When needed, I have used the 100%-40%-40% rule. Based on the following in Reg Guide 1.92
For response spectrum analysis, in which each of the three spatial components are calculated
separately, Chu, Amin, and Singh (Ref. 17) concluded that for an SCC subjected to the action of the three
components of an earthquake motion, the representative maximum response of interest of the SSC can be
satisfactorily obtained by taking the SRSS of the corresponding representative maximum response
for each of the three components calculated separately.
The SRSS procedure for combining the responses to the three components of an earthquake
motion is based on the consideration that it is very unlikely that the maximum response for each of the
three spatial components would occur at the same time during an earthquake.
The 100-40-40 percent rule was originally proposed as a simple way to estimate the maximum
expected response of a structure subject to three-directional seismic loading for response spectrum
analysis, and is the only alternative method for spatial combination that has received any significant
attention in the nuclear power industry. The results of the 100-40-40 spatial combination have been
compared with the SRSS spatial combination. Generally, they indicate that the 100-40-40 combination
method produces higher estimates of maximum response than the SRSS combination method by as much
as 16 percent, while the maximum under-prediction is 1 percent.
