The following reference works are cited in this example problem:
Belzer P. et al. (October, 1990). Alternate Modal Combination Methods in Response Spectrum Analysis. NUREG/CR-5627, BNL.
Morante, R., Y. Wang. (December, 1999). "Reevaluation of Regulatory Guidance on Modal Response Combination Methods for Seismic Response Spectrum Analysis." NUREG/CR-6645, BNL-NUREG-52576.
U.S. Nuclear Regulatory Commission. (July, 2006). Combining Modal Responses and Spatial Components in Seismic Response Analysis, Revision 2. Regulatory Guide 1.92.
Gupta, A.K. (1993). Response Spectrum Method in Seismic Analysis and Design of Structures. CRC.