Etabs Mass Summary By Story Better Today
If you have a mechanical penthouse, the default summary lumps the penthouse floor mass into the roof story below it. This makes it look like your roof weighs 2x what it should. Better means decoupling these.
The ETABS Mass Summary by Story is not just a diagnostic tool—it is the foundation of your seismic safety. To make it better, you must:
The next time you run ETABS, spend 15 minutes on the Mass Summary. Compare it, question it, and improve it. Your future self—and the building's occupants—will thank you.
About the Author / Further Resources: For a deep dive, download the official CSI Analysis Reference Manual (Chapter 3: Mass and Weight). Or, check your local building code (ASCE 7, IBC, Eurocode 8, IS 1893) for specific mass combination factors—they vary, but the principles above remain universal.
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.
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.
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 table is a critical output that aggregates the total mass and weight at each level, forming the basis for seismic and dynamic analysis. Overview of Mass Summary
The Mass Summary by Story table provides the cumulative and individual mass for each story based on the Mass Source defined in your model. It typically includes: UX, UY, UZ: Total mass in the X, Y, and Z directions.
Sum UX, Sum UY: Cumulative mass from the top story down to the current level. XCM, YCM: Coordinates of the Center of Mass for each story. How to Access the Table To view the mass summary after running an analysis: Navigate to the Display menu. Select Show Tables.
Expand Analysis Results > Structure Results > Mass Summary > Mass Summary by Story. Improving Mass Accuracy ("Better" Mass)
To ensure your mass summary is "better" (more accurate), verify your Mass Source definition:
Self-Mass: Ensure "Element Self-Mass" is checked if you want ETABS to automatically calculate mass from material density and volumes.
Additional Mass: Use "Additional Mass" to account for non-structural elements like partitions or cladding that aren't modeled as physical objects.
Specified Load Patterns: For seismic mass, it is common practice to include of Dead Load and a percentage (e.g.,
) of Live Load by selecting "Specified Load Patterns" in the Mass Source dialog. Key Applications
Seismic Base Shear: The total weight (W) used to calculate base shear ( ) is derived from this table.
Drift Checks: Code-compliant drift checks must be based on the Center of Mass coordinates provided in this summary.
Soft Story Check: Mass distribution across stories is a factor in identifying structural irregularities like soft stories.
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). etabs mass summary by story better
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
To generate a professional Mass Summary by Story report in ETABS that is "better" than a basic data dump, you should focus on verifying the Mass Source and providing clear context for seismic design Draft Report Structure 1. Project Overview & Calculation Basis Mass Source Definition: Explicitly state the load patterns used (e.g., Dead Load + Live Load) Lumped Mass Logic:
Confirm that "Lump Lateral Mass at Story Levels" is enabled to ensure masses are correctly aggregated at each floor for seismic analysis Coordinate System:
Note the global origin used for Center of Mass (CM) and Center of Rigidity (CR) calculations CSI Knowledge Base 2. Story Mass Summary Table Extract this table by navigating to
Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story Cumulative Mass X (Sum to N) Total Mass
Check that the mass values are in the correct units (e.g., kilograms or tonnes); ETABS may require manual conversion (dividing by gravity if using weight units) 3. Torsional Sensitivity (CM vs. CR) For a superior report, include the Centers of Mass and Rigidity table to identify eccentricity Table Path:
Display > Show Tables > Analysis Results > Structure Results > Centers of Mass and Rigidity CSI Knowledge Base
Explain that the distance between CM and CR (eccentricity) directly influences the torsional shear on the building Fawad Najam 4. Quality Control & Verification Mesh Verification:
Mention if manual meshing was used for walls and slabs, as auto-meshing can sometimes lead to lumping errors CSI Knowledge Base Diaphragm Check: Confirm that Rigid Diaphragms
are assigned to all floors to ensure the story mass is reported accurately CSI Knowledge Base
Unlocking Precision: Why "Mass Summary by Story" is Better for ETABS Analysis
As a structural engineer, you know that the soul of a seismic analysis lies in how you define your building's mass. While ETABS offers various ways to view your model's weight, the Mass Summary by Story report is often the gold standard for high-accuracy design.
Here is why this specific report is better for your workflow and how to master it. 1. Superior Accuracy in Seismic Weight Verification
The primary benefit of the Story Mass Summary is its role in "Mass calculated manual" verification. By breaking down the weight floor-by-floor, you can easily compare ETABS’ output against manual calculations for:
Slab Weight: Effective area (total minus openings) times thickness and unit weight.
Vertical Components: The summary accounts for half the mass of columns/walls above and below the floor, ensuring vertical continuity.
Adjusted Top Stories: It correctly identifies that the top story has no vertical components above it, preventing the overestimation of roof mass. 2. Streamlined Diaphragm and Lateral Load Analysis
The Story Mass Summary is essentially the "heartbeat" of your lateral force distribution.
