Constraint equation vs Rigid elements

[josf1988]

thread727-341008

I am performing transient analysis on a frame like structure which supports immense amount of force, but these forces are acting some where around the frame in space, so I need to connect these force to the frame through rigid elements and transmit them through rigid elements to the frame. I did it in Ansys workbench through remote forces but workbench has limited capabilties. you cant create nodes and define elements. Workbench instead creates MPCs which calls CE ( constraint equations ) automatically. In order to confirm i performed a similar analysis with a simpler model in both workbench and NX NASTRAN. In workbench using the CE it creates automatically and in Nastran using RBE2 elements, but the results were totally different. Now this is the answer to the question asked in the previous thread form where I came here. So if the results are different that means they both have to different, but theoretically they shouldn't be. So which one is correct.
I ve included and attachment as well. Help me with this. I am new to FEA, so excuse me if I have said something stupid above.
Thanks
Joseph George

 

[BlasMolero]

Dear Josep,
VERY IMPORTANT: to transmit forces & moments with NX NASTRAN you need to use RBE3 rigid elements, not RBE2. The RBE3 element is a powerful tool for distributing applied loads and mass in a model. Unlike the RBAR and RBE2 elements, the RBE3 doesn’t add additional stiffness to your structure.

With the Nastran solvers, RBE2 and RBE3 elements behave differently:

• RBE2 — The active degrees of freedom of the single core node (independent) enforce the active degrees of freedom of the leg nodes (dependent). Therefore, the active degrees of freedom of all nodes included in an RBE2 element are considered rigidly connected.

•RBE3 — The weighted average of the active degrees of freedom of the leg nodes (independent) enforces the active degrees of freedom of the single core node (dependent). This weighting is set to 1 and cannot be changed in NX; all nodes contribute to the motion equally. Unlike the RBE2, the RBE3 element is flexible (not infinitely rigid), due to the allowable freedom of the leg nodes. Common uses for this element type include adding and distributing mass without adding stiffness, and distributing a load from multiple points to a single point.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
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[josf1988]

Dear BlasMolero
Thank you for your valueable reply.
I decided to resolve all the forces onto the CG as forces and moments and apply the loads instead of transmitting through Rigid elements.

Regards
Josf1988