Etabs Mass Summary - By Story

The "Where is my Mass?" Mystery Here is the most common panic moment for engineers: "I assigned a 10-inch slab, beams, columns, and walls... so why does the Mass Summary show almost nothing on the roof?"

If you use Rigid Diaphragms, your Mass Summary will show a single point mass at the CM. This is fine for simple boxes. But if you have a podium slab or a building with a hole (an atrium), using a rigid diaphragm will overestimate torsion because mass is artificially concentrated.

If you modeled a concrete slab but forgot to assign Self Weight Multiplier = 1 (under ), ETABS will only include the superimposed dead loads (finishes, partitions) and completely ignore the 150 pcf concrete you just modeled. etabs mass summary by story

ETABS doesn't just use "dead load." It calculates mass based on your load combinations. Specifically, for seismic analysis (per ASCE 7, IS 1893, or Eurocode 8), it typically uses:

But here is a hard truth:

If you’ve ever run a seismic analysis in ETABS, you’ve probably scrolled past the Mass Summary by Story table. It looks harmless enough—just a few columns of numbers under the Display > Show Tables > Analysis > Results > Structure Output menu.

In fact, most convergence errors, unexpected mode shapes, and even incorrect base shears can be traced back to a single misunderstood line in this table. The "Where is my Mass

But when he ran the modal analysis, the building had massive torsion. He spent days adding shear walls.

He forgot to check the Mass Summary. The summary showed that 70% of the building's mass was concentrated on the 5th floor (UX value was huge). No amount of shear walls on floors 1-4 could fix that torsion without massive foundation changes. He had to add stiffness at the 5th floor . But if you have a podium slab or