The force/time profiles that you add in a Time History analysis are the unbalanced forces. While Caesar II can handle a non-zero starting point, doing so in my opinion implies an unbalanced force to start with. I guess you can experiment with it to see if the results make sense.
_________________________
Loren Brown
Director of Technical Support
CADWorx & Analysis Solutions
Intergraph Process, Power, & Marine
12777 Jones Road, Ste. 480, Houston, TX 77070 USA