UniformExcitation output analysis

[whzju]

Hi everyone,

I am performing some transient analysis on a pile foundation recently.
I use the "UniformExcitation" to apply the acceleration by fixing the bottom boundary.
But, I am confused about the output. The opensees said when using "UniformExcitation", the results are relative value.
What does this represent?
If anyone is familiar with this, could you please explain to me.

Many thanks,
Huan

[STKO Team]

When you use UniformExcitation, if you plot the displacement of a node, that would be the relative displacement, i.e. the displacement relative to the ground displacement. That is, you will see your model move, but with the base fixed.
If instead you use MultiSupporExcitation, then the output will be absolute, i.e. the ground motion is included in the output.

[whzju]

Thanks for the reply.
Is this also the same for the acceleration?
So the output is the relative acceleration for a fixed ground bottom and if we want the actual acceleration at the ground surface we need to add the input acceleration at the ground bottom?