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Question about Gap Elements

Question about Gap Elements

Question about Gap Elements

I want to now if there is a way to use gap elements in linear static analysis, my problem is: I am modeling a mast climbing working platform, the tower is consist of four 2*2 HHS members which are confined in horizontal plane with 8 rollers. there is 1/32Inch gap between rollers and HSS  because of the size of model (2 million Dof) we don't want go nonlinear. It seems that the gap element is pulling my structure.(basically I need a kind of gap element that behaves only in compression in linear static analysis)

RE: Question about Gap Elements


Gap elements are nonlinear so the short answer is, no, unfortunately. A gap in a linear analysis works as a spring, I believe. Your model is large but fairly simple, if I understand things correctly. Why don't you just remove the gaps in tension?

Another thing. I think that in NX Nastran there is a "small" deformation option that runs nonlinear problems in linear mode. Perhaps somebody who knows NX can help you with that part if you have NX.

Good Luck


RE: Question about Gap Elements


Gap Elements are nonlinear but a simplified version of elements is available also for linear analysis (SOL 101). It requires just a few changes to your input file and an additional PARAMETER CDITER. The full description of the procedure may be found on NASTRAN Manuals, try and search for 'Gaps in SOL  101' or 'Linear Gap Elements'.
Hope this helps; otherwise let me know.



'Ability is 10% inspiration and 90% perspiration.'

RE: Question about Gap Elements

Thomas suggested to remove the gap elements which are in tension, the problem is because of fairly complex mechanisems and loading in my model I am not exactly able to say which gap element is working in tension.
I am using NEi nastran, I tried List/output/query/stresses on gap element, but it dosen't give any result.

any suggestion would be appericiated

RE: Question about Gap Elements

Hi Sherbrooke,

If you want to avoid nonlinear (due to the large model size), I would recommend replacing the gap elements with springs elements (in the NEiModeler you can do this by creating a spring property, and going to Modify-Update Elements-Type, and choose all the gaps, and then pick the new spring property).

Then run the analysis once, and determine which spring elements are in tension, remove those, and re-run the analysis.

Thank you,

Noran Tech

RE: Question about Gap Elements

I am  also try to use GAP element in linear static analysis. Can you guide me regarding the changes that I have to made in the bdf file.

RE: Question about Gap Elements

Nothing really needs to change.  Just realize that in a linear solution the gaps will act like springs and will take both tension and compression loads.  If you are using NEi Nastran I recommend you use surface contact.

RE: Question about Gap Elements

I am using linear gap in my models. By the examples, references guide and other presentations on that subject, I am able to use it correctly, but I have to admit that I don't really know what I am doing. Is there someone who can explain how it works? I mean how does Nastran makes the iterations, what really means Shut/Open? How does Nastran know that one grid point has to stop when the "gap" is shut?

Thank you very much for any help!

RE: Question about Gap Elements

I'm in teh same boat with you maitreskywalker, i've just started to look into using linear gaps for a contact situation in SOL101, havn't even got to using it correctly yet, nevermind knowing how it works, maybe that should be the other way around!

RE: Question about Gap Elements

I don't exactly know how linear gaps works, but I think the linear gap works as the same way as the linear contact when SOL101 is used. The funcion is nonlinear although it is "hidden" (I mean you are not able to define gap parameters and nonlinear parameters). If I have understood correctly, there is only one increment so the load is immidiately 100 %. Then the solver iterates as many times as needed. However, linear contact seems to work fine.

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