But here is a hard truth:
Self-weight assignment.
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
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?" But here is a hard truth: Self-weight assignment
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. so why does the Mass Summary show almost nothing on the roof
But when he ran the modal analysis, the building had massive torsion. He spent days adding shear walls.
