Applying an earthquake acceleration to model

[yingying]

I am analysing an earth dam under a seismic acceleration - what is the correct way of applying the acceleration?

I am using the following the define the acceleration:

set accelFile "eq2accel.csv"
set cFactor 1
timeSeries Path 1000 -dt $dT -filePath $accelFile -factor $cFactor
pattern UniformExcitation 2 1 -accel 1000

I believe the UniformExcitation is applied to all fully fixed nodes, do I fix one base node or fix all the base nodes? When I run the model with all the base nodes fixed, the output accelerations I get from the crest of the dam are much greater than expected, however when I fix only one base node, the crest accelerations are smaller than expected.

[selimgunay]

Typically you need to fix all the base nodes unless there is a good reason to release the fixity in some of them, e.g. rocking, etc. There may be other reasons for the accelerations larger than expected. Also please note that if you use the standard recorder, recorded accelerations are relative accelerations and not the absolute

[Nikaili]

selimgunay wrote:
> Typically you need to fix all the base nodes unless there is a good reason
> to release the fixity in some of them, e.g. rocking, etc. There may be
> other reasons for the accelerations larger than expected. Also please note
> that if you use the standard recorder, recorded accelerations are relative
> accelerations and not the absolute

First, I apologize for my question, I am pretty new to OpenSees. You mention the accelerations you obtain in this was are relative. How would I extract the absolute values?

[selimgunay]

You need to add the applied ground motion to the recorder using the timeseries command

recorder Node -file nodesA.out -timeSeries 1 -time -node 1 2 3 4 -dof 1 accel

[fmk]

when you used uniform acceleration it does not apply any acceleration to the base nodes .. what it does is create a forcing function F(t) = -ag(t) M 1, M being the mass matrix and ag being the ground motion and 1 a vector of 1's and 0's, 1 in the vector if the corresponding dof is in the dirn specified in the input ..as Selim points out, the results obtained are relative, you have to add the ground motions imposed to get the total acceleration.

[vpatnana]

recorder Node -file nodesA.out -timeSeries 2 -time -node 1 2 3 4 -dof 1 accel (I applied timesSeries 2 for groundmotion)

I am not able to get the total acceleration response even timeseries is used, and in the output "TimeSeries *getTimeSeries(int tag) - none found with tag: 2" is showing even when I changed to timeSeries 1. Please help me.

[selimgunay]

1 or 2 stands for the tag of the timeseries that you have previoulsy defined. Therefore, instead of 1 or 2, you should use the tag of the timeseries that you have previously defined.