equalDOF/rigidLink beam or bar

[mohsenVazirizade]

As we can see in the Manual, there are three commands for Multi-Point Constraints.
1 rigidDiaphragm
2 equalDOF
3 rigidLink
3-1 rigidLink bar
3-2 rigidLink beam

In this topic I want to shed light on the difference between the second and third ones:
if you see this topic: http://opensees.berkeley.edu/community/ ... =2&t=63430
I stated that rigidLink bar A B and equalDOF A B 1 2 3 are the same. As you can see in the manual, it is asserted that only the translational degree-of-freedom will be constrained to be exactly the same as those at the master node by using rigidLink bar and here nodes A and B are constrained on transitional directions, x y and z.

In the following I provide a 3D example:
4 cantilever columns are modelled and then a force is exerted in node 5.

node 1 0. 0. 0.
node 2 4. 0. 0.
node 3 0. 4. 0.
node 4 4. 4. 0.
node 5 0. 0. 5.
node 6 4. 0. 5.
node 7 0. 4. 5.
node 8 4. 4. 5.
fix 1 1 1 1 1 1 1
fix 2 1 1 1 1 1 1
fix 3 1 1 1 1 1 1
fix 4 1 1 1 1 1 1

pattern Plain 1 Linear {
load 5 100. 0. 0. 0. 0. 0.
}


when we use
rigidLink bar 5 6 or equalDOF 5 6 1 2 3 (the resualts are exatly the same)
as we expected, the force is distributed between the columns, which are involved, equally.
reaction for nodes 1 to 4
-50 0 0 0 -250 0
-50 0 0 0 -250 0
0 0 0 0 0 0
0 0 0 0 0 0
displacement for nodes 5 to 8
0.0013 0 0 0 0.0004 0
0.0013 0 0 0 0.0004 0
0 0 0 0 0 0
0 0 0 0 0 0
_____________________________________________________________
To recap of this part:
rigidLink bar A B = equalDOF A B 1 2 3
1 2 3 means transitionally constrained

_____________________________________________________________
The problem is here:
According to manual http://opensees.berkeley.edu/wiki/index ... nk_command
using command, rigidLink beam, both the translational and rotational degrees of freedom are constrained. Therefore, based on which is provided earlier, we expect that:
rigidLink beam 5 7 and equalDOF 5 7 1 2 3 4 5 6 to be the same. On the following some examples are provided seperately:


Using rigidLink beam command
rigidLink beam 5 7
reaction for nodes 1 to 4
-99.7013 0 0 0 -374.2530 -0.5974
0 0 0 0 0 0
-0.2987 0 0 0 -125.7470 -0.5974
0 0 0 0 0 0
displacement for nodes 5 to 8
0.0016 0 0 0 0.0004 0.0002
0 0 0 0 0 0
0.0009 0 0 0 0.0004 0.0002
0 0 0 0 0 0

Rotationally Constrained
equalDOF 5 7 4 5 6
reaction for nodes 1 to 4
-100 0 0 0 -375 0
0 0 0 0 0 0
0 0 0 0 -125 0
0 0 0 0 0 0
displacement for nodes 5 to 8
0.0016 0 0 0 0.0004 0
0 0 0 0 0 0
0.0009 0 0 0 0.0004 0
0 0 0 0 0 0

Transitionally and Rotationally Constrained
equalDOF 5 7 1 2 3 4 5 6
reaction for nodes 1 to 4
-50 0 0 0 -250 0
0 0 0 0 0 0
-50 0 0 0 -250 0
0 0 0 0 0 0
displacement for nodes 5 to 8
0.0013 0 0 0 0.0004 0
0 0 0 0 0 0
0.0013 0 0 0 0.0004 0
0 0 0 0 0 0
____________________________________________________
As it can be seen, rigidLink beam 5 7 provide almost the same results as equalDOF 5 7 4 5 6 rather than equalDOF 5 7 1 2 3 4 5 6 (this negligble difference ie because of the fact that the constraint object constructed for the beam option assumes small rotations). Thus, we can conclude that rigidLink beam just constrains rotational degree of freedom rather than both transitional and rotational.

To recap, if my examples and description are clear, please let me know why the manual expresses that "beam: both the translational and rotational degrees of freedom are constrained."

[mohsenVazirizade]

I don't think the following description about the rigid link command is correct. is there any idea?


http://opensees.berkeley.edu/wiki/index ... nk_command
beam both the translational and rotational degrees of freedom are constrained.

[fmk]

equalDOF 5 7 1 2 3 4 5 6 IS VERY DIFFERENT from the beam constraint .. you are asking that all the dof have the same exact value, this is what you are seeing .. the rigidBeam command is just like adding a rigid beam between the nodes (similar to rigid diaphragm) .. have a look at the source code if you are stuck .. as your loading is not symmetric your structure is going to twist regardless of the beam constraint .. which is what you are seeing.