Engineering & Analysis Methods > Panel Buckling

Plate Buckling Approach for Non-Uniform and/or Biaxial Loads

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Phil:
Yes, the "entered buckling length" is the X and Y buckling span that is entered on the sizing form buckling tab.

When you import a finite element model, HyperSizer attempt to read the geometry of each component and determine on its own what the proper buckling lengths should be.  If the component has a simple shape (i.e. relatively rectangular and singly connected), it usually does a good job with this.  However, if the component shape is very complex (for example elements on either side of a wing or something like that), then sometimes, the buckling length that HyperSizer chooses is not appropriate.

Open your FEM in the HyperSizer graphics window and turn on buckling lengths on the left hand side (the icon looks like a set of cross-hairs).  You can select components and adjust their buckling lengths on the sizing form if they do not look to be correct.

One thing to realize about HyperSizer's buckling analysis is that it calculates buckling assuming uniform loads on a representative rectangular area.  Typically buckling waves go between hard points like frames or shape control members.  So, it is important to make sure that your analysis is using the correct buckling lengths in order to get accurate results.  Your buckling length can actually be larger or smaller than the actual component area.  For example, say a component is defined so that it spans multiple frames.  In this case, the buckling length should not be the component length, but rather should be the frame spacing.  HyperSizer cannot automatically determine this, but the user should change the buckling length manually to the frame spacing.

Finally, in the case where you have compression and tension in the same panel, HyperSizer reduces the buckling length so that it only covers the compressive portion of the panel.  This is done automatically by HyperSizer and the user does not have to change anything.  HyperSizer does a pretty good job of this in most cases, but sometimes it can be over conservative in the statistical loads analysis if the loads are varying wildly over a component.  In this case, it is usually the best practice to split the component up so that you don't have wildly varying loads within one component.

Let me know if this helps.

garyjh:
This is a question with regards to the shear that is used when analysing a panel.

Currently I am analysing a panel with loads obtained from an FE model. The majority (60% of area) of the panel is under positive shear. The positive shear average, std deviation & peak is greater in magnitude than the negative shear average, std deviation & peak. Yet the design to loads uses the lower magnitude negative shear average & std deviation.

Normally I would use the greatest magnitude of average shear with the negative Nx & Ny. Why does hypersizer not do this? This may lead to critical positive shear loads being missed.

Phil:
You are technically correct, there is a possibility that a load combination could be missed if you only combine the postive shear with the postive Nx/Ny and the negative shear with the negative Nx/Ny (which is what HyperSizer does now).

The best workaround that I can suggest right now is to break the component in question into two different components, one that is all in positive shear and one that is all in negative shear.  Then you will be ensured to get all of the loads accounted for.

Note that if all of the component is in Nx compression, then the software should use this average compressive Nx load for both the "tension" and the "compression" loop.  Then the value that is reported will be that corresponding to the lowest margin of safety.

In other words... lets say

Nx_compression = -1000
Nx_tension = 0

Nxy_negative = -400
Nxy_postive = +600

HyperSizer would then analyze the following:

1) Nx = -1000; Nxy = -400
2) Nx = -1000; Nxy = +600

And would return the minimum margin from these two conditions.

However, if part of the structure is in tension and part in compression

Nx_compression = -1000
Nx_tension = +500

Then HyperSizer would analyze the structure with the following conditions:

1) Nx = -1000; Nxy = -400
2) Nx = +500; Nxy = +600

So it would never catch what could be a worse case Nx = -1000, Nxy = +600...

To catch this, you would need to re-define your component so that you don't have the wide variation of load.

I hope this helps.

Phil

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