CQCX is the variant of the complete quadratic combination working without adjusting the sign of the result values. It's considered an evolution of the CQC rule. The following formulas are used in CQCX:

Rk,tot,max = √abs(ΣΣRk,i*ρij*Rk,j) and

Rk,tot,min = - √abs(ΣΣRk,i*ρij*Rk,j)

Hint: Later investigations and discussions with earthquake specialists showed that most likely a procedure without sign adjustment is actually to be considered in a more modern approach, and it's the correct procedure. This procedure is offered in RM Bridge under the name CQCX. Therefore, it is recommended to always use CQCX instead of CQC.

communities.bentley.com/.../cqc---complete-quadratic-combination

References:

[1] Der Kiureghian, A. “On response of structures to stationary excitation”, Report No. UCB/EERC-79/32, Earthquake Engineering Research Center, University of California, Berkley, California (1979).

[2] Der Kiureghian, A. “A Response Spectrum Method for Random Vibrations, Report No. UCB/EERC-80/15, Earthquake Engineering Research Center, University of California, Berkley, (1980).

[3] Wilson, E.L., Der Kiureghian, A., Bayo, E.P., “A Replacement of the SRSS method in seismic analysis, Earthquake Engineering and Structural Dynamics, Vol. 9, p. 187- 194, (1981).

[4] Newmark, N.M., Rosenblueth, E., “Fundamentals of Earthquake Engineering”, Prentice Hall Inc., (1971).