The classic methods to combine modal responses to obtain total response are:
1.- Absolute Sum: R=SUM abs(Rij); this tend to overestimate the
response, it is very conservative.
2.- SRSS; it is accurate when the modes are well separated but has an
objection: under estimate the response when the modes tend to have similar
frequencies and reach their maximum value almost simultaneously.
3.- Mixed Method, were the absolute sum is used when two modes frequencies
are within 10% of each other, otherwise use SRSS; this method is designed
to overcome the objection in SRSS empirically.
4.- CQC is used to overcome the SRSS objection basing the calculation on
what would arise for a stationary random response.
See: A Finite Elements Dynamics Primer. NAFEMS.