RBE3 element Femap advice 1

[Naffoussi]

I am working on a structure which will lift a container(in Femap). I have modelled the slings using a RBE3 6DoF element and fixed the node of lifting(see attached picture)The structure is meshed with a plate element. And when i try to analyse it Femap tell me that there is 6 FATAL Errors associated to the C-BUSH (3DoF) element that i placed between a RBE2 and a RBE3 element.

I would like some help from someone who is a professional in Femap. If I replace RBE3 by RBE2 structure will be rigid and that's not good. There is not an element that represent sling in Femap ??

http://files.engineering.com/getfile.aspx?folder=f7a62f9c-3605-4a6c-90b4-42ad68865c25&file=MASS1.png

http://files.engineering.com/getfil...8-acf3-4b10-a97d-041665ce76d9&file=RBE2_2.png

Thanks,
SE

 

[dwallace1971]

Probably has something to do with the formulation of the RBE's. The way you have it built you have two independent nodes of the RBE's joined by a spring. You might try replacing your RBE3 with a series of rod elements, with the "cable" option checked. That will create a tension only two-force member (cable).



"On the human scale, the laws of Newtonian Physics are non-negotiable"

 

[dwallace1971]

Also, I don't understand why you've gone to the trouble of modeling the holes in your part. You could simply eliminate the holes and, likewise, the RBE2's, simplifying your model.

"On the human scale, the laws of Newtonian Physics are non-negotiable"

 

[Naffoussi]

thank you for answers,

I used RBE2 element to model screws and make the hole rigid, but here the problem is related to RBE3 and not to RBE2, you mean I have to remove the RBE2 element and place a node at the center that I connect with RBE3? and I did not put two independent nodes on the RBE's Joined by spring.

each spring element is connected to one independant node on one side and to one dependant node on the other side.

Femap dislike maybe the way i defined the constrained node on the slave node of the RBE3 (slings.

i attached another picture to better explain the story of nodes.

(sorry for my english level)

Thanx

http://files.engineering.com/getfile.aspx?folder=5045c253-e180-4535-8815-2a10f8aa03e8&file=MASS2.png

 

[dwallace1971]

What is the error message you're getting? In the picture RBE2.png, are the red spider-web elements RBE2'S?

"On the human scale, the laws of Newtonian Physics are non-negotiable"

 

[rb1957]

what's special about a sling, that you can't use a ROD element ?

 

[dwallace1971]

What is the purpose of your model?

"On the human scale, the laws of Newtonian Physics are non-negotiable"

 

[Naffoussi]

yes the red spider is a RBE2 element, and there is nothing special with slings it's just because i never used rod element...the purpose of the model is to check the structure under lifting a container (6000Kg) thru the slings. I think that i will keep the RBE3 element and put a C-BUSH element with two nodes on the constrained node because i can't make a dependant node constrained...i will tell you tomorow if that works.

Best regards

 

[BlasMolero]

Hello!,
If the target is to study the structure, not the ropes, forget RBE3 elements, you can use CGAP NODE-TO-NODE elements of Tension-Only to simulate the guys, FIXING THE LIFTING NODE. You will have to play with the value of tension stiffness, a reasonable value could be say 1e6 N/mm (not bigger than 1000 times the stiffness of near elements). Use 1.0 for compression stiffness.

But you need to run a nonlinear static SOL106 analysis, this is the key when using CHAP elements to account for large displacements effect.

Best regards,
Blas.

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

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[BlasMolero]

Ah!,
After writting I realized that using rigid elements will cause incorrect results in buckling and differential stiffness analyses because the large displacement effects are not calculated.
Exceptions are zero length elements (e.g.,to simulate a hinge) and rigid elements constrained so that they don’t rotate.

If solving a nonlinear analysis try to minimize the use of RBE2 elements.

Best regards,
Blas.

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

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[inline6]

a sling wont transfer moments and needs only axial stiffness which will be K=EA/L just use ROD elements and be done with it. If you have never used them before maybe a good time to start. It doesn't look like any will go into compression so no need for fancy gaps or tension only rods but it is easy to check the assumption.

 

[MemoOurs]

Did I see it well? You constrained RBE3 dependant node? It must be free to move accordingly to the motion of all other (independent) nodes. RBE3 is ideal for load applications but can't be constrained (SPC). To do that, you could use RBE2 or add CBUSH with high stiffness’s and constrain other node.
Cheers