The .frc file contains two columns of data - time and load. CAESAR II has a processor (under Interfaces) to convert the .frc file to a force response spectrum. (Comment lines start with *.) If you wish to use the time history data directly, you will have to first convert the time column to milliseconds. Once adjusted, that time history data may be attached to your dynamic input as a Time History Definition. Under Name, provide the name of the file but precede the name with a #. For example, for a file named time_history, provide the name #time-history. That file (time_history) should reside in your CAESAR II data folder.