Displacement and Acceleration Recorder

[naba077]

Hi all,
I'm sorry to post this simple question, but I'm a bit confused with the relative and absolute displacement and acceleration. I defined an acceleration and force in the following manner. What will be my recordings, absolute or relative?? Forces are defined as a product of mass and acceleration at each node. Thanx in advance.

"
timeSeries Path 1 -dt 0.005 -filePath $GMdir/Nigata_EW.dat -factor $g

pattern Plain 1 1 {
foreach SupportNode $iSupportNode Mass $imass {
load $SupportNode [expr $Mass*(-1.0)] 0. 0. 0. 0. 0.;
}
}
"

[fmk]

Relative and absolute are terms used to describe whether the results are absolute or relative to the ground motion .. OpenSees is simply giving the results for the equation of motion, e.g. for linear model for example it it simply solving the equation of motion M.a + C.v + K.u = P(t) given boundary conditions equation. If you look at Prof Chopras book, Dynamics of Structures, depending on the P(t) these can be either uniform (section 9.4) or absolute (section 9.7) .. it depends on the matrix equation you have set up.

In OpenSees the users can obtain absolute or relative by using load patterns provided to help them set up the loads and boundary conditions .. when using UnformExcitation pattern the loads are applied -M*ground accelerations at free nodes by load pattern and results are thus relative and when using MultipleSupport the actual ground movement is imposed at boundary of the model and results are thus absolute.