Gap elements as constraints (floor)? 1

[jacasalr]

Hello everybody,

I am analyzing a tank (polymer concrete) that has a particular geometry with 4 supports (legs). These legs are simple supported to foundations so I have been wondering how to properly constraint this structure in the vertical direction, especially because a zero constraint in this direction would result in unreal tension stresses at the supports.

I have recently used gap elements by copying nodes on the bottom surface of the supports and then connecting them through the closest link function of Femap with NX Nastran. However, I have read recently that, when using gap elements, I should activate the option "Gap as contacts" in the Bulk Data Option of Nastran Static Solver. Could you guys please tell me your opinions about using gap elements to constraint a simple supported structure? Also your experience and/or comments about how to properly use them in Femap/NX Nastran would help me a lot.

Thanks in advance.

 

[rb1957]

I think NASTRAN has a nervous break-down if your model is completely supported by "iterative" elements (like springs, gaps, contacts).

When I had such a model I have added a rigid body constraint to the model. Constrain 1 node in all 6 dofs … so long as your model has stiffness in all 6 directions at this node. Then check that no (significant) load is being reacted at this node. Then you're GTG.

another day in paradise, or is paradise one day closer ?

 

[karachun]

Without option "Gap as contacts" your GAPs turn into springs in SOL101.

 

[jacasalr]

Thanks karachun! So, if they turn into springs, does that mean that they will have stiffness both in compression and tension? (I previously defined the gaps by only having compression stiffness).

 

[jacasalr]

Thanks rb1957! I will try a new model by constraining only one node in all 6 DOF's as you told me! Thanks again!

 

[karachun]

Yep, therefore GAP Elements have nonlinearity only if you force them using option "Gap as contacts".
Here is video and an article from Blas blog. You can see that Femap have custom tool to make many grounded gaps automatically, based on geometry and attached mesh.

https://iberisa.wordpress.com/2011/...-ensamblaje-con-contactos-surface-to-surface/

https://www.youtube.com/watch?v=6idlmTFzgbg

 

[jacasalr]

Amazing! Thanks a lot, karachun!