Buckling Analysis

[Anonymous]

Hi,

i'm trying to verify the results of buckling analysis performed by robot.

For a simple frame the results are very different from the exact solution.

The exact solution should be: Pcr=1.82(EI)/(L^2)=1.82(30x8.33)/(25)=18.19KN

(E=30.000MPa, L=5.0m, Sections 10x10cm, I=833.33cm4)

Robot gives Pcr=74.43

Are the results given by robot mean something else than the global instability coeficient and the critical load.?

Thank you.

[t.sautierr]

With the thread given by rafal you understand that in general case (not a buckling shape that is actually the buckling of a single element), RSA is calculating the buckling lenght according to Euler's Formula, but is has no physical meaning related to the single element. Maybe it shall remove this information in case of global buckling .... 

For your portal : it fixed at the bottom which a bit uncommun, but it would have been hinged at the bottom and fixed at the top, remember that the buckling lenght of the column is at least 0.7 but is generally greater than 2 and can go theoretically  to the infinity .... so 4 doesn't chock me.

[Anonymous]

And again, appreciate your answer! 🙂

- the model is exactly how its gonna be working in our current project (rigid column bottom, no external/internal bracings)

- yes, i understood that it's mathematical forms of possible bend shapes.

-i still wonder how i can apply buckling analysis in structural design practice.

A was aiming to have complex structures as for analyzing buckling length of single members in it. It would take more sophisticated results and making positive effects on the purchasing costs. As i see it now - i can use it for single bars and maybe 2D models only. Plz correct me if i'm wrong.

All my threads here addressed to the practical usage of this advanced robot feature nor of doubting or criticizing it.

[t.sautierr]

The critical coefficient is one parameter included in the structural anaysis process as described in the EN 1993-1-5. So it not a conclusion itself, and several ways can be taken after having evaluated the critical coeff.

Usually this procees leads to design a structure safer versus second order effect for a minimum material.

Search for docs on the structural analysis for instance : http://www.qub.ac.uk/structural_eurocodes/eurocode3_handouts/3_Structural_analysis_handout.pdf.

[Anonymous]

Would i make correct statement by saying Buckling analysis is rather refering to Direct Analysis Method which is used for global stability analysis of the whole structure with taking in consideration second order effects (P-deltas) and non-linear behaviors?

[t.sautierr]

I don't know DAM sorry, I work with the french code and had no time to study it till now 😞

[Anonymous]

Hello,

First of all I would like to apologize if I posted question at wrong place.

Does anybody have any experience with buckling analysis of the plate elements?

Basically I would like if someone did that kind of analysis to share tips and tricks.

Thanks

Ivan

[Artur.Kosakowski]

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@Anonymous wrote:
Would i make correct statement by saying Buckling analysis is ratherrefering to Direct Analysis Method which is used for global stability analysis of the whole structure with taking in consideration second order effects (P-deltas) and non-linear behaviors?

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IMHO DAM is actually used for design of elements (each of them separately) of a structure rather than for its overall stability. If I understand its principle correctly instead of assigning buckling lengths to columns based on stiffness of adjoining elements you "increase" loads and set them as 1L and actually buckling analysis is what you use for global stability analysis of the whole structure.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[t.sautierr]

Post your starting model and we will be able to react, there are no "general" tricks

[Anonymous]

Dear Sir, do we need to reduce the inertia for all structural members before running buckling analysis? If i am not wrong design code like ACI and EN1992-1-1 mention about cracked inertia in analysis in order to take into account cracking. 

[Anonymous]

This video can help u to understand Buckling analysis in robot .

good luck 

[anthonymctigue]

Thanks very much for the video! I have been using the buckling analysis tools in Robot for a while now to determine both local and global buckling modes for structures on demolition projects and it is an excellent sanity check that I have correctly understood that buckling modes for complete structures determined in Robot are a combination of local and global modes. That leads me to my next hypothetical question. If one could theoretically ask Robot to calculate an indefinite number of buckling modes, my understanding is that eventually, Robot would calculate the local buckling modes for all individual members in the structure and that the critical load reported for the bar that initiates the mode would be pretty close to that determined by code calculations? To give an example, if I looked up the critical force column that is normally hidden in the buckling analysis results table for your Mode 2 and 3 (i.e. local buckling of the bottom braced bays), this would almost exactly equal the design compression capacity you would get for these members calculated with an effective length factor of 1 using Eurocode 3 (or in my case, AS 4100) with the partial safety factor for strength stripped out of the calculation which would also almost exactly equal the axial force in that member under your ULS load case multiplied by the critical coefficient? Anthony