Hello, i´m starting to use Buckling Analysis (Analysis type) to find if is possible to know in a three-dimensional model if my structure is sway or non sway. I´m asking your help to verify if my thought is correct:
I go to analysis case and in every load combinations that I will use to design structure elements, I’m selecting Buckling analysis, using a number of modes = 3xstories. After run calculations I go to tables->critical loads and for each combination i´m founding the lower coeff, this will be my critical coefficient. Now depending country codes, but generally, in elastic analysis, if crit. coef > 10 my structure is non sway.
If this is correct i´m wondering how to know if the structure is non sway in both directions or just in one direction, should i have to think in the same way but finding the corresponding excitement mode to each direction?
Other question, in this last table, when I ask critical forces it shows me bar forces, the same to buckling length. It’s correct to think that displayed bar buckling length corresponding to lower critical force in each bar is the one to perform local second order effects calculations?
Last question, after this, should I transform combinations to linear combinations again before of design any element?
Best regards,
RRufino
Solved! Go to Solution.
Solved by Pawel.Pulak. Go to Solution.
It is enough to run buckling analysis for the combination with the highest vertical load.
Then it is necessary to display buckling shapes to find buckling modes corresponding to global buckling in X or Y and not to some local effects.
Buckling shapes can be displayed selecting buckling load case, activating display of deformation and activating display of modes instead of displacements - appropriate icon shown on the attached screen capture.
Depending on structure type and display layout this icon may be not displayed - in such case using from pull-down text menu: Loads>Select Result Type>Modes
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Regards,
Sorry for omitting it.
Other question, in this last table, when I ask critical forces it shows me bar forces, the same to buckling length. It’s correct to think that displayed bar buckling length corresponding to lower critical force in each bar is the one to perform local second order effects calculations?
No, in general case these results cannot be used for this porpose.
Critical forces displayed in this table for each mode for any member under compression are calculated multiplying compression force in appropriate member by critical coefficient of appropriate mode.
Buckling lengths are calculated from above mentioned critical forces and Euler's formula.
Detailed description of these calculations is given in this forum topic:
Last question, after this, should I transform combinations to linear combinations again before of design any element?
No, it is not necessary. Buckling analysis load cases contain both results of static analysis and buckling analysis.
Other topics, where global buckling is discussed:
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Regards,
Thanks again pp2008, now other question refering to local analysis:
Using R/C member type definitions and assembling to each one the bucking schemes, is there any table who has the every bars buckling lenght? I'm not seing this possibility on any table at Geometry or RC design tabs...
Best Regards,
RRufino
No, the information related to buckling length of RC members is not displayed in any summary table. It is given in calculation notes of specific columns.
Regards,
Hi, can you add to the wish list this feature:
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Roman Zhelezniak
Robot Evangelist & Passionate Civil Structural Engineer
Hi Romanich,
your suggestion added to the wish list 🙂
Regards,
In RSA 2016 the eigenvectors for buckling analysis are by default displayed with the normalization to 1 with no change in the UI.
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please i have one question more,
I did all the above procedures and i got (alpha cr) less than 10 for the most critical vertical load combination, but how i got from robot the amplified (moment and reactions) by alpha r which is = (alpha cr) / (alpha cr-1)??
i did 2nd order analysis (P-delta) for this load combination and i got little bit amplified moment but not with same ratio if i apply (alpha r) manually as in above equation, why?
many thanks in advance
I did all the above procedures and i got (alpha cr) less than 10 for the most critical vertical load combination, but how i got from robot the amplified (moment and reactions) by alpha r which is = (alpha cr) / (alpha cr-1)??
I'm not sure if I understand you correctly but what about creating a load combination with such load factor?
i did 2nd order analysis (P-delta) for this load combination and i got little bit amplified moment but not with same ratio if i apply (alpha r) manually as in above equation, why?
The results of second order analysis cannot be the same as the results of linear static analysis with additional load factor as the deformation of the model is different. Mind that for the 2nd order analysis you don't 'increase' actual loads to obtain larger bending moments.
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thanks for your answer Mr. Artur,
I'm not sure if I understand you correctly but what about creating a load combination with such load factor?
it is good idea and i will do it, but robot did the major and complicated job by calculating alfa cr, why it stops at this stage and did not continue to amplify the straining actions like other programs same as Fastrak for example?
The results of second order analysis cannot be the same as the results of linear static analysis with additional load factor as the deformation of the model is different. Mind that for the 2nd order analysis you don't 'increase' actual loads to obtain larger bending moments.
sorry if i was not clear in that point, i know 2nd order analysis results for sure not same as linear static analysis, but why robot does not consider or include the global instability amplification factor (sway sensitivity) in large displacement or p-delta option of nonlinear analysis??
please see attached BS 5950 instability file and two screen shots from fastrak program to clear my point of view.
thanks alot Mr. Artur
My understating is that there are 3 methods you can decide on one of them. I'm not familiar with the Sway-check method and Robot cannot do the Amplification moment one but you can run the second order analysis.
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