Etabs Mass Summary By Story -

| Mass Source Option | Key Components | Primary Use Case & Notes | | :--- | :--- | :--- | | | Structural element mass (material density) + Additional assigned mass. | General dynamic analysis; not aligned with seismic code requirements (e.g., 1.0DL+0.5LL). | | From Loads | Mass derived from specified load patterns (e.g., 1.0 DL + 0.5 LL). | Standard choice for seismic design based on gravity load representative value. Uses only vertical loads. | | From Self, Specified Mass, and Loads | Combines element mass and loads. | Must be used with caution to avoid double-counting the self-mass of structural elements. |

Before running the analysis, you must tell ETABS how to calculate the building's mass. Computers and Structures, Inc. Define > Mass Source Specified Load Patterns

If your "Mass Source" (Define > Mass Source) is set to include "Element Self Mass" AND "Loads," but you’ve also manually assigned mass, you might be double-counting. etabs mass summary by story

The 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

If a story has no diaphragm assigned, ETABS will still calculate the mass based on joint locations, but it will not display a single, unified Center of Mass coordinate for that story. Assign a rigid or semi-rigid diaphragm to resolve this. 3. Double Counting Loads | Mass Source Option | Key Components |

You can access the Mass Source definition by going to the . Here, you choose from three core options, and your selection directly dictates what appears in the "Mass Summary by Story" table:

: The specific level of the building (e.g., Story1, Story2, Roof). Ux : The total mass activated in the global X-direction. Uy : The total mass activated in the global Y-direction. | Standard choice for seismic design based on

Note on RZ: represents the torsional mass moment of inertia. This value dictates how the building twists during a seismic event and is critical for evaluating accidental eccentricity and torsional irregularities. Step-by-Step Mass Verification Workflow

Codes require that you accumulate 90% of the total building mass in the modes you consider for response spectrum analysis.