Lateral torsional buckling?

It's pretty straightforward to run an eigen buckling analysis in SAP. Go to the Define-Analysis cases menu and add a new case with Buckling as the analysis type. You'll need to specify which loads are to be used for your buckling analysis. Output gives the buckling factors for each mode. You can check the buckling modes graphically using the "Show deformed shape" button, or get them from the output tables.

Buckling factor of 5, for example, means that the load could increase up to 5 times before causing the system to lose stiffness by buckling. You can run the buckling with frame/beam or finite element models.. assuming you meant to say finite elements instead of Final elements

If it's an arbitrary section that will laterally displace and/or twist with any amount of load (i.e. a tiny one), then eigenvalue analysis won't work correctly. On the other hand, if the beam does have a trivial solution, it will, but you'll have to build up your beam using shells or solids. Be sure to verify your results A LOT, using beams with known buckling loads, to make sure it's not "garbage in, garbage out."

Keep in mind that your answers will be purely elastic. In other words, the stress you're getting is the equivalent of Fcr from AISC 2005 Eq. 2-4 for example, NOT a buckling stress or load that accounts for inelastic buckling. If the stress is high enough, you have to figure out a way to adjust for inelastic buckling and Spec. equations usually provide a fairly easy way to do this.

If you have a system without a trivial solution, your only option is to do a nonlinear static analysis, or a series of them, to determine the buckling load.

Thank you 271828,
but you are talking abaut the second step-> how to play with results, but I need information how to "build" correct model (which elements, material, analysis, etc. should I use(run)). Can you recommend me this basic info.


Drile007

More info on your beam?

Curently I don't have exact problem, but one day I was wondering about the [∧] and [∨] beam shape. I was thinking about I beam with large openings too...
Thanx 271828 for your attention!


Drile007

Well, in general, you can probably get away with using SAP's shell element. For example, if you were doing an I-shaped beam, you could build up the flanges and web with shells.

The key to doing anything like this is to start simple and work your way up, verifying that it is doing what you want at each little step. For example, you can probably find the LTB solution for a rectangle. Then figure out the LTB for an I-shape.