1-DOF eigenvalue

[endryus]

I have a simple 1-DOF model. I can't get the eigenvalue.
someone can help me?

# ------------------------------
# OpenSees input file
# Andrea Mordini - 2006
#
# Units: N, m, s
# ------------------------------
wipe;
# ------------------------------
# Start of units and constants
# ------------------------------
# space units
set m 1.;
set cm [expr $m/100];
set mm [expr $m/1000];
# time units
set s 1.;
# constants
set pig [expr 2*asin(1.0)];
set g [expr 9.80665*$m/pow($s,2)];
set large 1.e20;
set small 1.e-20;
# force units
set N 1.;
set kN [expr $N*1000];
# mass units
set kg [expr $N*pow($s,2)/$m];
# stress units
set MPa [expr $N/pow($mm,2)];
set GPa [expr $kN/pow($mm,2)];
# ------------------------------
# End of units and constants
# ------------------------------
# ------------------------------
# Start of model generation
# ------------------------------
model basic -ndm 2 -ndf 3
# nodes
# Node tag x y
node 1 0.00 0.00 -mass 0 0.00000 0
node 2 1.00 0.00 -mass 0 100.00000 0
# SP constraint definition
# SPC tag Dx Dy Rz
fix 1 1 1 1
# section
# Section tag E A J
section Elastic 1 [expr 205000*$MPa] [expr 1*pow($m,2)] [expr 0.01*pow($m,4)]
# transformation
geomTransf PDelta 1
# elements
# Element tag inode jnode intp sect transf
element dispBeamColumn 1 1 2 5 1 1
# ------------------------------
# End of model generation
# ------------------------------
# ------------------------------
# Start of static/modal analysis
# ------------------------------
# loads
# Analysis parameters
system BandGeneral
constraints Transformation
numberer RCM
test NormDispIncr 1.0e-12 10 3
algorithm Linear
integrator LoadControl 1
analysis Static
# Analysis
analyze 1
puts "static analysis completed"
#Eigen Analysis
eigen 1
puts "modal analysis completed"
# ------------------------------
# End of static/modal Analysis
# ------------------------------

[stefania]

for the analysis you must put something like

test EnergyIncr 1.0e-8 20 3
algorithm Newton
system SparseGeneral -piv
constraints Transformation
integrator Newmark 0.5 0.25
numberer RCM
analysis Transient

[endryus]

It does not work. In theory, the eig analysis should not be related with the previous analysis (if there is not change in stiffness)...

[silvia]

the eigen solver is limited to solving N-1 eigenvalues.
\please add a negligible mass to another dof, like 1e-9, and try it.
please let me know if it works.

[endryus]

Actually, for a 9-dof system I have to add not only 1 fake mass but 8 masses to obtain 9 eigenvalues!
Can you explain why?
could you please add it to the documentation?
thanks and welcome back!

andrea

[silvia]

it's a strange eigensolver I guess.
i'll put it in the documentation right awya.