BandArpackSolver::Maximum number of iteration reached.

[gf135]

Dear friends,
thank you very much for the continued work and support of OpenSees, a great software package.

I'm having difficulty with the attached file. It's a simple 3D reticular structure built with linear elasticBeamColumn elements. I am doing a dynamic analysis but the problem I have starts when calculating the eigenvalues which gives me a first lambda value in the negative domain. This, in turn, causes the failure of subsequent operations.

Lines 290 and 291 describe two elements that are arranged as a lateral stiffness cross in the frame defined by nodes 25-29-41-45 in the yz plane. When these lines are commented out, the program runs well but when I introduce these elements, I obtain first the error:
"BandArpackSolver::Maximum number of iteration reached. "
and then a negative first eigenvalue.

I've played around introducing these types of crosses in other frames and sometimes they work and sometimes I obtain the same error. I have not detected a solid error pattern and I do not think that there are fundamental structural mistakes in the definition.

Am I doing something wrong?

Any help that you can provide will be greatly appreciated!
Thanks,
Guillermo

FILE:

model basic -ndm 3 -ndf 6

node 1 0 0 0
node 2 2.5 0 0
node 3 4.5 0 0
node 4 7 0 0
node 5 0 3 0
node 6 2.5 3 0
node 7 4.5 3 0
node 8 7 3 0
node 9 0 7 0
node 10 2.5 7 0
node 11 4.5 7 0
node 12 7 7 0
node 13 0 9.5 0
node 14 2.5 9.5 0
node 15 4.5 9.5 0
node 16 7 9.5 0
node 17 0 0 3.5
node 18 2.5 0 3.5
node 19 4.5 0 3.5
node 20 7 0 3.5
node 21 0 3 3.5
node 22 2.5 3 3.5
node 23 4.5 3 3.5
node 24 7 3 3.5
node 25 0 7 3.5
node 26 2.5 7 3.5
node 27 4.5 7 3.5
node 28 7 7 3.5
node 29 0 9.5 3.5
node 30 2.5 9.5 3.5
node 31 4.5 9.5 3.5
node 32 7 9.5 3.5
node 33 0 0 7
node 34 2.5 0 7
node 35 4.5 0 7
node 36 7 0 7
node 37 0 3 7
node 38 2.5 3 7
node 39 4.5 3 7
node 40 7 3 7
node 41 0 7 7
node 42 2.5 7 7
node 43 4.5 7 7
node 44 7 7 7
node 45 0 9.5 7
node 46 2.5 9.5 7
node 47 4.5 9.5 7
node 48 7 9.5 7
node 49 0 0 10.5
node 50 2.5 0 10.5
node 51 4.5 0 10.5
node 52 7 0 10.5
node 53 0 3 10.5
node 54 2.5 3 10.5
node 55 4.5 3 10.5
node 56 7 3 10.5
node 57 0 7 10.5
node 58 2.5 7 10.5
node 59 4.5 7 10.5
node 60 7 7 10.5
node 61 0 9.5 10.5
node 62 2.5 9.5 10.5
node 63 4.5 9.5 10.5
node 64 7 9.5 10.5

# Masses
set NodalMass 2050.1325

mass 1 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 2 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 3 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 4 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 5 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 6 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 7 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 8 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 9 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 10 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 11 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 12 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 13 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 14 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 15 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 16 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 17 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 18 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 19 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 20 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 21 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 22 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 23 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 24 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 25 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 26 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 27 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 28 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 29 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 30 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 31 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 32 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 33 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 34 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 35 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 36 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 37 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 38 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 39 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 40 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 41 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 42 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 43 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 44 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 45 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 46 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 47 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 48 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 49 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 50 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 51 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 52 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 53 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 54 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 55 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 56 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 57 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 58 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 59 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 60 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 61 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 62 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 63 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0
mass 64 $NodalMass $NodalMass $NodalMass 0.0 0.0 0.0

fixZ 0.0 1 1 1 1 1 1

geomTransf Linear 1 1.0 0.0 0.0
geomTransf Linear 2 0.0 0.0 1.0
geomTransf Linear 3 0.0 -1.0 0.0

# Columns
set ACol 0.0348
set ECol 30000000000.0
set GCol [expr $ECol*0.43]
set IyCol 2.0E-4
set IzCol 2.0E-4
set JCol [expr $IyCol+$IzCol]

element elasticBeamColumn 1 1 17 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 2 2 18 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 3 3 19 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 4 4 20 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 5 5 21 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 6 6 22 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 7 7 23 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 8 8 24 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 9 9 25 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 10 10 26 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 11 11 27 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 12 12 28 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 13 13 29 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 14 14 30 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 15 15 31 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 16 16 32 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 17 17 33 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 18 18 34 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 19 19 35 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 20 20 36 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 21 21 37 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 22 22 38 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 23 23 39 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 24 24 40 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 25 25 41 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 26 26 42 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 27 27 43 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 28 28 44 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 29 29 45 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 30 30 46 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 31 31 47 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 32 32 48 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 33 33 49 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 34 34 50 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 35 35 51 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 36 36 52 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 37 37 53 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 38 38 54 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 39 39 55 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 40 40 56 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 41 41 57 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 42 42 58 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 43 43 59 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 44 44 60 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 45 45 61 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 46 46 62 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 47 47 63 $ACol $ECol $GCol $JCol $IyCol $IzCol 1
element elasticBeamColumn 48 48 64 $ACol $ECol $GCol $JCol $IyCol $IzCol 1

