linear buckling / strand7 2

[jazzmine]

Hi guys!
I have problem. I am new in this program.
I have to do a linear buckling analyses of rectangular simple supported plates with circular and rectangular holes in various positions subjected to axial compression, so i can calculate buckling coefficient k.

what i do with restraint ? In y direction I mark Dy Dz and Rx Ry , in x direction I mark Dx Dz and Rx Ry for simple supported plate.

the load was applied directly to the nodes.

my question is : are this restraint ok? and what do I have to do with FREEDOM CASES?

I first do linear analysis and then buckling analysis. How to calculate buckling coefficient k with buckling load factor?

THX!!

 

[IDS]

jazzmine - I really think you'd get more from doing some more experimentation on your own. The stresses you have generated are for a uniform compressive stress of 4500 combined with a flexural stress of +-500. Most likely there was an existing stress and you clicked the "add" button, rather than "apply".

Note that the stress is applied as a series of stepped pressures, so the stress on the outer plates will be +-475 (i.e. the stress on the mid point of the plate), not +-500. Also all the top and bottom edge nodes are restrained in the X and Y directions in your model, which is very different to the conditions in Roark Table 15.2.3.

The buckling stress is just the maximum applied stress (projected to the edge of the plate) times the buckling factor found by the analysis. You can calculate the axial load and bending moment using the formulae given in your previous post (assuming that W is the elastic section modulus).

By the way, be careful with abbreviations, it looks like you are from a non-English speaking country, so use of abbreviations may cause confusion where we have different conventions.



Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[jazzmine]

Ok. Thanks for your advices and help!

 

[IDS]

Ok. Thanks for your advices and help!

Good luck! Feel free to keep the discussion going if you have any more questions, and I would be interested to see how your final results compare with the Roark values, and also what effect the central hole has.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[jazzmine]

I apply bending as you said. I guess that I should multiply bucklig load factor with 0,5 MPa ? If I calculate stress ratio with formula which is given in Roark`s table (15.2.3), I get α=0,5 for bending. So I shoud compare my results with the first line in that table. Can you give me your opinion?
Thanks

 

[IDS]

I apply bending as you said. I guess that I should multiply bucklig load factor with 0,5 MPa ? If I calculate stress ratio with formula which is given in Roark`s table (15.2.3), I get α=0,5 for bending. So I shoud compare my results with the first line in that table. Can you give me your opinion?

Yes, I agree with both points. I said previously that the Roarke table did not cover the pure bending case, but that was wrong. α is the maximum stress divided by the stress difference, so α = 0.5 is pure bending and α = infinity is pure compression.

I'm getting a k value about 6-7% lower than the Roark value for the pure bending case.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[jazzmine]

I need your advice about combination of compression and bending. What is your opinion about restraints, should I leave them like for bending? I made two models, first one with the restraints like for compression and second one like for bending. For first model, Mode 1: buckling load factor is 0 and Mode2: 17,3741. And for the second model: Mode 1: 17,3741. If I multiply buckling load factor with 1,5 MPa (I add compression load to the bending load) and compare results with the Roarke table, the difference is about 2%.
Thanks.

 

[IDS]

I'd suggest keeping restraints to the minimum that models the restraints in the actual structure and provides sufficient restraint to allow the analysis to solve. In the case of getting a buckling factor of zero (or very close to it) have a look at the mode shape (set the deflection magnification to 10%) and confirm that this corresponds to a deformation that can't happen in practice. In this case it shows rotation of the plate in the XY plane about the centre point, which is clearly not a real buckling mode. Having done that you may choose to do another run with added restraints, as you did. This gives identical results, and also good agreement with the Roarke results, so you can be confident that the added restraints are not restraining a buckling mode.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[jazzmine]

Thank you.