DOF stability problem

Question:

Hello,


client has a problem with the model in dynamic analysis.


Client defined the modulus of subgrade reaction under the foundation and he has got the: DOF stability problem.

We can avoid it by putting rigid supports under the piles (Rxyz and Fxyz rigid) where the data can be easily used for design the footing. Unfortunately, client can not make  the foundations more rigid anymore and we still have DOF stability problem, so there is a problem.
 

For example, In LUSAS Bridge, it was enough - to define at least one support fully rigid - so that the 3D model had data for the matrix - but here it does not work. 

I suppose there is a way to go, but I do not really know how to do it in MIDAS. I would ask for help.

Please see the model.


Regards,


Answer:

Hey,

The model wasn't attached.  Kindly send the same.

Regards,
Nandeep
Technical Manager, MIDAS

Please find the model.

Hey,

That model has certain separate structures which are not connected to the main part.  Hence, these do no have any restraint in X and Y direction.  Thus, the instability.  Some such structures are shown in the image below.
Personally, I think its wrong to do eigenvalue analysis of disconnected structures and all such structures should be removed or ignored since these would have individual time periods, much different from the main structure time period.  However, if these structures are to be kept, then they need to be restrained.
For all such structures, at least any one node would have to be restrained in Dx, Dy and Rz directions.  This has been done in the attached model, but thats in Civil 2019.  This restrain needs to be applied at following node list for proper behavior (List:  74 158 166 170 206 212 219).
Kindly let us know if further assistance would be required.

Regards,
Nandeep
Technical Manager, MIDAS
Creation date: 10/17/2018 10:47 PM (dklee@midasit.com)      Updated: 10/18/2018 8:17 AM (dklee@midasit.com)
Files   
DataImage54.png
Model 4 - modulus of subgr reaction.rar
Model 4 - modulus of subgr reaction.zip