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Re: Appendix (B5-2-b), AISC 1989 Spec

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>In my 12 years of structural experience I noticed that my structural
>colleagues (with more experiance that I have) never bother to check their
>design using Appendix B5-2-b with regard to tube sections. There is several
>reasons for them not to "bother" with it, and the main reasons usually are:
>1. extra time to calculate the member using Appendix B5-2-b
>2. more involved calculating procedures as established by AISC when Appendix
>B5-2-b is used.
>However, these two reasons should not come into the picture as the tube
>member designed using Appendix B5-2-b simply does not safisfy AISC
>requirements when compared with "standard" AISC design method.
>Take this example under consideration:
>Member is T12X12X5 (tube)
>L(x) = 24.5 ft
>L(y) = 16.35 ft
>F(x) = -154.6 kips
>M(x) =  36.1 k-ft
>M(y) = 27.2 k-ft
>F(y) = 36 ksi
>Allowable Stress Increase = 1.33
>Max K*L/r = 200
>According to "standard" AISC design method
>f(a) / F(a)     = 0.468
>f(bx) / F(bx) = 0.320
>f(by) / F(by) = 0.211
>Sum           = 0.999     The member WORKS
>Going through similar analysis using Appendix B5-2-b the following results
>are noted:
>f(a) / F(a)     = 0.493
>f(bx) / F(bx) = 0.373
>f(by) / F(by) = 0.250
>Sum           = 1.116   The member is OVERSTRESSED by 11.65%. This overstress
>  might be significant by not making the structure safe ....
>Does anyone has an opinion why structural engineers willingly design unsafe
>structures using tube-sections only because it take more work and it is too
>much "bother".....
>Your response would be appreciated.
>Karen .....
I think their are two things wrong with your example.  First, for most
tubular sections Fy = 46 ksi.
Second, Appendix B5-2 isn't applicable until the section is classified as
"slender".  My quick check of the flange width to thickness using b=bf-3t
resulted in a b/t of 35.4.  The compact section value for Fy = 36 ksi was
31.7.  However the value (noncompact limit) to classify the section as
slender was 39.7.  Therefore, a check per Appendix B5-2 isn't required.

Scott A. Jensen P.E.                               	email:  saj5(--nospam--at)
Lockheed Martin Idaho Technologies	208-526-0544
P.O. Box 1625		        fax:	208-526-2681
Idaho Falls, ID 83415-3650