Etabs Mass Summary By Story Better -

The Mass Summary by Story table in ETABS is often considered "better" or more reliable than other mass tables because it provides a comprehensive accounting of all structural mass assigned to a level, regardless of whether a diaphragm is present. Why "Mass Summary by Story" is Often Preferred

Completeness: Unlike the Mass Summary by Diaphragm table, which only includes mass associated with specific diaphragm assignments, the story summary captures every ounce of mass (columns, walls, beams, and slabs) assigned to that story level.

Total Seismic Weight: It is the standard reference for verifying the total seismic weight of the structure against manual calculations.

Modal Analysis: It helps you verify which parts of the structure (e.g., superstructure vs. substructure) are contributing to specific Modal Mass Participation Ratios. Where to Find It You can access this table by following this path in ETABS: Go to the Display menu. Select Show Tables. etabs mass summary by story better

Navigate to Analysis Results > Structural Results > Other Definitions > Mass Data > Mass Summary by Story. Key Data in the Table The table typically includes the following columns: Story: The specific floor level.

Mass X & Mass Y: The translational mass in global directions.

Mass Moment of Inertia: The rotational mass about the Z-axis. The Mass Summary by Story table in ETABS

Centers of Mass (XCM, YCM): The geometric center where the story mass is concentrated. Common Issues to Watch For Center of rigidity - ETABS - CSI Knowledge Base

Step 5: Create a Custom Mass Summary Table (The "Better" Version)

Now, the exported table alone is still not great. Create an Excel pivot table that:

Subtracts diaphragm overlap mass (manual—use Display > Show Forces/Stresses > Shell > Mass to verify distribution).
Breaks down mass by load pattern (Dead vs Live vs Snow).
Calculates cumulative seismic weight from top down (required for base shear distribution).

Your custom table should have these columns: | Story | Mass DL (kip-s²/in) | Mass LL (reduced) | Mass Total | Seismic Weight (W) | Cumulative W | |-------|---------------------|-------------------|------------|--------------------|---------------| Your custom table should have these columns: |

1. Purpose

The ETABS Mass Summary by Story provides the lateral seismic mass (typically in ton-sec²/mm or kip-sec²/in) used for modal analysis (eigenvalue extraction) and equivalent lateral force (ELF) procedures. Understanding it correctly prevents underpredicting seismic demands.

How ETABS calculates story mass (overview)

ETABS sums nodal masses from distributed area loads, line loads, and point masses attached to elements.
Mass contributions are mapped to story levels based on node elevations.
Mass participation factors are computed from modal eigenvectors; ETABS reports both translational and rotational contributions per mode and cumulative.

Step 4: Run Analysis & Extract the Enhanced Summary

After updating the mass source and stories:

Run full analysis.
Go to Display > Show Tables > Analysis > Results > Structure Output > Mass Summary.
Click "Export to Excel" — do not rely on the on-screen table for critical review.