Etabs Mass Summary By Story May 2026
Mastering the ETABS Mass Summary by Story: A Guide to Accurate Seismic Analysis
3. The Center of Rigidity vs. Center of Mass: The Rotational Narrative
Perhaps the most critical column in the Mass Summary table is the Center of Mass (CM) location relative to the Center of Rigidity (CR). While the CR is often calculated separately, the Mass Summary provides the precise location of the resultant inertial force for each floor.
The offset between the CM and CR creates the eccentricity that drives torsional forces during an earthquake. A deep analysis of the Mass Summary tracks the trajectory of the CM coordinates ($X_m, Y_m$) up the building height. etabs mass summary by story
- The Drift: If the CM coordinates drift erratically from floor to floor relative to the geometric center, it suggests an asymmetrical application of superimposed loads (e.g., heavy mechanical equipment or cladding loads applied only on one side of the façade).
- The Implication: High eccentricity values evident in the summary table alert the engineer to potential torsional irregularities before a single modal analysis is run. It dictates where "accidental eccentricity" load combinations will be most punitive.
D. Center of Mass (CM)
- X and Y Coordinates: These columns show the location of the center of mass for that specific story.
- Why this matters: You must compare the Center of Mass (CM) with the Center of Rigidity (CR). If they do not align, the building will experience torsion during a seismic event. Large eccentricities here indicate potential torsional irregularities.
4. Application in Seismic Analysis
2.1 Mass vs. Weight in ETABS
- Weight ($W$): Force due to gravity ($W = m \cdot g$). Used for gravity load combinations and P-Delta effects.
- Mass ($m$): Inertial property ($m = W/g$). Used for dynamic analysis (eigenvalue extraction, time history).
ETABS automatically converts defined dead, live, and superimposed loads into mass based on user-defined mass source definitions (per ASCE 7 or similar). Mastering the ETABS Mass Summary by Story: A