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Active Member
kwarr155
Posts: 6
Registered: ‎12-29-2010

Your model may not be tied down warning

570 Views, 6 Replies
12-29-2010 07:42 AM

I am performing a Linear Static analysis on an assembly with some surface contact and am getting  "Warning: your model may not be tied down enough or you may have a change in stiffness somewhere..."  The analysis will eventually converge and a solution is found but does this effect the results in any way.  If so, how can I fix this because the DOF it says to check is fixed and shouldn't be moving.  I would appreciate any advice.

 

Thanks

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Valued Contributor
dharhay
Posts: 60
Registered: ‎04-11-2008

Re: Your model may not be tied down warning

12-29-2010 07:54 AM in reply to: kwarr155

Have you reviewed the displacements of your model?  Sometimes when this warning happens you do get stress results but the displacements are very very large.  You then find some other method of applying constraints that ties things down better for the solver.

Dave H
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Active Member
kwarr155
Posts: 6
Registered: ‎12-29-2010

Re: Your model may not be tied down warning

12-29-2010 08:19 AM in reply to: dharhay

The displacements seem to be reasonable for the model and I have tried different constraints but usually still get this warning anyhow. 

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Valued Contributor
dharhay
Posts: 60
Registered: ‎04-11-2008

Re: Your model may not be tied down warning

12-29-2010 10:10 AM in reply to: kwarr155

Have you tried the Sparse solver?

Dave H
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Active Member
kwarr155
Posts: 6
Registered: ‎12-29-2010

Re: Your model may not be tied down warning

01-06-2011 11:56 AM in reply to: kwarr155

Yes I have tryed the Sparse solver.  I think that it is the default solver used when surface contact is involved.  Would the mesh size have any effect on this?

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Employee
Employee
xli
Posts: 99
Registered: ‎12-04-2009

Re: Your model may not be tied down warning

01-06-2011 02:04 PM in reply to: kwarr155

Do you have contact elements defined in the model? In linear stress static analysis model, contacts often servers as supports or constraints of whole structure parts besides connectors. Therefore, during the process of nonlinear iteration of searching for active contact elements for the solution, at certain state in the iteration matrix solver could report the whole structure is not well constraint ... something like that as you saw because some contacts are open (not contribute stiffness to whole structure). But model may eventually end up tied down enough at the end of that iteration. There is other cases, Algor simulation has some automatic stabilizing scheme in contact iteration that might make structure solvable and event reach to a solution.

 

 

 

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Active Member
TommyBreaux2629
Posts: 7
Registered: ‎04-22-2010

Re: Your model may not be tied down warning

09-25-2013 09:16 AM in reply to: xli
What is most aggravating to me is that with the warning the software gives a specific DOF to investigate. So you go to the DOF find the node associated with it then go back into the results environment and look at the node. The node is usually in the middle of a large surface and has nothing to do with contact surfaces or a location where there might be an " abrupt change in stiffness". At least if they are going to give a warning and specify a location it ought to have something to do with the problem.
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