# Beams
set ABea 0.045973
set EBea 30000000000.0
set GBea [expr $EBea*0.43]
set IyBea 2.0E-4
set IzBea 2.0E-4
set JBea [expr $IyBea+$IzBea]

element elasticBeamColumn 49 17 18 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 50 18 19 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 51 19 20 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 52 21 22 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 53 22 23 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 54 23 24 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 55 25 26 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 56 26 27 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 57 27 28 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 58 29 30 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 59 30 31 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 60 31 32 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 61 17 21 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 62 21 25 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 63 25 29 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 64 18 22 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 65 22 26 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 66 26 30 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 67 19 23 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 68 23 27 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 69 27 31 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 70 20 24 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 71 24 28 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 72 28 32 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 73 33 34 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 74 34 35 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 75 35 36 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 76 37 38 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 77 38 39 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 78 39 40 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 79 41 42 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 80 42 43 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 81 43 44 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 82 45 46 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 83 46 47 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 84 47 48 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 85 33 37 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 86 37 41 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 87 41 45 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 88 34 38 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 89 38 42 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 90 42 46 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 91 35 39 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 92 39 43 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 93 43 47 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 94 36 40 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 95 40 44 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 96 44 48 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 97 49 50 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 98 50 51 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 99 51 52 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 100 53 54 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 101 54 55 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 102 55 56 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 103 57 58 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 104 58 59 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 105 59 60 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 106 61 62 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 107 62 63 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 108 63 64 $ABea $EBea $GBea $JBea $IyBea $IzBea 2
element elasticBeamColumn 109 49 53 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 110 53 57 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 111 57 61 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 112 50 54 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 113 54 58 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 114 58 62 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 115 51 55 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 116 55 59 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 117 59 63 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 118 52 56 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 119 56 60 $ABea $EBea $GBea $JBea $IyBea $IzBea 1
element elasticBeamColumn 120 60 64 $ABea $EBea $GBea $JBea $IyBea $IzBea 1

# Retrofits(Bracings)
set ARet 0.01
set ERet 30000000000.0
set GRet [expr $ERet*0.43]
set IyRet 2.0E-4
set IzRet 2.0E-4
set JRet [expr $IyRet+$IzRet]

# THESE LINES ARE THE REINFORCEMENT

#element elasticBeamColumn 121 25 45 $ARet $ERet $GRet $JRet $IyRet $IzRet 1
#element elasticBeamColumn 122 41 29 $ARet $ERet $GRet $JRet $IyRet $IzRet 1

recorder Node -file NODE.TXT -time -node 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 -dof 1 2 disp

set xDamp 0.02; # damping ratio (0.02-0.05-typical)
set lambda [eigen 1]
puts "Lambda = $lambda"
set omega [expr pow($lambda,0.5)]
set Tperiod [expr 2*3.141592/$omega]; # period (sec.)
puts "Period of the Structure: $Tperiod"
set alphaM 0.0; # stiffness-prop. RAYLEIGH damping parameter; D = alphaM*M
set betaK 0.0; # stiffness proportional damping; +beatK*KCurrent
set betaKcomm [expr 2*$xDamp/$omega]; # mass-prop. RAYLEIGH damping parameter; +betaKcomm*KlastCommitt
set betaKinit 0.0; # initial-stiffness proportional damping; +beatKinit*Kini

set Gravity 9.81
set PI 3.141592
set QkeAngle 95.0324
set QkeAngleRAD [expr $QkeAngle*$PI/180.0]
set CQkeAngle [expr cos($QkeAngleRAD)]
set SQkeAngle [expr sin($QkeAngleRAD)]
set GaccelX "Series -dt 0.0125 -filePath ACC.DAT -factor [expr $Gravity*$CQkeAngle]"
set GaccelY "Series -dt 0.0125 -filePath ACC.DAT -factor [expr $Gravity*$SQkeAngle]"
pattern UniformExcitation 1 1 -accel $GaccelX
pattern UniformExcitation 2 2 -accel $GaccelY

constraints Transformation
numberer RCM
system ProfileSPD
integrator Newmark .5 .25 $alphaM $betaK $betaKcomm $betaKinit
#test NormUnbalance
algorithm Linear
analysis Transient
analyze 120 0.125

[silvia]

thank you for posting this.
This is a silly problem with the eigensolver. whenever there are two eigenvalues that are the same it is unable to make a choice so it choses something else, like a negative eigenvalue.

please replace the eigen-command line with the following:
set lambdaN [eigen 2]
set lambda [lindex $lambdaN 0];

so you have it calculate two eigenvalues and then you choose only one.

this worked on my running your file. please let me know if it works for you, too!!!

[gf135]

Fantastic!
Thanks, Silvia!
Guillermo