The mass moment of inertia about the global X, Y, and Z axes.
The is far more than a routine output table. It is the structural engineer’s first line of defense against incorrect seismic modeling. By understanding how mass is calculated—from the mass source definition to the lumping onto diaphragms—you can diagnose hidden errors, validate your model’s behavior, and confidently proceed to response spectrum analysis.
Navigate through: > Mass Data > Mass Summary by Story . Key Components of the Summary
: A brief comparison between ETABS results and a manual "check" of the total building weight to ensure no loads were missed or double-counted. How ETABS Lumps Mass
The offset between the CM and CR creates the that drives torsional forces during an earthquake. A deep analysis of the Mass Summary tracks the trajectory of the CM coordinates ($X_m, Y_m$) up the building height. etabs mass summary by story
The coordinates of the Center of Mass for that specific story.
Next she compared imposed loads used to compute masses. The mass summary reminded her of the live load reduction she’d applied per code: residential floors used a reduced live load factor, but the rooftop mechanical area still used full live load. That explained a spike at Story 11 where the plant room sat. She adjusted the live load assignment—moving some equipment to a lower floor—and watched the mass numbers settle into a more uniform gradient.
Why do engineers look at this table? Here are three primary reasons:
: The vertical mass active in the global Z direction (only populated if vertical mass is included in your mass source). The mass moment of inertia about the global X, Y, and Z axes
The vertical mass active at that story level. This is used primarily if you are running a vertical seismic or time-history analysis.
Navigate to the window on the left side of your screen. Expand the Tables tab. Expand Analysis Results , then open Model Results .
In the tree view, go to > Structure Output > Other Output Items > Table: Mass Summary by Story . Key Data in the Table The table typically includes the following columns: Story : The specific floor level.
Extracting this data in ETABS takes only a few clicks. Follow these steps: By understanding how mass is calculated—from the mass
Are you getting in your table, or are the numbers just not matching your manual estimates ? What elements are included in your Mass Source definition ?
If a penthouse or a specific floor has heavy equipment (HVAC, tanks), the mass summary will clearly show a spike in weight at that level, requiring increased lateral force design in those stories. 5. Interpreting the Data: Practical Example
Any lumped masses explicitly assigned to joints or frames by the user.