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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 .....