CAESAR II provides two options for combining the missing mass correction with modal dynamic results SRSS and Absolute. The Absolute Combination method provides the more conservative result and is based upon the assumption that dynamic amplification is going to occur simultaneously with the maximum ground acceleration or force load. Literature (References 1, 2) states that the modal and the rigid portions of the response to typical dynamic loads are actually statistically independent, so that the SRSS Combination method is a more accurate representation of reality. Because the SRSS Combination method is most closely aligned to reality, CAESAR II defaults to this missing mass combination method.
References
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A. K. Gupta, Response Spectrum Method in Seismic Analysis and Design of Structures, CRC Press, 1990
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K. M. Vashi, "Computation of Seismic Response from Higher Frequency Modes," ASME 80-C2/PVP-50, 1980
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O. E. Hansteen and K. Bell, "On the Accuracy of Mode Superposition Analysis in Structural Dynamics," Earthquake Engineering and Structural Dynamics, Volume 7, John Wiley & Sons, Ltd., 1979