Dave,
Thank you for the answer.
But this was all clear for me.
The typical floor response spectrum beginns with a verly low frequency
about 0,1 HZ with a low acceleration.
Its clear this frequency with a low acceleration will give big
displacement (pseudostatic anchor movement).
If the user beginn his spectrum input at about 1 Hz, then the pseudo anchor movement much lower.
We to consider anchor movement from a spectrum with a frequency
at the ramp to the first big acceleration in the spectrum and not the first input cell in the spectrum.