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Re: AISC Appd. 'B' - Qs

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Q-sub-a applies only to stiffened compression elements and is applicable
for HSS sections.  Per LRFD Appendix B, b-sub-e should be calculated only
when b/t is greater than 238/SQRT f.  Per your numbers 39.9 is less than
238/SQRT f, therefore b-sub-e equals b.  Q-sub-a is equal to 1.0.  Your
conclusion appears to be correct.

Q-sub-s applies only to unstiffened compression elements and is not
applicable for HSS sections.  Q-sub-s is equal to 1.0.

For background theory, I recommend you look at Salmon & Johnson's 4th
Edition, Section 6.18.

Rick Drake, SE
Fluor Daniel, Aliso Viejo, CA

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                    richard lewis                                                                                  
                    <rlewistx@juno.      To:     seaint(--nospam--at)seaint.org                                                 
                    com>                                                                                           
                    07/31/01 02:50       cc:                                                                       
                    PM                                                                                             
                    Please respond       Subject:     AISC Appd. 'B' - Qs                                          
                    to seaint                                                                                      
                                                                                                                 . 
                                                                                                                   




I have a question regarding using AISC Appendix 'B'.  I have a HSS
10x10x1/4 column with axial load and bending.  The b/t ratio of 39.9
exceeds the noncompact requirement which makes me check Appd. 'B'.  A
tube is a stiffened compression element which uses Section B5.2.b to
calculate the effective width.

As I understand it the 'f' value is the stress in the flange of the tube
from bending and axial load.  This is used to calculate the effective
width of the tube.  In my case the 'f' value is 19.7 ksi.  This makes the
effective width 'be' = 9.51 inches.  The real width of the tube flange is
9.30 inches (39.9*the thickness of 0.233 inches of the flange) so I can
use the real section properties of the tube because these are less than
the 'be' properties.  'Qa' will then equal 1.0.   Is this correct?

How do I calculate 'Qs' for the tube column.  Appd. 'B' Section B5.2.a is
for unstiffened elements.  What is the equation for a stiffened element
of a tube?

Rich
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