Plain GroundMotion command

[aminrahmaani]

Dear fmk,

Would you explain more the following "Note", mentioned in description of Plain GroundMotion command (OpenseesWiki):

"For earthquake excitations it is important that the user provide the displacement time history, as the one generated using the trapezoidal method will not provide good results."

You mean if I use acceleration time history as the input, I won't get correct results?

Thanks.

[fmk]

it depends how the motion has been processed .. it is always safest to use the ground displacements provided.

[aminrahmaani]

fmk,

I modeled a soil column including three elements. I used multi-support excitation command to excite the model at the base of the soil column. The base of the model were fixed in all direction except in the direction of excitation. Here is the BC at the base:

fix 1 0 1 1
fix 5 0 1 1
fix 9 0 1 1
fix 13 0 1 1

I got correct results for displacements at each level but the accelerations are totally incorrect ( I got the acceleration of 3000 6000 m/s^2). The constraint handler that I'm using is "Penalty". When I use "constraint Transformation" everything is ok (However, I get 0 acceleration at the base, very strange!!!!). Would you please tell me what I should do to solve the problem? I have to use constraint penalty.

Thanks.

[fmk]

you get 0 accelerations at the base with transformation becuase i don't bother setting the accelerations there, only the displacements. the Penalty is different because it adds modes to the problem.

[aminrahmaani]

So how can I avoid adding those modes?

[fmk]

if you don't want them you have one option .. don't use the penalty method!