Etabs Mass Summary By Story Better May 2026
ASCE 7 allows reduced live loads for seismic mass (e.g., 25% of floor live load in storage areas). The default summary does not apply these reductions automatically. You need a better mass source.
The default ETABS Mass Summary by Story is a starting point, not a finish line. A better mass summary is one that:
By taking 10 minutes to define a proper mass source, adjust story definitions, and validate with base reactions, you transform a confusing default output into a reliable engineering tool. Your plan checkers—and your building’s safety—will thank you.
Next Step: Open ETABS right now, go to Define > Mass Source, and verify your multipliers. You may be surprised at what you find.
Need a template? Download our free "Better ETABS Mass Summary Excel Checker" (link below)
Have a specific mass summary bug? Describe your model in the comments.
To create a professional and comprehensive ETABS story mass summary report, you should organize your findings into a clear structure that includes modeling assumptions, primary data tables, and verification checks. 1. Report Structure & Components etabs mass summary by story better
Project Overview: Briefly describe the structure, including the number of stories, heights, and general building type.
Mass Source Definition: Explicitly state how the mass was derived (e.g., element self-mass plus specified load patterns like 100% Dead Load and a percentage of Live Load).
Assumptions: Mention if masses were "Lumped at Story Levels" to ensure they are properly associated with floor diaphragms. 2. Primary Data Table: Story Mass Summary
The core of your report should be a table extracted from ETABS under Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story. Mass X (kg/kN) Mass Y (kg/kN) Cumulative Mass X Cumulative Mass Y
Note: If your model uses diaphragms, also include the Center of Mass and Rigidity table to show eccentricity. 3. Verification & Quality Checks
A "better" report includes validation to ensure the model is accurate: ASCE 7 allows reduced live loads for seismic mass (e
Base Reaction vs. Total Mass: Compare the total vertical base reaction ( Fzcap F sub z
) under dead load cases to the total cumulative mass. These should align closely; a significant discrepancy often indicates unassigned masses.
Participating Mass Ratio: Include a check for Participating Mass Ratios (found under Analysis Results > Modal Results). Ensure that at least 90% of the mass is participating in the principal directions for seismic analysis. 4. How to Generate in ETABS
In ETABS, tracking and verifying building mass is a critical step for seismic and dynamic analysis
. Users frequently encounter two different tables to evaluate this: Mass Summary by Story Diaphragm Mass Summary Mass Summary by Story
table is generally the superior and more reliable choice for extracting the total seismic weight of a structure. Direct Comparison: Story Mass vs. Diaphragm Mass Mass Summary by Story Diaphragm Mass Summary Scope of Mass mass assigned to the story (slabs, beams, columns, walls). By taking 10 minutes to define a proper
Only captures mass associated with nodes assigned to a specific diaphragm. Best Used For
Total seismic base shear verification & manual weight checks.
Evaluating torsional eccentricities (Center of Mass vs. Center of Rigidity). Handling of Off-Diaphragm Mass Includes it flawlessly. Ignores it, leading to underreported building mass. Sensitivity to Meshing High; requires proper node alignment to lump correctly.
High; unmeshed or improperly assigned shells will be ignored. Why "Mass Summary by Story" is Usually Better Mass Summary by Story
table provides a much more holistic view of your structure's mass. The primary reasons it is considered the superior reference for total building weight include:
Mass Summary by Story is not the same as Mass Summary by Diaphragm ETABS 2016
| Column | Description | Engineering Significance | |--------|-------------|--------------------------| | Story | Floor level | Check mass jumps between stories | | UTotal (or UX, UY, UZ) | Translational mass (same in X & Y for symmetric buildings) | Lateral seismic mass: dead + appropriate live load + partition + equipment | | RTotal (or RX, RY, RZ) | Rotational mass moment of inertia about vertical axis | Affects torsional response (accidental eccentricity) | | Self Mass | Mass from element self-weight | Based on assigned materials | | Additional Mass | User-defined mass (shell uniform load, point masses) | Often missed – check carefully |