Multiple Support Excitation

[kyungtae79]

Hi, all

I am checking multiple-support excitation under a SDF system.

For a SDF system, there are only 2 nodes.
Node 1 is a base node and node 2 is a top node as a cantilever column.

I applied a record of displacement data at node 1 for multipleSupport excitation.

However, after running this model , displacement responses at node 1 is not the same as the input displacement.

Since the output is absolute values, the input displacements should be the same as absolute displacement at the base.

The responses are half of inputs.

Do you know why??

[fmk]

let me guess .. you have also fixed the node in the dirn of the motion using the fix command and you are using the Penalty constraint handler.

[kyungtae79]

Exactly, I fixed that direction of dofs at the base node and used penalty constraints.

If I released the direction at the node, the base displacement became the input but responses of the top node were different.

Is that why??

[fmk]

yes .. you were only imposing 1/2 the displacements at the base.

[kyungtae79]

How can I impose the full displacements?

Just use a factor of 2?

Otherwise, do I need to change the constraints or release the dof?

[fmk]

i thought you already figured it out .. DO NOT FIX THE NODE IN THE DIRN OF THE MOTION IF YOU ARE DOING MULTI-SUPPORT .. if you need to do gravity analysis firts, fix it and then remove it before the transient analysis.

[kyungtae79]

Thank you.

When I released the direction and used constraints of penalty.

Displacements of the base node look good.

However, in terms of acceleration, responses of a top node became quite different. (even looked like wrong).

[fmk]

the accelerations at any node in a transient analysis is related to disp and vel at the node, these are given by the integration scheme you employed, e.g:

[url
http://opensees.berkeley.edu/wiki/index ... ark_Method
[/url]

it is good you ask, but you need to know the theory behind what you are doing.

[kyungtae79]

I am sorry to bother you again.

Would you check it out?

Since I applied an input to the base in the X direction.

I released the corresponding dof and used constraints of Penalty.

I did not scale the input.

If I changed the constraints into Transformation, I got almost zero displacements...

Why..?

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

wipe
model BasicBuilder -ndm 3 -ndf 6

node 1 0. 0. 0. -mass 0. 0. 0. 0. 0. 0.
node 2 0. 0. 10. -mass 180.617 180.617 180.617 0. 0. 0.
node 3 0. 0. 20. -mass 180.617 180.617 180.617 0. 0. 0.


fix 1 0 1 1 1 1 1

geomTransf Linear 1 0. 1. 0.
element elasticBeamColumn 1 1 2 3.669 33094834.944 13789514.56 2.305 1.778 0.872 1
element elasticBeamColumn 2 2 3 3.669 33094834.944 13789514.56 2.305 1.778 0.872 1


set nEigenI 1
set nEigenJ 1
set lambdaN [eigen [expr $nEigenJ]]
set lambdaI [lindex $lambdaN [expr $nEigenI-1]]
set lambdaJ [lindex $lambdaN [expr $nEigenJ-1]]
set omegaI [expr pow($lambdaI, 0.5)]
set omegaJ [expr pow($lambdaJ,0.5)]
set lambda1 [lindex $lambdaN 0]
set omega1 [expr pow($lambda1,0.5)]
set PI 3.141592654
set freq1 [expr $omega1/2/$PI]
puts "Natural frequency = $freq1"
set xDamp 0.05
set MpropSwitch 1.0
set KcurrSwitch 0.0
set KcommSwitch 0.0
set KinitSwitch 1.0
set alphaM [expr $MpropSwitch*$xDamp*(2*$omegaI*$omegaJ)/($omegaI+$omegaJ)]
set betaKcurr [expr $KcurrSwitch*2.*$xDamp/($omegaI+$omegaJ)]
set betaKcomm [expr $KcommSwitch*2.*$xDamp/($omegaI+$omegaJ)]
set betaKinit [expr $KinitSwitch*2.*$xDamp/($omegaI+$omegaJ)]
rayleigh $alphaM $betaKcurr $betaKinit $betaKcomm
puts "rayleigh $alphaM $betaKcurr $betaKinit $betaKcomm"

# recorder
recorder Node -file topNode_multi.acc -time -node 3 -dof 1 2 3 accel
recorder Node -file topNode_multi.txt -time -node 3 -dof 1 2 3 disp

# Multiple Support Excitation
set GMfact [expr 1.]
pattern MultipleSupport 1 {
set DispSeries "Series -dt 0.02 -filePath xdisp_center.out -factor $GMfact"
groundMotion 1 Plain -disp $DispSeries
imposedMotion 1 1 1
}

#constraints Penalty 1.0e+18 1.0e+18
constraints Transformation
numberer RCM
system BandGeneral
test NormDispIncr 1.0e-3 20 1
algorithm ModifiedNewton
integrator Newmark 0.5 0.25
analysis VariableTransient

analyze 877 0.02 0.00015 0.02 20

[fmk]

i don't have your motion .. post the first 10 lines.

[kyungtae79]

Unit : meter
dt : 0.02

numbers are quite small...


from 0.02 sec.

0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.000001
0.000001
0.000001
0.000001
0.000000
0.000000
0.000000
-0.000001
-0.000002
-0.000003
-0.000003
-0.000004
-0.000005
-0.000006
-0.000006
-0.000007
-0.000007
-0.000007
-0.000007
-0.000008
-0.000008
-0.000008
-0.000008
-0.000008
-0.000008
-0.000009
-0.000009
-0.000009
-0.000009
-0.000010
-0.000010
-0.000011

....

[fmk]

you need to either:


1) move the eigen value command to after the pattern command .. remember you don't have a fixity in the 1 dirn until after you have created the sp.

2) fix the node 1 1 dof before the eigen command, then remove it before you do
the analysis.

[kyungtae79]

Thank you for your help.

I solved this problem.

But, I have a question on your second suggestion.

After fixing the dof, how can I remove or change the fixity?

[fmk]

use "remove sp nodeTag? dof?"

[url]
http://opensees.berkeley.edu/wiki/index ... ve_Command
[/url]

[andreascj]

hi fmk and Silvia,
is it necessary to remove the restraints of fixed base nodes in the direction of motion for multi support? I tried some examples where i can get the response even with the base nodes remained fixed and when i record the displacements at fixed base nodes, i can obtain the input displacements.
BUT it is very surprising that when i record, velocity and accelerations at the fixed base nodes, i get just zeros!
I even tried with remove sp and still could not record the vel and accln?
IS there any bug in the recorders? i am using the V 2.2.0
Thanks