Jaewan,

For more details, see REG GUIDE 1.92 "Combining Modal Responses and Spatial Components in Seismic Response Analysis". Also following materials will be useful for your study.
-WRC Bulletin 352 Independent Support Motion Method of Modal Spectra Seismic Analysis
-REG GUIDE 1.60 Design Response Spectra for Seismic Design
-REG GUIDE 1.61 Damping Values for Seismic Design

You can get copies for all of them from Changyu or Samin.

Sun Wee
from Calgary