Buckling in l member

[Helena Goni]

What’s the correct way to study bucking in an L member? I’m studying it with the following methods, and results are very different:

Case 1: Stability analysis according to 6.3.1. I define in RF-Steel effective lengths from RF-Stability. Ratio: 1.05
Case 2: Stability analysis according to 6.3.4. Ratio: 1.03
Case 3: 7 GDL. Ratio: 1.97
Case 4: I use RF-IMP to generate pre-deformed model and I study resistance in RF-Steel. Ratio: 1.92

[Frank Faulstich]

Hello,

the differences between case 1 and case 2 are small and, in my opinion, are due to the different methods.

It is very interesting that case 3 shows a much larger ratio. I would guess that the problem is not purely a buckling problem, but that lateral torsional buckling also plays a role.

Case 4 should actually come out with something similar to cases 1 or 2. You might want to check the FE mesh.

Perform the test with a slightly smaller load so that the ratio in case 1 is less than 1.0. The load in the range above 1 is not necessarily the same for different methods.

Check whether the same cross-section classes were used for the verification in cases 1 and 4.

Frank Faulstich

[Helena Goni]

Thanks Frank!

I’ve performed the test with a slightly smaller load, and this is the result:
Case 1: Stability analysis according to 6.3.1. I define in RF-Steel effective lengths from RF-Stability. Ratio: 0.96
Case 2: Stability analysis according to 6.3.4. Ratio: 0.95
Case 3: 7 GDL. Ratio: 1.24
Case 4: I use RF-IMP to generate pre-deformed model and I study resistance in RF-Steel. Ratio: 1.18

I’m really surprised because ratio in cases 3 and 4 has decreased a lot! The first test was considering a factor 0.60, and now I’ve changed it to 0.55:


This is the model and the mesh:


I’m testing the left member.

[Frank Faulstich]

Hello Helena, the FE mesh looks good. It should be fine enough.

Do you see any torsion in the deformation in case 3?

The difference in ratio from case 4 to case 1 seems a bit large to me. If you attach the model here, I'll take a closer look.

Frank Faulstich

[Helena Goni]

Hi Frank, it seems to me there is torsion in case 3, but I think there is also torsion in Stability. I'm not sure.

I attach the model. Thank you!

[Frank Faulstich]

Hi Helena, this is a really interesting example. It is more complicated than it looks at first view.

In my opinion, the results of cases 3 and 6 are correct.

Let's have a look at the deformation from LC 2:



So the whole system is twisting. This results in these bending moments:



In the calculation with 7DOF, these are also present:




I think, with the procedures according to 6.3.1 and 6.3.4 this effect cannot be considered correctly.

Frank Faulstich

[Helena Goni]

Ok, thank you so much!

You’re right, what I’ve written here as case 4 is in the RFEM file case 6 (I forgot to change the number). Just in case someone want to check the results.