AISI Cold Formed Code Checks

[ToadJones]

When using RISA to design cold formed C and Z shapes, does the AISI 2007 code check include limit states of yielding, local buckling, distortional buckling, and lateral torsional buckling or does the effective width method in the code take care of these limit states?

I ask because there doesn't seem to be a way to look at RISA's output and determine which bending limit state is controlling the design.

 

[JoshPlumSE]

Yielding = Yes
Local Buckling = Yes
Lateral Torsional Buckling = Yes
Distortional Buckling = ?? I'm not sure. I see the term in the AISI manual, but I don't know what sections of the code it is applicable to.

Specific limitations on cold formed code checks mentioned in the RISA-3D help file are:
1) Kt in section C3.1.2.1 (lateral torsional buckling strength resistance) is assumed to be 1.0.
2) All conditions listed for the use of C3.1.3-1 are assumed satisfied (when user has specifically entered an R value, that is).
3) Section C3.1.4 is not considered in the calculation of Mn (beams having one flange fastened to a standing seam roof system).
4) Effects of shear stiffeners for section C3.2.1 are not considered.
5) Only strong axis bending and strong axis shear are considered for equation C3.3.1 (combined bending and shear).
6) Section C3.4 (web crippling) is not considered in the calculation of the member strength.

 

[ToadJones]

Josh-
Thanks, as usual, for the input.

I am still unclear on how RISA is performing a local buckling check?
I actually don't even see provisions in AISI for the check.

 

[JoshPlumSE]

You know what the Q value is in the AISC code, right? It's essentially a multiplier based on how much of the cross section will be buckling. AISC usually applies this as a direct reduction on member. A very simple implementation for cross sections that should not experience very much local buckling.

AISI using a slightly more advanced concept. They look at each element (web, flange, et cetera) and figure out how much of that element they can count on after local buckling has occurred.... Very similar in concept to AISC at this point. Then AISI goes a step further and develops effective cross sectional properties (I_effective, A_effective, S_effective, et cetera) for the section. These effective properties are then used in most of their code check equations.

The AISI code is almost completely built around the concept of local buckling. So, any implementation of these code provisions would have to include local buckling to be of any use at all to an engineer.