[en] Individual transient structural dynamic responses are required to be combined for extreme load nuclear power plant design. This combination is associated with individual events that are postulated to occur simultaneously. The heuristic formula called the SRSS rule has been used for combining peak dynamic primary structural responses to obtain a 'probable' response combination. The meaning of 'probable' has not been quantitatively defined, and in addition, the SRSS rule has a problem of interpretation when the frequencies of the individual time history responses are different. When the individual responses to be combined have significantly different frequencies, and a cyclic repetition of the higher frequency response occurs, then a 'probable' response combination rule may require the addition of the peak individual responses. The simple SRSS rule does not reflect this and can therefore be applied improperly. The conditions under which the SRSS rule is valid as well as what rules should be used when SRSS is not valid are subjects addressed in this paper