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.
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.
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
The structural weight and mass distribution are the core pillars of seismic and dynamic analysis in ETABS. Understanding the Mass Summary by Story table is not just a matter of pulling software data—it is a critical step in verifying that your building will behave exactly as intended during a lateral event.
The following breakdown details how to access this data, why it matters, and how to optimize your modeling to ensure your mass summaries are as accurate as possible. 📥 How to Access the Mass Summary by Story in ETABS
To locate the data after running an analysis, follow this simple path in the software menu: Go to Display on the top menu bar. Select Show Tables. Expand the Analysis Results folder. Expand Structure Results.
Click on Centers of Mass and Rigidity or look for specific mass tables depending on your software version.
This will generate a clear grid displaying your stories, their respective masses in orthogonal directions, and the mass moment of inertia ( MMIcap M cap M cap I 🔍 Why the Mass Summary by Story is So Critical
The mass of your building is the primary driver of inertial forces during an earthquake. The Mass Summary by Story report serves several high-stakes engineering purposes:
In ETABS, the Mass Summary by Story table is a critical output for verifying your building's seismic weight and ensuring your Mass Source is defined correctly. It provides a lumped mass per level, which is more intuitive for global checks than viewing individual joint or element masses. How to Access the Table
You can find this table after running an analysis by navigating to: Display > Show Tables
Analysis > Results > Structure Results > Mass Summary by Story Why This Table is "Better"
Seismic Weight Verification: It allows for a quick manual comparison against estimated building weights (e.g., Dead Load + % Live Load) to ensure no significant loads were missed. etabs mass summary by story better
Identifying Modeling Errors: If the "Total Mass" does not match your base reactions, it often indicates elements meshed between stories or incorrect labeling of points.
Superstructure vs. Substructure: It helps you isolate the mass of specific parts of the building to calculate precise mass participation factors for complex structures. Key Columns & Interpretation Description Story The name of the specific floor level. Mass X / Mass Y
The lumped translational mass in the global X and Y directions. Weight The total vertical weight lumped at that story level. Cumulative Mass The total mass of that story plus all stories above it. Important Tips for Accuracy Mass Source
In ETABS, the Mass Summary by Story is a critical diagnostic table used to verify the seismic weight and mass distribution of a structure. This data is essential for calculating the base shear during seismic analysis. Key Components of Story Mass
ETABS calculates story mass based on your defined Mass Source.
Element Self-Mass: Automatically calculated from the volume and material density of structural members.
Additional Mass: Manual assignments for non-structural elements like cladding or partitions.
Specified Load Patterns: Mass derived from weight (typically Dead Load + a percentage of Live Load). How ETABS Lumps Mass
The software follows a specific logic to distribute mass at each level:
Tributary Distribution: ETABS "lumps" mass at the center of each floor by taking half the weight of the story below and half from the story above.
Master Node: Translational and rotational masses are concentrated at a master node, usually at the center of mass, to capture dynamic effects.
Mesh Sensitivity: Improper auto-meshing of walls or slabs can lead to incorrect mass calculations. Manual meshing is often recommended for higher accuracy. Accessing and Exporting the Report To generate and review this data for your project report: Additional Mass Frame Objects
Mass Summary by Story table in ETABS is widely considered the superior tool for verifying a building's seismic weight compared to other output tables like "Story Mass" or "Mass Summary by Diaphragm". It provides a non-cumulative breakdown of mass (UX, UY, UZ) assigned to each level, which is critical for calculating base shear and checking modal mass participation. Why it is considered "better" Comprehensive Attribution : Unlike diaphragm-based tables, the Mass Summary by Story
includes all masses at a story level, even those not strictly attached to a diaphragm (such as points or shells outside the diaphragm's extent). Design Verification
: It allows engineers to easily check consistency against manual calculations. High-accuracy models typically show less than a 2% difference
between this table's output and manual seismic weight estimates. Lumped Mass Logic
: ETABS calculates these values by lumping half the mass from the story below and half from the story above at the floor level, providing a realistic distribution for lateral force analysis. Key Review Points & Tips Mass Source Configuration : The accuracy of this table depends entirely on your Mass Source The Mass Summary by Story table in ETABS
definition. To align with most seismic codes (like ASCE 7-16), ensure you include 100% of Dead Loads and a percentage of Live Loads (typically 25% for storage or specific occupancy). Unit Conversion
: Output values often require conversion; for example, dividing by 1,000 to match specific project units (e.g., from kg to Tonnes). Meshing Warning
for walls or slabs can sometimes lead to incorrect mass distributions. Many experienced reviewers prefer a Manual Mesh
to ensure all elements are properly connected and their masses are accounted for correctly in the summary. Consistency Check : Use this table to cross-reference against the Story Forces
table to ensure the applied un-decomposed loads match the calculated reactions. Common Pitfalls to Avoid
The ETABS Mass Summary by Story is not a passive output; it is a sanity check, a diagnostic radiograph, and a dynamic Rosetta stone. To use it better:
Master the Mass Summary, and you will no longer wonder why your building twists unexpectedly or why your base shear seems low. You will know.
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
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:
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 Audit your Mass Source before running analysis
What is ETABS Mass Summary by Story?
The Mass Summary by Story report in ETABS provides a detailed breakdown of the mass properties of a building model, organized by story. This report is essential for understanding the distribution of mass in the building, which is critical for seismic design, wind design, and other dynamic load analyses.
How to Access Mass Summary by Story in ETABS
To access the Mass Summary by Story report in ETABS, follow these steps:
Understanding the Mass Summary by Story Report
The Mass Summary by Story report provides the following information:
Interpreting the Mass Summary by Story Report
The Mass Summary by Story report is useful for:
Tips for Using Mass Summary by Story in ETABS
By following these guidelines and using the Mass Summary by Story feature in ETABS, structural engineers can gain valuable insights into the mass properties of their building designs, ultimately leading to safer and more efficient structures.
Now, the exported table alone is still not great. Create an Excel pivot table that:
Display > Show Forces/Stresses > Shell > Mass to verify distribution).Your custom table should have these columns: | Story | Mass DL (kip-s²/in) | Mass LL (reduced) | Mass Total | Seismic Weight (W) | Cumulative W | |-------|---------------------|-------------------|------------|--------------------|---------------|
Before we can make it better, we must understand what the software is doing under the hood.
When you run Analyze > Run Analysis > Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story, ETABS reports:
The Scenario: A firm in a high-seismic zone (Seismic Design Category D) ran ETABS and got a base shear of 1,500 kips. The design was proceeding.
The Problem: A junior engineer noticed the Mass Summary by Story showed only 1,200 kips of mass at the ground floor, while the architectural takeoff showed 1,800 kips.
The Investigation:
DEAD + 0.25 LIVE.Partition load (15 psf) was missing entirely.The Fix: They updated the mass source to include PARTITION and converted cladding to Additional Mass. The new Mass Summary showed a 28% increase in seismic weight.
The Result: Base shear increased from 1,500 kips to 2,100 kips. The original columns were undersized. A potential collapse was avoided.
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