After running modal analysis, ETABS provides "Modal Participating Mass Ratios." For a valid model, the sum of modal mass ratios in the X and Y directions must exceed 90% of the total mass from the Mass Summary. If not, the analyst must add more modes.
Before analyzing the vertical distribution, one must scrutinize the genesis of the values. The Mass Summary is often misunderstood as a simple summation of geometry. In ETABS, mass is a derived property, distinct from the "Weight" used for gravity design. The deep analysis begins with the Mass Source definition.
The engineer must distinguish between Dead Load (DL) and Superimposed Dead Load (SDL). The mass summary captures the "real" mass—the construction materials—plus a portion of the live load (typically 25% to 50% depending on the seismic code) representing the probable mass during a seismic event. An anomaly in the total mass usually points to a misinterpretation of the "Mass Source" dialog—specifically the inclusion or exclusion of "Additional Mass" assignments versus pattern-loaded live loads. If the summary does not reflect the intended Mass Source combination, all subsequent spectral analyses are rendered null. etabs mass summary by story
If the mass summary is wrong, the building’s period (T) and modal participation ratios will be wrong.
The specific floor label (e.g., "Story1", "Story2", "Roof", "Base"). The specific floor label (e
Engineers must define a Mass Source via Define > Mass Source:
Incorrect mass source is a leading cause of erroneous dynamic results. Incorrect mass source is a leading cause of
Occasionally, loads are applied to elements that are not properly connected to the diaphragm (e.g., a beam modeled without a support connection or a slab not meshed with walls).
Your base (foundation level) often has a mass value if you have a basement slab or ground floor. In elevated structures (e.g., podium slabs), ensure mass is not double-counted between the base and first floor.