AISC slenderness check vs AISC combined axial and bending check 2

[tnteng]

Can anyone tell me why the AISC code has both the slenderness check and the check for the loading case of combined bending and axial? Dosen't satisfying the equations H1-1, H1-2 & H1-3 act as a check on buckling. If so then why does the slenderness check need to be done?

The slenderness check in section B-7 that slenderness ratios should be less than 200, but if it is larger, the allowable stress shall not exceed equation E2-2. Can someone please explain this requirement?

Does anyone know where I can see the derivation of equation H1-1.

Thanks in advance for any help with these questions.

Tony Billeaud
Mechanical Engineer

 

[csd72]

I am no expert on either the ASD or LRFD versions, but I think I know what you are getting at:

I believe that the slenderness limits come from the ASD codes, which do not fully allow for secondary effects and therefore may become un conservative at higher slenderness.

The LRFD code specifies second order analysis provisions and therefore it probably isnt applicable, but it has stayed in and therefore should be adhered to. By reference, the Australian codes have dropped these slenderness limits about a decade ago.

It is also these second order effects that are the reson why you check this buckling twice as an out of straightness will have moments imposed by both bending stress and axial stress superimposed. This effects can be greater than the sum due to second order.

The formula H1-1 is similar to that for most steel codes throughout the world.

 

[nutte]

The slenderness check in B7 (KL/r should be less than 200) is a guideline. Like you note, if it exceeds 200, you must use equation E2-2, which is the Euler buckling equation. You want to avoid going higher than 200, but you're not required to.

The commentary to the specification (located after the spec chapters and appendices) has good information pertaining to this and your question about the interaction equations in Chapter H.