A question about stablility

[mfarzad]

Hi,
In the below example,we have two nodes:one is rigid and another is free.however in the rigid node,if you release rotational DOF,OpenSees solve it!!!
you know that in this case structure will become unstable,but opensees solve it without any warning message.
please help me.
thank you.

# --------------------------------------------------------------------------------------------------

# Example 1. cantilever 2D
# static pushover analysis with gravity.
# all units are in kip, inch, second
# elasticBeamColumn ELEMENT
# Silvia Mazzoni & Frank McKenna, 2006
#
# ^Y
# |
# 2 __
# | |
# | |
# | |
# (1) 36'
# | |
# | |
# | |
# =1= ---- -------->X
#

# SET UP ----------------------------------------------------------------------------
wipe; # clear opensees model
model basic -ndm 2 -ndf 3; # 2 dimensions, 3 dof per node
file mkdir Data; # create data directory

# define GEOMETRY -------------------------------------------------------------
# nodal coordinates:
node 1 0 0; # node#, X Y
node 2 0 432

# Single point constraints -- Boundary Conditions
fix 1 1 1 1; # node DX DY RZ

# nodal masses:
mass 2 5.18 0. 0.; # node#, Mx My Mz, Mass=Weight/g.

# Define ELEMENTS -------------------------------------------------------------
# define geometric transformation: performs a linear geometric transformation of beam stiffness and resisting force from the basic system to the global-coordinate system
geomTransf Linear 1; # associate a tag to transformation

# connectivity: (make A very large, 10e6 times its actual value)
element elasticBeamColumn 1 1 2 3600000000 4227 1080000 1; # element elasticBeamColumn $eleTag $iNode $jNode $A $E $Iz $transfTag

# Define RECORDERS -------------------------------------------------------------
recorder Node -file Data/DFree.out -time -node 2 -dof 1 2 3 disp; # displacements of free nodes
recorder Node -file Data/DBase.out -time -node 1 -dof 1 2 3 disp; # displacements of support nodes
recorder Node -file Data/RBase.out -time -node 1 -dof 1 2 3 reaction; # support reaction
recorder Drift -file Data/Drift.out -time -iNode 1 -jNode 2 -dof 1 -perpDirn 2 ; # lateral drift
recorder Element -file Data/FCol.out -time -ele 1 globalForce; # element forces -- column
recorder Element -file Data/DCol.out -time -ele 1 deformation; # element deformations -- column

# define GRAVITY -------------------------------------------------------------
pattern Plain 1 Linear {
load 2 0. -2000. 0.; # node#, FX FY MZ -- superstructure-weight
}
constraints Plain; # how it handles boundary conditions
numberer Plain; # renumber dof's to minimize band-width (optimization), if you want to
system BandGeneral; # how to store and solve the system of equations in the analysis
test NormDispIncr 1.0e-8 6 ; # determine if convergence has been achieved at the end of an iteration step
algorithm Newton; # use Newton's solution algorithm: updates tangent stiffness at every iteration
integrator LoadControl 0.1; # determine the next time step for an analysis, # apply gravity in 10 steps
analysis Static # define type of analysis static or transient
analyze 10; # perform gravity analysis
loadConst -time 0.0; # hold gravity constant and restart time

# define LATERAL load -------------------------------------------------------------
# Lateral load pattern
pattern Plain 2 Linear {
load 2 2000. 0.0 0.0; # node#, FX FY MZ -- representative lateral load at top node
}

# pushover: diplacement controlled static analysis
integrator DisplacementControl 2 1 0.1; # switch to displacement control, for node 11, dof 1, 0.1 increment
analyze 1000; # apply 100 steps of pushover analysis to a displacement of 10

puts "Done!"