ETABS assigns all mass within a story to the diaphragm at that level. Points:
Many users assume ETABS uses the same load combinations for mass. It does not. By default, ETABS uses Dead Load (1.0) and Live Load (0.0). For seismic design, you must define a proper mass source via:
Define > Mass Source > Add New Mass Source.
For most codes, set:
The ETABS Mass Summary by Story is not just a table—it is the DNA of your structural model. Garbage mass leads to garbage periods, garbage base shear, and ultimately, an unsafe design.
Take five minutes to review this summary before every analysis. It will save you weeks of debugging hidden errors and ensure your building stands strong when the ground shakes.
By understanding where your mass comes from and how ETABS organizes it by story, you move from being a software operator to a true structural analyst.
Understanding the ETABS Mass Summary by Story In structural engineering, specifically when performing seismic or dynamic analysis, the distribution of mass is just as critical as the stiffness of the members. ETABS provides a dedicated output called the Mass Summary by Story to help engineers verify that their model’s inertia is correctly accounted for. etabs mass summary by story
This report is the primary tool for checking the "Mass Source" definition and ensuring that the dead loads, live load percentages, and self-weight are accurately represented at each floor level. How to Access the Mass Summary Table
Once you have successfully run your analysis in ETABS, follow these steps to view the data: Navigate to the Model Explorer on the left-hand side. Go to Tables > Analysis > Results > Structure Results. Double-click on Mass Summary by Story.
Alternatively, you can go to the top menu: Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story. Breaking Down the Data Fields
When you open the table, you will see several columns. Understanding these is vital for a precise seismic analysis:
Story: The name of the floor level defined in your story data.
Mass X / Mass Y: This represents the translational mass in the X and Y directions. In most standard buildings, these values will be identical unless you have defined specific directional mass properties. ETABS assigns all mass within a story to
Mass Source: Indicates which "Mass Source" pattern (defined in Define > Mass Source) was used to calculate these values.
Center of Mass (X, Y): The coordinates representing the "centroid" of the mass for that specific story. This is used by ETABS to calculate accidental eccentricity and torsional effects.
Cumulative Mass: The total mass from the top of the building down to that specific story. Why the Mass Summary Matters 1. Seismic Base Shear Verification The total seismic weight (
) of the building is the sum of the masses across all stories multiplied by gravity (
). If your Mass Summary shows significantly lower values than your manual "take-off" calculations, your seismic base shear ( ) will be underestimated, leading to an unsafe design. 2. Identifying Modeling Errors
A common mistake in ETABS is "double-counting" mass—for example, including the self-weight in the Mass Source definition while also having a Dead Load pattern that includes self-weight. By checking the Mass Summary, you can see if the totals look abnormally high. 3. Torsional Sensitivity By understanding where your mass comes from and
By comparing the Center of Mass (CM) in this table to the Center of Rigidity (CR) (found in a separate table), engineers can determine the eccentricity of the building. If the CM and CR are far apart, the building will experience significant rotation during an earthquake. Tips for Accuracy
Include Lateral Mass: Ensure your Mass Source includes 100% of the Dead Load and the code-specified percentage of the Live Load (usually 25% for storage or per local IBC/ASCE codes).
Check the Units: Always verify if you are looking at Mass (e.g., ) or Weight ( ). ETABS reports Mass in this specific table.
Diaphragms: To get an accurate story-by-story breakdown, ensure you have assigned diaphragms to your floor slabs. Conclusion
The ETABS Mass Summary by Story is more than just a list of numbers; it is the "sanity check" for your building’s dynamic behavior. Before trusting your period of vibration or seismic drift results, always validate that the mass distribution matches your engineering intent.