Has anyone here used FEM design?

[bojoka4052]

I was given a previously made model of a beam that buckles. I need to change the length of the beam and objects connected to it, and I also need to change the material data. Then I need to find a critical buckling load.

Straight forward stuff, I just have a short deadline and have limited access to help finding out everything as I have never used this programme before. Could anyone give me some pointers on; ?

- Finding buckling load

- Changing geometry

- Changing material data

 

[271828]

There are various approaches.

One is to build the beam using shell elements and then run a linear eigenvalue buckling analysis. The problem with this is that it might predict many modes that are not of interest.

Some of these problems have behavior that requires more thinking than hitting "go" and taking what the program spits at you, however.

Another approach:

1. Build the beam using shell elements.

2. Apply a tiny force to causes a tiny deflection in the direction you are interested in. For a beam, this is probably a tiny lateral load at midspan at the compression flange.

3. Apply the load. For a beam this is probably the vertical load. Run two analyses: linearly elastic and geometrically nonlinear. The nonlinear can be a small-displacement analysis with geometric stiffness matrix, not iterating with moving node locations and regenerated elastic stiffness matrix.

4. Select a DOF such as the lateral displacement of top flange at midspan.

5. Run each analysis and record the linear and nonlinear deflections. Increase the load and repeat.

6. Plot the curves. Study what is happening as you increase the load. When the nonlinear analysis is having large nonlinear deflections you're at a buckling load. If the deflection goes negative or the analysis won't converge, then you're at or over the buckling load. Look at the deflected shapes and think as you're going along.

 

[rb1957]

which s/ware ?

changing material "should" be easy ... a new material property, or easier, overwrite the current property IF used only for this beam.
I suspect the beam (that is changing material) is only part of the model, so a new material would be needed, then update (edit?) the beam elements.

Changing length may be very easy (just change grid/node co-ords) or very difficult (if changes in back-up model are required).

another day in paradise, or is paradise one day closer ?

 

[centondollar]

If the structure is complicated (a beam in a multi-story frame or space frame), you need read the software manual, use google or check out a video on Youtube regarding buckling analysis in the software in question. Your employer can hardly expect you to meet "a short deadline" if you have not been trained in use of the software used.

If it is a beam that can be approximated as one of Euler's four cases, or if it is a simple (say, less than 5 members) frame, just do a hand calculation using tables (Euler buckling) or Berry's stability functions and the slope-deflection method.

For a beam, the simplest way to run a buckling analysis in software is to model the beam as a beam (shell modelling will add no significant amount of accuracy) and solve the eigenvalue problem. User "271828" suggests geometrically non-linear postcritical analysis (iterative procedure where you create an initial imperfection from static analysis, and then run an analysis that increases load and updates stiffness step-by-step until a significant deflection occurs), but it is probably not needed for a beam, since you will account for imperfections by utilizing a code buckling curve, slenderness coefficient and reduction factor in ordinary design.

PS. A nonlinear analysis might be needed for shell structures, arches, extremely thin unstiffenedplates, unusually slender girders or complex combinations of any of the aforementioned structures, but those are more advanced structures and not relevant to your case.

 

[bojoka4052]

How can I change the grid/node coordinates?

What if I just scale my beam to get it to the size I want?

 

[3DDave]

Is it FEM-Design from BuildSoft?

 

[rb1957]

"How can I change the grid/node coordinates?" ... this seems to be an extraordinarily novice question.

I think you need a bunch of time understanding the s/ware before using it. And I'm not (IMHO) being a d1ck about this ... FEA is a very dangerous tool. Would you pick up a chain saw and attack the tallest tree in the forest without any practice, or even knowing if the chain saw was safe to operate ?

another day in paradise, or is paradise one day closer ?