ASTM A500? Does AISC or AISI control design?

[lma2h]

I am determining the allowable bending stress for a 1 1/2x1 1/2 tube using ASTM A500 Grade B steel. This is referenced in both AISC 9th Edition and AISI Cold Formed Steel Design Manual. For AISC the bending equation is typical, but for AISI there is an omega value to devide your allowable by. This greatly effects the allowable bending stress of the member. Any ideas which one should govern and why?

 

[haynewp]

What is the thickness?

 

[miecz]

Is the shape cold formed?

 

[lma2h]

The thickness is 14 GA.

According to AISC 9th Edition Table 3 (Page 1-92) all A500 Grade B steel is cold formed. Initially this is why I thought that I would need to design according to AISI. But in AISC it provides load tables for structural tubing with A500 Grade B. So which should govern, AISC or AISI.

If calculating using AISI there is an omega factor that would reduce my nominal flexural strength by .6 (1/1.67) compared to the nominal flexural strenghth by AISC design.

This is just confusing to me which to use when it is referenced in both. Thanks for the help.

 

[UcfSE]

The omega factor of 1.67 in the AISI is the factor of safety you apply to the nominal moment strength Mn when using ASD. Using the AISC 9th will not give you nominal moment strength directly. The result using F3 gives you the allowable moment, which is what you get after you divide the nominal moment by the factor of safety.

 

[aggman]

The omega factor is the factor of safety. It’s the same as it is in AISC 9th ed. In the AISC 9th ed if the member is compact then the allowable stress is .66*Fy. If it’s non-compact then its .6*Fy. As long as the section falls within the non-compact limits in Table B5.1 you can use AISC. If it doesn't then you will need to use AISI. Reason being is that the flange or the web can actually buckle before yielding is reached.

 

[aggman]

Or what UcfSE said. He just be me to it.

 

[Taro]

As a general rule: If it's 1/8" thick or greater, use AISC. If it's thinner, use AISI.

 

[lma2h]

I now see the error of my ways. Indeed AISC and AISI are actually the same just presented in a different manner.

As an engineer that has just been introduced to this board and am posting for the first time, I would like to say thanks for everyone's responses and thanks for this board.