If the mass is calculated incorrectly:
At the bottom of the modal analysis output, ETABS shows the sum of mass in the direction of excitation (UX, UY). For a correct model, the sum of UX mass from the Mass Summary should exactly equal the sum of participating mass in the first translational mode (usually > 90% of total mass). etabs mass summary by story
Navigate to: Define > Mass Source
In ETABS, the mass of the structure directly influences the calculation of: Since If the mass is calculated incorrectly: At the