The following five common topics in the modal Floor Response Spectrum (FRS) method are examined: 1) The relationship between the effective mass and the total mass; 2) Estimation of the base shear; 3) Identification of a local mode; 4) The uniform acceleration to be applied in the equivalent static analysis method; 5) Correction for the residual rigid modes. This paper shows the usage of effective mass in determining the base shear. A quantitative method based on the ranking of the importance of a mode and the importance of a mass in a mode is given, in addition to graphic animations, to exclude a local mode. It also provides a more realistic static method with a less overly conservative acceleration.

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