Extracting Mass and Stiffness of ShellMITC4??

[madtehrani]

Hi all,
I know how to extract the mass and stiffness. But, my problem is when I use the ShellMITC4 to model a plate. It does not work and gives me errors.

Any help would be appreciated.

I use the following simple code.

# Units: kg, m, sec
# ----------------------------
# Start of model generation
# ----------------------------
wipe
model basic -ndm 3 -ndf 6

# $matTag $E $v $rho
nDMaterial ElasticIsotropic 1 200e9 0.3 8050

# $matTag $threeDTag
nDMaterial PlateFiber 2 1

# $secTag $matTag $h
section PlateFiber "7" 2 0.01

set nx 2
set ny 2

# generate the nodes and elements
block2D $nx $ny 1 1 ShellMITC4 "7" {
1 0 0 0
2 0.1 0 0
3 0.1 1.0 0
4 0 1.0 0
}

# define the boundary conditions
fixY 0.0 1 1 1 1 1 1;


#==========================================================================
# PERFORM EIGEN ANALYSIS
#==========================================================================

set eigenvalues [eigen 10]
set omega {}
set f {}
set T {}
set pi 3.141593

puts " "
puts "PERIODS (sec):"

foreach lam $eigenvalues {
lappend omega [expr sqrt($lam)]
lappend f [expr sqrt($lam)/(2*$pi)]
lappend T [expr (2*$pi)/sqrt($lam)]
puts "[expr (2*$pi)/sqrt($lam)]"
}


#==========================================================================
# EXPORT MASS & STIFFNESS MATRICES
#==========================================================================

# STATIC ANALYSIS --------------------

test NormDispIncr 1.0e-6 6 0
algorithm Newton
system FullGeneral
constraints Transformation
integrator LoadControl 1.0
numberer Plain
analysis Static
analyze 1
printA -file "kMatrix.out"

wipeAnalysis
loadConst -time 0.0


# TRANSIENT ANALYSIS --------------------

test NormDispIncr 1.0e-6 6 0
algorithm Newton
system FullGeneral
integrator CentralDifference
constraints Transformation
numberer Plain
analysis Transient
analyze 1 1.0
printA -file "mMatrix.out"

wipeAnalysis


return

[fmk]

it is working in the sense that the matrices are popping out .. it fails in the transient analysis because of the 0 terms in the mass matrix .. if you look you wil see that there are probably only mass terms associated with the translational dof.

[madtehrani]

Thank you Frank for your response.
I think the output of the mass matrix is fine even though I get that error because of the 0 masses. However, when I use the output mass and stiffness to do eigen analysis in MATLAB my frequencies are not match with OpenSees. The OpenSees frequencies are correct.
I tried to reconstruct the stiffness matrix and used with the mass output from OpenSees. These combination gives me the right frequencies. I believe there are some issues with the output stiffness matrix.