different people, all of which have some merit. They are
     
(1) h is the clear distance between the flanges (height of the web) and 
t is the thickness of the flange (t=tf).
(2) h is the clear distance between the flanges (height of the web) and 
t is the thickness of the web (t=tw).
(3) h is the width of the compression element which, in the context 
Table 5.1, is the width of the flange and t=tf.
     
I think that (2) is the correct interpretation and this is backed up by 
the Example 12 and Page 2-220 of the green book (9th Edition). However, 
it does seem unusal that the local buckling capacity of a flange is 
related to the stiffness of the web. Can someone please expand this.
     
Andrew
     
--
     
Andrew Abbo - AndrewA(--nospam--at)formsys.com
Formation Design System - http://www.formsys.com
Maxsurf for marine design, Multiframe for structural design, Neoform for
     
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