When you open the table, it typically contains the following columns. Let’s break down each one:
The ETABS Mass Summary by Story is not just another output table; it is your model's truth serum. Before you invest hours interpreting drift ratios or designing rebar, spend two minutes reviewing this summary.
Ask yourself:
A correct Mass Summary by Story ensures every subsequent analysis—from story shears to modal participation factors—sits on a foundation of reality. Use it wisely, and your designs will be both efficient and safe.
The ETABS Mass Summary by Story is a crucial output used to verify the seismic weight and inertia of a structure. It provides a detailed breakdown of the mass and weight assigned to each level, which is essential for calculating base shear and checking for irregularities. Key Components of the Mass Summary Report
The standard report typically includes the following columns and data points:
Story Name: Identifies each floor level defined in the model. etabs mass summary by story
Mass X and Mass Y: The translational mass assigned to each story in the global X and Y directions, respectively.
Mass Moment of Inertia (MMI): The rotational mass calculated about the center of mass, used for dynamic and seismic analysis.
Cumulative Mass: The total mass from the top of the building down to the current story. Story Weight: The vertical weight (
) calculated for each floor, often verified against manual calculations. Proper Content for Documentation
When presenting a "Mass Summary by Story" in a structural report, you should include:
Mass Source Definition: Explicitly state how the mass was derived (e.g., Element Self Mass, Additional Mass, or Specified Load Patterns like When you open the table, it typically contains
Summary Table: The direct export from the Display > Show Tables > Structure Output > Other Output Items > Mass Summary by Story.
Center of Mass (CM) and Center of Rigidity (CR): Documentation of these coordinates to demonstrate any inherent torsion or eccentricity in the building.
Verification: 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
Mass is typically calculated based on material density and element volume. It is then "lumped" at story levels based on: Mass Summary for ETABS Stories | PDF - Scribd
The Mass Summary by Story is essential for:
To generate a Mass Summary by Story in ETABS: A correct Mass Summary by Story ensures every
The specific steps may vary depending on the version of ETABS you are using. Always refer to the software's user manual or online support for detailed instructions.
Your base (foundation level) often has a mass value if you have a basement slab or ground floor. In elevated structures (e.g., podium slabs), ensure mass is not double-counted between the base and first floor.
After you verify the mass summary, run a modal analysis. The Mass Participation Factor report depends entirely on the Mass Summary.
The total seismic weight of the structure is: $$W = g \times \sum_i=1^n m_i$$ where $m_i$ = U1 (or U2) mass of story i, and $g$ = gravitational acceleration.
Example check: If ETABS reports total mass = 100,000 N·s²/m, then $W = 100,000 \times 9.81 = 981,000 N = 981 kN$. This must match the sum of dead load plus applicable live load per ASCE 7.