The linear buckling analysis

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  • Last Post 15 April 2019
James02dai posted this 08 April 2019

Hi, there, I had problems with my linear buckling analysis by using line body model. I applied force on the end and applied fixed support on the other end. I found that the force is consistent even though I changed the model length. Can anyone point out the mistakes I made in my analysis? Below picture shown the model of I created.

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jj77 posted this 08 April 2019

It looks very strange (deformation). I would not use both a displacement and a force (use one of them) in the linear static analysis, use just one or the other (fixed support is OK).

James02dai posted this 08 April 2019

Hi,jj77, I used displacement because I want to make a buckling model of one end fixed, one end is simple support. If I did not apply displacement , then may I know what constraint should be used for the simple supported end?

jj77 posted this 08 April 2019

If you attach your model I will have a look - use the attach button next to your previous post to attach and archived model .wbpz which can get by going to File/Archive in WB.

James02dai posted this 08 April 2019

Hi, jj77, here is the model I want to create. The 3rd model is the one I tried.  Bottom end is fixed and top end is simple supported. And the beam is I section beam. I would appreciate you soo much if you could solve my doubt. Thanks.

James02dai posted this 08 April 2019

N=2 is the one that I had attempted.

jj77 posted this 08 April 2019

Well it works OK for a square section, which gives the same results as expected as the Euler buckling load formula (K=0.7) - but it does not give same results for I section

James02dai posted this 08 April 2019

So what is the method for the I beam?

jj77 posted this 08 April 2019

Not sure what you mean but as I said and as you can see yourself from your analysis it does not produce correct results - perhaps someone else has some more feedback, but I do not know exactly why 

James02dai posted this 08 April 2019

Okay, thanks for your comment. As I am quite new to this software , I thought that the I section should be the same for the square section but it does not generate correcte results. Anyway thanks very much.

jj77 posted this 08 April 2019

Well not sure what you mean with "I thought that the I section should be the same for the square section".

 

But an I section is different to a square section in terms of the sectional properties (Ixx.Iyy, Warping conts., J,Area,....). 

 

And Eluler load is proportional to Ixx,Iyy,.. so they will of course be different buckling loads for a square section (SS 200mmX200mm) and an I section (say UB 203mmX102mmX23mm)

 

Also look on the buckled mode shape that you show it does not make any sense what so ever so it is not me saying it only, just look on your screen and be critical

James02dai posted this 08 April 2019

what I mean is the boundary condition applied to square section beam should be the same for I section beam. Thanks for the comment.

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jj77 posted this 08 April 2019

If one restrains the rotational dof then it is OK (not sure 100% why that is).

 

Add a command snippet in analysis settings - where you define loads and BC:

 

nsel,all   ! selects all nodes
d,all,ROTY,0    ! fix ROT dof which is rotation about Y axis which points along beam

James02dai posted this 15 April 2019

I had tried higher mode ,and it works for I section column,

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