SDOF and soil column connection

[yegoroff]

Hi,

I am modelling a soil column (ElasticIsotropic and PressureDependMultiYield02 materials) with an attached SDOF to it.
I attach the SDOF to the soil column using the following method:
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set MidTopX [expr {int((($ColumnWidth/$MeshSize)+1)*($MeshSize/2))}]; #coordinate of the middle point at the surface of soil column
set MasterNode $NodeTags([expr {(int($ColumnWidth/$MeshSize)+1)}],1) #tag of this node

set n1 100000001; #bottom of SDOF
set n2 100000002; #free end of SDOF

model BasicBuilder -ndm 2 -ndf 2

node $n1 $MidTopX 0.0
node $n2 $MidTopX 1.0

equalDOF $MasterNode $n1 1 2
#Masternode has 3 DOF, therefore, equalDOF needed

mass $n2 $M 1e-18
#mass assigned to node 2 (top of SDOF) in X direction

uniaxialMaterial Steel01 20000001 0.118623 1.949551 0.03
uniaxialMaterial Viscous 20000002 0.139626 1
uniaxialMaterial Parallel 20000003 20000001 20000002

element truss 80000001 $n1 $n2 1 20000003
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Using this method, I get exactly the same displacements at the top of my SDOF for two completely different models.
What could be the reason for this?

[fmk]

suprised you get any results as second node has no stiffness in second direction. though that 1e-18 on mass might be saving you as something in the tangent matrix.
if sdof model just use a zero length element in dirn you want.

is ground motion acting in both directions, if only one which?