Dear Sara,
I agree with Gerrit that the displacement, velocity, and acceleration of a DOF should not be changed directly without considering whether the equations of motion are consistent with the specified motion.
If you simply wish to prescribe a platform surge motion time-history, you can use FAST’s seismic loading module, which was developed for a related purpose. Please see my post dated Aug 29, 2012 in the forum topic found here for more information: Software Releases.
Best regards,