Ajay,

If you are aware of the details of what happens in a response spectrum analysis, kindly ignore my post. If not, pl. read more about it . Two sources are:

1) Dynamics of structures by Clough and Penzien.

2) Dynamics of structures: Theory and applications to earthquake engineering by Anil K Chopra.

3) Three dimensional static and dynamic analysis of structures by Wilson.( this book is downloadable if you search google).

Regards