eigen values and vectors

blackard · Post by **blackard** » Mon Sep 26, 2005 12:25 pm

I am trying to use the eigen command, and it gives me errors. The following script works fine without the line with the eigen command. But when I put in the line "eigen 2" all sorts of errors are obtained.

It is also not clear from the manual how to record the eigenvalues and eigen vectors for continuum elements. Any suggestions?

wipe

model basic -ndm 2 -ndf 2

node 1 0.0 0.0
node 2 72.0 0.0
node 3 72.0 24.0
node 4 0.0 24.0

fix 1 1 1
fix 2 0 1
fix 3 0 1
fix 4 1 1

nDMaterial ElasticIsotropic 1 3000.0 0.0

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 2

initialize

system BandGeneral

constraints Transformation

numberer RCM

test NormUnbalance 1e-12 20 1

algorithm Linear

integrator DisplacementControl 1 1 0.001

analysis Static

#-----------------------------------------
#Recorder
#-----------------------------------------

recorder Node -file Displ.out -node 1 2 3 4 -dof 1 2 disp
recorder Node -file Reactionss.out -node 1 2 3 4 -dof 1 2 reaction

#-----------------------------------------
#Perform the analysis
#-----------------------------------------

analyze 1

berktaftali · Post by **berktaftali** » Mon Sep 26, 2005 2:18 pm

You have to define mass. See the manual for the command...

blackard · Post by **blackard** » Tue Sep 27, 2005 8:13 am

My mistake

I put masses at the nodes, and I still get alot of error messages (which I don't really understand).

When I want to find eigenvectors and eigenvalues, do I have to be using a particular integrator, solver, algorithm, etc. in the analysis portion of the script? Will OpenSees even calculate eigenvalues when performing a static analysis?

silvia · Post by **silvia** » Tue Sep 27, 2005 1:42 pm

yes, you need to specify analysis information.
if you provide enough information, the solver will work.

berktaftali · Post by **berktaftali** » Tue Sep 27, 2005 10:40 pm

eigen command should work fine without any analysis definition (provided that mass & stiffness is defined properly). In fact, it defines its own analysis, solver, algorithm etc.

See eigenAnalysis function in commands.cpp...

blackard · Post by **blackard** » Thu Sep 29, 2005 5:39 am

So does the eigen command work with the quad element?
I can't get it to work with the stdBrick element either.
In both cases I have a working script, until I add the command:

eigen 2

Then the thing won't run.

What am I missing?

Post by **fmk** » Thu Sep 29, 2005 9:53 am

with the default Arpack eigen routines you can only request n-1 eigenvalues (n the number of unknowns with mass) .. so in your case you can only ask for 1.