But when he ran the modal analysis, the building had massive torsion. He spent days adding shear walls.
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. etabs mass summary by story
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. But when he ran the modal analysis, the
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. But here is a hard truth: If you’ve