Bar 2 or Bar 3 elements 2

[mohsinkhn785]

Hello,

I have made a stowage unit model, as attached in the file. Can anyone help me out on how to add bar 2 or bar 3 elements at the feet of the unit as shown in the file? The purpose of attaching the feet to the bar is to fix the stowage unit as fixed on an aircraft floor.

Thank you.

 

[rb1957]

I would not use BARs (beams) but three rods, in the orthogonal directions (X, Y, Z), at each point.

Stiffness (Area of the rod) … I use anything from 0.1in2 to 1in2 (1in2, 1in long, AL, gives stiffness of 1E7 … pretty typical).

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[mohsinkhn785]

@rb1957 (Aerospace).. Thank you for your reply. its highly appreciated. How would it differ if BARS were used instead of rods, and secondly, in FEMAP, I get an option to use standard or the NASTRAN element, any explanation wud be helpful? Thank you.

 

[rb1957]

RODs allow only axial load, BEAMs allow transverse shear.

My usual approach is "lazy" … three RODs and fully constrain the unconnected nodes.
Yes, I could constrain just the axial freedom.

If you use BEAMs then constrain only the axial freedom (may have been obvious to you).
If you use BEAMs and constrain all translation freedoms then you allow transverse shear and your reactions are no longer orthogonal and "wrong".
If you use BEAMs and fix the ends (constrain all 6 dof) your results are definitely Wrong.

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

 

[mohsinkhn785]

Hi, Could you please let me know the steps on how to attach these rod elements to the feet as in my file?

 

[rb1957]

?

you have a node at the foot of your structure, (X,Y,Z)

create 3 nodes off-set by 1 in each direction, (X+1,Y,Z), (X,Y+1,Z), (X,Y,Z+1)

connect these three nodes to the foot node, with RODs, area = 1 (or 10 or 0.1), material = Al

constrain the three new nodes in translation (X,Y,Z). Using RODs you can constrain in all 6 dof. If you use beams (because you want to fix the foot), then constrain in only 3 dof.

each ROD will react load in one direction = pinned support with finite stiffness.

check displacement of foot node, should be "small".

check effect of changing ROD stiffness (area), should be "small".


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

 

[mohsinkhn785]

If I constrain 3 dofs when using the beam, it causes transverse shear, but then in the last few lines of your comment, you mentioned that I can use beams and constrain 3 dof. How can this validate correct reactions?

 

[mohsinkhn785]

This is how the apex node is joined to the three-rod nodes created as shown in the attachment. Please verify if this is right

 

[rb1957]

nope.

the node you want to restrain is 0 (X, Y, Z).

create three nodes …
1 (X+1, Y, Z)
2 (X, Y+1, Z)
3 (X, Y, X+1)

create three RODs …
0-1, 0-2, 0-3
Area either 1 or 0.1 in2, Al ( E = 1E7)

Constrain nodes 1,2,3 in X, Y, Z (redundant, but easy)

each ROD reacts load in one of the global axes.


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

 

[mohsinkhn785]

So, the O node is the origin node (0,0,0)? In total there should be 12 nodes?

 

[rb1957]

nope …
"0" is the node you want to constrain, presumably 4 of them ?
yeap …
12 additional nodes

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

 

[mohsinkhn785]

Hi, This is how I have obtained the other offset nodes from one of the foot nodes. Since, I am pretty new to FEMAP, Can u guide me how to join a rod to these 3 nodes obtained to get the rods 0-1, 0-2,0-3?

Thank you

 

[rb1957]

you have four nodes …
1 on the structure
and 2,3,4 are offset in the three directions.

so RODs are N1-N2, N1-N3, N1-N4.
area = 1in2 (or 0.1in2) … try and see the difference
material Al, E = 10E6psi

add nodal constraint (pinned) to N2, 3, 4.

if this is not clear, read the manual, do the tutorials, watch the YT videos.

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

 

[mohsinkhn785]

yes,this is very clear. I did it and constrained N2,3,4 as full fixity. Secondly, the three nodes at each feet were plotted as you suggested (x+1,y,z),(x,y+1,z),(x,y,z+1). Can I plot as (x+5,y,z) or +1 is the standard?

 

[rb1957]

+1 or +5 not much difference … +5 will be a "softer" constraint (stiffness = AE/L)

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