Autodesk Robot Structural Analysis

[#1]

I have Type 3 Instability in my model and I read this thread: http://forums.autodesk.com/t5/Autodesk-Robot-Structural/instability-type-3-RX-direction/m-p/3364875

Now, I do not think I have any extremely short members, because "Tolerance of  structure model generation" is set to 30 mm. There are no rigid links, diaphragms, etc. Also the results are correct, so I don't think there is any mechanical instability. The longest member is 12 meter long. Is it possible that stiffness difference between a 30mm member and 12,000mm member triggers Type 3 Instability message or it must be something else?

[#2]

Send us this model to check

[#3]

Yes, this is possible especially when a 30mm bar has got a large section and the 12 m a small section assigned. You may try to make a quick test and assign the same section to all bars. If it is only the size of the sections that causes the very large difference in the stiffness then the instability will no longer be reported. Please mind that this may not be enough (to get rid of the  instability warning) in cases the difference in lengths is too large.

[#4]

Sorry fot the late response, I was on vacation.

As Artur suggested, I did a small test and applied the same section to all members and then the Istability type 3 warning did not show up.

I am trying to figure a way to solve this problem: Would it help to divide long members into shorter members?

Rafal, I have sent you the model.

[#5]

No, this will probably not help. Form what you wrote I'd say that you can just ignore the warning (unless you want to use similar sections for all bars of the model) after carefully checking the deformation shape of the structure under applied load as well as values of nodal displacements and rotations.

[#6]

I am trying to figure a way to solve this problem: Would it help to divide long members into shorter members?

Rafal, I have sent you the model.

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I guess if you enhance geometry accuracy in your model it willbe fine.

Check this: on dummy model select everything and make intersection, open Bars table on Values tab, sort it by length,

find the shortest ones, correct these places on original model.

[#7]

Should not "Tolerance of structure model generation", which is set in this case to 30mm, take care of this and assure that no element is shorter than 30mm?

[#8]

I would suggest to correct geometry manually \ by Correct options in Robot or to correct analytical model in RST.

[#9]

Patrick,

I would be careful using 30mm as model tolerance, as every time you recalculate the structure nodes/panel edges will move and therefore you will need increasingly larger tolerance every time.  

Tony

[#10]

Rafal,

Now I am not sure if I understand what "Tolerance of structure model generation" does. Is it not supposed to merge any nodes that are less than 30mm apart?