If you have ever run a seismic analysis in ETABS, you have undoubtedly stared at the Mass Summary by Story table. It appears deceptively simple: a list of stories, masses in X, Y, Z, and a summation. Yet, for many engineers, this table is a source of quiet anxiety.
Is the mass correct? Did the software double-count the dead load? Why is the seismic weight different from the gravity takeoff? And most importantly—how can you get a better, more reliable Mass Summary?
In this article, we will move beyond the default settings. We will explore how to extract, validate, and optimize the ETABS Mass Summary by Story to ensure your dynamic analysis (Response Spectrum, Time History) and modal results are physically accurate. etabs mass summary by story better
After updating the mass source and stories:
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. If you have ever run a seismic analysis
A "better" Mass Summary is not just a number—it is a deliverable. You should export the table and compare it to your design basis.
| Column | Description | Engineering Significance | |--------|-------------|--------------------------| | Story | Floor level | Check mass jumps between stories | | UTotal (or UX, UY, UZ) | Translational mass (same in X & Y for symmetric buildings) | Lateral seismic mass: dead + appropriate live load + partition + equipment | | RTotal (or RX, RY, RZ) | Rotational mass moment of inertia about vertical axis | Affects torsional response (accidental eccentricity) | | Self Mass | Mass from element self-weight | Based on assigned materials | | Additional Mass | User-defined mass (shell uniform load, point masses) | Often missed – check carefully | Is the mass correct
Instead of relying on ETABS’ automatic story grouping, define Groups (e.g., Group-Roof-Beams, Group-Penthouse-Walls). Then modify the Mass Summary by Group table. This gives you control to exclude non-structural masses that artificially inflate the story summary.