quad elements

[Olympia]

Dear all,

I'm trying to solve a simple 2d problem in which I have modeled a 20m height by 100m length elastic soil domain using quad elements with nDMaterial ElasticIsotropic material. In order to run time history analysis, I excite the base nodes with
a= 9.81*sin(2*pi*t) but when I plot the results, they are not what I expect.

My basic question is why the acceleration results of the base nodes as derived by the following recorder command are not identical with the input motion? The fact is that neither the results provided by the demonstration examples are not identical..
What is the acc provided by the OpenSees in each node (absolute, relative)?

recorder Node -file acceleration.out -time -dT $motionDT -nodeRange 1 $numNodes -dof 1 2 accel


Many thanks in advance,
Olympia

[vesna]

The acceleration recorded with your recorder is relative acceleration. If you want to record the absolute acceleration look at Example 2 at the bottom of this page:
http://opensees.berkeley.edu/wiki/index ... e_Recorder