Response spectrum analysis gives you a peak modal response for each vibration mode of a structure — but those peaks don't happen at the same instant, so you can't just add them. The modal combination rule you choose is how you reconcile that: it turns a vector of per-mode peaks into a single design value for base shear, drift, or member force. Get it wrong and you either badly under-design (missing coupled-mode interaction) or waste steel chasing a bound that will never occur.

This video walks through the three workhorse rules that every seismic engineer should know cold: ABS (absolute sum — conservative upper bound, assumes all modal peaks coincide), SRSS (square root of sum of squares — assumes modes are statistically independent, valid when natural frequencies are well-separated), and CQC (complete quadratic combination — includes cross-modal correlation coefficients, essential when frequencies are closely spaced, as in 3D buildings with torsional modes near translational ones).

The reason CQC exists at all is the failure mode of SRSS on modern asymmetric buildings: when two modes sit within ~10% of each other in frequency, their responses are correlated, and SRSS silently under-predicts. CQC's cross-terms fix that. Expect the video to cover when each rule is code-permitted (ASCE 7, IS 1893, Eurocode 8 all have opinions), and the physical intuition for why closely-spaced modes need special treatment.