Imposed displacement time series ground motion

[nitadorakis]

Hi

I have a problem with a simple model which is not structural but maybe someone could help me here, because in soil-modelling subforum nobody helps me.

I try to impose displacement time series at all the base nodes of a rectangular 2D soil layer that is simulated with ElasticIsotropic nDMaterial for the quad elements that are used.
All base nodes are fixed in 2 dimensions, as well as the nodes of the right boundary.
The displacement time series is sinusoidal with period 1sec and amplitude 1, is applied in x-direction (=1st DOF), at 0.05 increments and is located in file displ.txt
In order to impose the ground motion Ι use:


set GMdirection 1 ;# ground-motion direction
set DtGround 0.05 ;# time-step for input ground motion
set IDloadTag 2;
set IDgm 0
# numXnode= number of base nodes in x direction
# totalNodes= total number of nodes

timeSeries Path 1 -dt $DtGround -filePath displ.txt
pattern MultipleSupport $IDloadTag {
for {set k 1} {$k <= $numXnode} {incr k 1} {
set IDgm [expr $IDgm +1]
groundMotion $IDgm Plain -disp 1
imposedMotion $k $GMdirection $IDgm
}
}

I use the following to record the absolute displacements in some nodes at the base (middle) and on the surface (middle, left and right end).

recorder Node -file displX.out -time -node [expr int(0.5*$numXnode)] [expr ($totalNodes- int(0.5*$numXnode))] [expr ($totalNodes-$numXnode+1)] [expr $totalNodes] -dof 1 -dT 0.05 disp


The problem is that at the node in middle of the base the recorded displacement is half of the imposed (i.e. the time series is sinusoidal with amplitude 0.5 instead of 1 which is the imposed).
The results in the other 3 nodes are also half of those expected.

I don't think that is a matter of relative-absolute displacement, because the results still don't make sense.
What could be wrong? I'm looking forward for some help.

[vesna]

This may happen if you assign double constrains on a node in the same dof. If you are assigning two constraints to the same node, penalty constraint assigns half of your total displacement to each one of them. Try not to fix a node in the direction in which you define "sp".

[nitadorakis]

Thank you

I was using constraints Penalty 1.0e18 1.0e18.
I tried with Transformation method and it worked fine. The response in now rational.
Could you please give a guideline when one should use the penalty method and when the transformation method?

[hesamgholamigmail]

Hello, I am performing nonlinear time history analysis on the bridge with SSI(Structure-Soil Interaction).this work is conducting by Multiple-Support excitation{ground motion-impose}.
how can I use of acceleration.txt file or velocity.txt file or dispalecement.txt file cause some resources recommended that must be applied [remove SP command] after multiple support excitation with disp.txt. but I have not information about accel or vel.
please, inform me. thankful