negative eigen values

[blackard]

I am getting negative eigen values using a quad element.
The script is quite basic, it sure looks like the element has nodes defined counterclockwise as required. The element works well when applying pure shear or pure axial loads. But the eigen values are:

-9.24198 -0.89034 -9.24198 0.89034

Here's the tcl script:





#units: kips, in, sec

wipe

model basic -ndm 2 -ndf 2

# node
# tag X(in) Y(in)
node 1 0.0 0.0
node 2 72.0 0.0
node 3 72.0 24.0
node 4 0.0 24.0

# node mass X mass Y
mass 1 0.0058 0.0058
mass 2 0.0058 0.0058
mass 3 0.0058 0.0058
mass 4 0.0058 0.0058



# node
# tag DX DY
fix 1 1 1
fix 2 0 0
fix 3 0 0
fix 4 1 1


nDMaterial ElasticIsotropic 1 3000.0 0.36


element quad 1 1 2 3 4 60 PlaneStress 1


pattern Plain 1 Linear {
# Node FX FY
load 2 1000.0 0.0
load 3 1000.0 0.0
}


eigen frequency 2


initialize

system BandGeneral

constraints Transformation

numberer RCM

test NormUnbalance 1e-12 20 1

algorithm Linear

integrator LoadControl 1

analysis Static


recorder Node -file Q4eigen.out -node 2 3 -dof 1 2 "eigen 2"


analyze 1

[silvia]

i believe you are outputting the eigenvectors.
if you just give the command:

eigen 2


you'll see that you get two positive eingenvalues.

[blackard]

I tried that, now I'm not getting any output file.
How can I actually record the eigen values?

[fmk]

use tcl as outlined in my response to your latest post

http://opensees.berkeley.edu/community/ ... .php?t=479

[blackard]

I am now getting numbers, but seemingly too many numbers.
The numbers of course vary depending on what I put in the recorder command. I am using a quad element with 4 of 8 dofs fixed.

My eigen command is:

eigen frequency 3


For the recorder I have tried a number of different combinations:

recorder Node -file Q4EigenVector1.out -node all -dof 1 "eigen 1"
recorder Node -file Q4EigenVector2.out -node all -dof 1 "eigen 2"
recorder Node -file Q4EigenVector3.out -node all -dof 1 "eigen 3"

recorder Node -file Q4EigenVector1.out -node all -dof 2 "eigen 1"
recorder Node -file Q4EigenVector2.out -node all -dof 2 "eigen 2"
recorder Node -file Q4EigenVector3.out -node all -dof 2 "eigen 3"

recorder Node -file Q4EigenVector1.out -node all -dof 1 2 "eigen 1"
recorder Node -file Q4EigenVector2.out -node all -dof 1 2 "eigen 2"
recorder Node -file Q4EigenVector3.out -node all -dof 1 2 "eigen 3"

are a few examples.

Each time I get somewhat different results. Usually what is recorded
is 4 numbers or 8 numbers. But in 2-D it seems that a vector should have 2 components.


What is the significance of

-node all

-node 1 4

-node 2 3

-node 1 2 3 4

Or other node lists?

Along the same lines, what is the difference in

-dof 1

-dof 2

-dof 1 2 3

etc.

in the recorder command.

At this point I am expecting 3 eigen vectors, but am getting
numerous numbers output in a variety of combinations. Nothing
looks like two components of one eigen vector.

HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[silvia]

but doesn't the size of the eingenvector depend on the number of nodes?

[blackard]

Actually, ya, I think there should be 4 components with 4 free dofs, not 2 like I had indicated. Some times I get 4, other times 8.

The biggest dilemma is which vector goes with which eigen value?

Also, I am presuming that the list of numbers corresponds to the list of dofs in the "-dof 1" or "-dof 1 2", etc. part of the recorder command?
I'm just not sure at all of what I am seeing in the output.

[silvia]

plot the output.
first mode should have no nodes, second mode has one mode, third mode has three, and so on, right?