The story-based mass summary provides a high-level overview perfect for peer reviews and senior principal approvals.
Why ETABS "Mass Summary by Story" is Superior for Seismic Design
definition. To align with most seismic codes (like ASCE 7-16), ensure you include 100% of Dead Loads and a percentage of Live Loads (typically 25% for storage or specific occupancy). Unit Conversion
The default is a starting point, not a finish line. A better mass summary is one that:
This will generate a clear grid displaying your stories, their respective masses in orthogonal directions, and the mass moment of inertia ( MMIcap M cap M cap I 🔍 Why the Mass Summary by Story is So Critical etabs mass summary by story better
Navigate to Display > Show Tables (or use the shortcut Ctrl + T ).
The "Mass Summary by Story" table in ETABS is far more than a simple list of numbers. It is a powerful validation tool that, when understood and used correctly, ensures the accuracy of your entire seismic design process.
: Mention if masses were "Lumped at Story Levels" to ensure they are properly associated with floor diaphragms. 2. Primary Data Table: Story Mass Summary
Accidentally adding live load to the mass source while also converting it via load cases. The story-based mass summary provides a high-level overview
Calculate the tributary area of a typical floor, multiply it by your expected dead load per square foot, and compare it to the table's Mass X value. The numbers should align within a 5% to 10% margin.
The translational mass active in the X and Y directions. In most standard buildings, these values will be identical unless you have directional mass modifiers.
In structural engineering, the accuracy of a building’s dynamic analysis hinges on how mass is distributed and accounted for. While various methods exist for auditing mass, utilizing the feature in ETABS (Extended Three-Dimensional Analysis of Building Systems) stands out as the superior approach for ensuring model integrity, seismic compliance, and computational efficiency. Precision in Seismic Force Calculation
Because the table calculates the center of mass ( Unit Conversion The default is a starting point,
Why is the mass in my penthouse higher than the roof? Why is the mass contribution from diaphragms different from my dead load takeoff? Why does the seismic weight not match my hand calculations?
To access the table, go to: Key Data Columns: Story: The level name.
Expand Analysis Results > Structure Results > Mass Summary > Mass Summary by Story .
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.
ETABS displays mass in (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 .