ETABS does not automatically know what to include as mass. You must define the Mass Source.
Typical seismic mass composition:
Warning: If you forget to define the mass source, ETABS uses the default (self-weight only). You will underestimate mass by 30–50%, leading to an unconservative design. etabs mass summary by story
Sometimes shown as UX Mass, UY Mass. This confirms the directional mass used for base shear calculation. ETABS does not automatically know what to include as mass
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. Typical seismic mass composition: