After running the modal analysis, compare the Modal Participating Mass Ratio. If the sum of UX mass participation is less than 90% for the first 10 modes, your Mass Summary is irrelevant—you have not captured enough dynamic response.
Do not trust the number until you complete this checklist.
If you have a mechanical penthouse, the default summary lumps the penthouse floor mass into the roof story below it. This makes it look like your roof weighs 2x what it should. Better means decoupling these. etabs mass summary by story better
You reach the roof. Here stands a tiny, lightweight acrobat.
In the realm of structural engineering, gravity is the ultimate critic, and inertia is its partner in crime. When analyzing a building in CSI ETABS, the Mass Summary by Story is often viewed as a mere formality—a single table to check before running a response spectrum analysis. However, treating this summary as a simple output is a missed opportunity. To be a better engineer, one must treat the Mass Summary not as a result, but as a diagnostic tool for model integrity, load path rationality, and dynamic behavior. After running the modal analysis, compare the Modal
Here is how to move from simply reading the Mass Summary to interpreting it with precision.
ASCE 7 allows reduced live loads for seismic mass (e.g., 25% of floor live load in storage areas). The default summary does not apply these reductions automatically. You need a better mass source. If you have a mechanical penthouse, the default
For underground structures or tanks, the mass of water and soil is significant. Assign these via Additional Mass or as a special load case with a multiplier of 1.0 in the mass source. Do not rely on DEAD for these.
ETABS displays mass in ton (metric) or kip-s²/in (imperial). Do not confuse mass (force/g) with weight (force). Seismic weight in kips is Mass (kip-s²/in) * 386.4 in/s².