Ajay,
Earthquake dynmaic spectrum (Response Spectrum)results can be combined mostly either by CQC method or SRSS method. In Caesar you will get combination by SRSS method. The full form of SRSS is "Square root of sums of square".

For any parameter like displacement will be calculated based on number of mode shapes considered by you. for example if you have considered 10 number of mode shapes then all the maximum peaks in each modes will be combined by SRSS formula already mentioned by Mr.Richard. Since this formula is using square value of the respective parameter, so the results will be always positive irrespective of +ve or -ve sign.

Anidya Ji is already Mentioned the best reference for understanding the response spectrum method.