Seismic lateral forces are directly proportional to a structure's mass (
Weight). An error in the mass summary directly leads to an incorrect seismic force. Modal Analysis:
In practice, the correct choice is for building structures where the seismic weight is defined by building codes as a function of loads (DL + LL). According to a 2022 comparative study, the From load with DL+0.5 LL option yields results closest to manually calculated seismic mass and complies with ASCE 7–10 recommendations.
This is the rotational mass (polar moment of inertia) about each axis.
By mastering the Mass Summary table, you move from simply "running the model" to truly understanding the behavior of your structure. etabs mass summary by story
The total mass accumulated from the top story down to the current story. The value at the base story represents the total seismic mass of the structure. Translational Mass vs. Rotational Mass
Depending on your local building code and the occupancy of the building, a percentage of the live load (often ) is factored into the seismic mass. 2. Accessing the Mass Summary Table
is a critical output in ETABS used to verify the dynamic characteristics of a building. It provides a breakdown of the calculated mass—including self-weight, superimposed dead loads, and a percentage of live loads (as defined in the Mass Source)—assigned to each floor level. 1. The Role of the Mass Source The accuracy of the mass summary depends entirely on the Mass Source definition. Engineers typically include: Self-Weight and Specified Load Patterns:
The Mass Summary is only as accurate as your definition. In ETABS, go to Define > Mass Source to configure this. Common practices include: Seismic lateral forces are directly proportional to a
) match your expected engineering units to avoid decimal placement errors.
A common pitfall in ETABS modeling is ensuring that all mass is correctly accounted for, especially for "null" areas (areas not part of a structural slab). If you apply loads to a null area, you explicitly assign a diaphragm to that null area for its mass to be included in the building's seismic mass for modal analysis. Otherwise, the load contributes to reactions but not to the building's mass matrix [7†L31-L34], making the structure seem lighter than it is.
The resulting table provides a clear breakdown, usually organized from the highest story down to the base. Setting Up the "Mass Source" Correctly
Right-click anywhere on the ETABS table and select Export to Excel . This allows you to quickly graph the mass distribution over the height of the building to spot sudden anomalies. According to a 2022 comparative study, the From
Interpretation of the "Mass" column is key. This value is the result of the mass source calculation applied to all elements and loads tributary to that story's diaphragm. The accuracy of this data is paramount for all subsequent dynamic procedures.
Automatically calculated from material density and volume. Additional Mass: Mass assigned to joints or frames.
Confirm that dead loads, live load reductions, and structural self-weight are being synthesized correctly.
The output will be presented as a spreadsheet-style table that can be exported, copied, or included in design reports.