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Re: Appendix (B5-2-b), AISC 1989 Spec
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- Subject: Re: Appendix (B5-2-b), AISC 1989 Spec
- From: saj5(--nospam--at)inel.gov (Scott A Jensen)
- Date: Tue, 14 Oct 1997 11:51:18 -0600
>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)inel.gov Lockheed Martin Idaho Technologies 208-526-0544 P.O. Box 1625 fax: 208-526-2681 Idaho Falls, ID 83415-3650
- References:
- Appendix (B5-2-b), AISC 1989 Spec
- From: Karen12959
- Appendix (B5-2-b), AISC 1989 Spec
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