By Story Better !full! - Etabs Mass Summary

| Component | Description | Key Details & Potential Pitfalls | | :--- | :--- | :--- | | | The total mass in translational and rotational directions per story. | Mass is "lumped" to the defined story levels following specific rules (e.g., identical point coordinates for meshed elements). | | Mass Summary vs. Mass Summary by Diaphragm | Different methods can yield different values. | Discrepancies often arise if mass is assigned outside a defined diaphragm. Ensure all mass-contributing elements are connected to a diaphragm. | | Mass vs. Weight | Weight = Mass × g . | ETABS uses mass for dynamic analysis ( m = W/g ) but may use weight directly for static base shear calculations per some codes. | | Story Mass vs. Group Total Mass | The sum of story masses often differs from the total of all structural elements' mass. | Hypothesis: In many cases, Story mass is less than Group All mass. Root Cause: This signals a modeling error. A common reason is surfaces with more than 4 sides that are not auto-meshed; the program may effectively ignore them. Resolution: Edit the geometry to use only 3- or 4-sided polygons. | | Reactions vs. Mass Summary | Reactions from a DEAD load case may not match the weight in the mass summary. | Hypothesis: The reactions and mass summary are not matching. Root Cause: This mismatch is possible when elements are meshed across story levels. Check diaphragm assignments and ensure points at floor levels are not shared with other stories. |

Let’s compare a example.

Name this mass source clearly, e.g., "ASCE7_Mass_Source" so reviewers know what you used.

Properly add Load Patterns (Dead Load, Superimposed Dead Load, Partition Load) with a multiplier of 1.0.

After assigning the correct Mass Source (via ), re-run the analysis. Now, open Display > Show Tables > Analysis Results > Structure Output > Mass Summary by Story . etabs mass summary by story better

Select "From Self and Specified Load Patterns" and create a custom combination:

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 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.

Base shear increased from 1,500 kips to 2,100 kips. The original columns were undersized. A potential collapse was avoided.

Reflects specified dead, live, and lateral load combinations automatically. | Component | Description | Key Details &

Mass Summary by Story is not the same as Mass Summary by Diaphragm ETABS 2016

Ensure that the sum of the effective modal masses in the X and Y directions equals at least 90% of the total mass listed in your Story Mass Summary. If they don’t match, your structure is either too flexible (need more modes) or your constraints (rigid diaphragms) are preventing proper mass participation.

Accurate mass calculation is the foundation of reliable seismic and dynamic analysis. In ETABS, structural engineers often grapple with different ways to verify the mass of a building. While checking individual element masses or global totals works, leveraging the table is a vastly superior workflow.

Compare total calculated mass against initial estimates in seconds. 2. Seamless Integration of Mass Sources Mass Summary by Diaphragm | Different methods can

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.

It directly impacts the natural period and mode shapes of the building.

The total mass participating in the X or Y direction. Xcmcap X sub c m end-sub Ycmcap Y sub c m end-sub : The location of the Center of Mass. Xcm,cumcap X sub c m comma c u m end-sub Ycm,cumcap Y sub c m comma c u m end-sub : The cumulative center of mass up to that story. 4. Strategies for Better Seismic Modeling A. Identify Mass Irregularities Early

Even with a perfect mass source, you may see strange results. Here is how to get better by debugging.