Lumped Mass Efficiency: ETABS can lump the mass of all nodes on a story to a "master node" at the center of mass. This simplifies the mass matrix and significantly reduces analysis time for high-rise buildings without sacrificing accuracy.
Drift Control: Since story drift is computed at the center of mass, having a clean, story-by-story mass breakdown is critical for verifying that your building stays within the strict 0.020h0.020 h 0.025h0.025 h seismic limits. 3. Better Troubleshooting of Mass Discrepancies
Have you ever noticed your base reactions don't match your mass? The Story Summary helps you pinpoint the culprit:
Element Meshing: It reveals if elements meshed between stories are lumping mass to the wrong level.
Null Areas: It helps identify if Superimposed Dead Loads (SDL) on null areas are failing to transfer to the seismic mass because a diaphragm wasn't assigned. How to Access the Report To get this better view of your structure’s data: Navigate to the Display menu. Select Show Tables. Go to Output > Other Output Items > Mass Summary by Story. Pro-Tip: The "Mass Source" Secret
For the best results, ensure your Mass Source is defined via Define > Mass Source. To avoid double-counting, don't check "Element Self Mass" if you are also including a Dead Load pattern with a 1.0 multiplier in your load list.
How are you currently verifying your building's seismic weight in ETABS? Let’s discuss your favorite verification techniques in the comments!
Explore the CSI Knowledge Base for detailed FAQs on mass application. Mass Source
In ETABS, the Mass Summary by Story report is a critical diagnostic tool used to verify the seismic weight and mass distribution of a structure. Understanding how to interpret and optimize this report ensures that your seismic analysis—specifically base shear and modal participation—is accurate. 1. Key Components of the Mass Summary Report
The report provides a tabular breakdown of mass at each story level, derived from the Mass Source definition. Otte International : Found under
Display > Show Tables > Other Definitions > Mass Data > Mass Summary by Story Calculation Logic If you have a mechanical penthouse, the default
: ETABS typically lumps masses at the floor level by taking half the weight of the story below and half the weight from the story above. Values Reported Translational Mass (UX, UY) : Used for lateral seismic force calculations. Rotational Mass (RZ)
: Represents the mass moment of inertia, crucial for capturing torsional effects. 2. Common Discrepancies and "Better" Interpretation
Users often find that the sum of story masses does not perfectly match base reactions. To get a "better" or more accurate summary, consider these factors: The "Half-Story" Rule
: The base level often includes half the mass of the lowest columns/walls. While this mass appears in the report and total reaction ( cap F sub z
), it does not participate in base shear because it is already "at" the ground. Meshing Issues
: Auto-meshing walls or slabs can sometimes lead to incorrect mass distributions. Manual meshing or ensuring all elements are properly connected to story nodes is recommended for precise results. Lumping Mass at Stories : Checking the option to "Lump Lateral Mass at Story Levels"
in the Mass Source definition simplifies the system into a multiple-degree-of-freedom (MDOF) model, which is often more practical for large buildings but less "rational" than distributed mass at every node. 3. Optimizing the Mass Source for Better Reports
To ensure your story mass summary is reliable, follow these best practices: Avoid Double Counting
: When defining the Mass Source from "Specified Load Patterns," ensure "Element Self Mass"
if your Dead Load pattern already includes a self-weight multiplier of 1.0. Include Imposed Loads : Per codes like
, include 100% of dead load and a percentage (typically 25% or 50%) of live load in the mass source for seismic weight. Unit Conversion
: Remember that ETABS results may need conversion; for instance, dividing weight-based results by the gravitational constant (
) to obtain true mass, or adjusting units (e.g., dividing by 1,000) for standard reporting. 4. Verification Checklist Center of rigidity - ETABS - CSI Knowledge Base
Why ETABS "Mass Summary by Story" is Superior for Seismic Design
In structural engineering, precision in mass distribution is the bedrock of reliable seismic analysis. While ETABS offers various ways to view model data, the Mass Summary by Story
report stands out as the most critical tool for engineers. It transcends simple data output by providing a high-level, synthesized view of a building’s dynamic characteristics. 1. Clarity in Vertical Mass Distribution Unlike individual element data, the Mass Summary by Story
aggregates all structural and non-structural loads—including dead loads and specified live load reductions—into a single value per level. This allows an engineer to immediately identify mass irregularities
. A sudden spike or dip in mass between floors can signal a modeling error or a significant architectural change that will disproportionately attract seismic forces, helping to catch "soft story" or "weight irregularity" issues early. 2. Direct Verification of the "Mass Source"
The accuracy of any dynamic analysis (Response Spectrum or Time History) depends entirely on the Mass Source
definition. This summary table is the primary diagnostic tool to ensure the software is "seeing" the weight as intended. By comparing the story mass against manual hand-calculations or architectural estimates, engineers can verify that self-weight, superimposed dead loads, and the appropriate percentage of live loads are being correctly converted into inertial mass. 3. Streamlined Center of Mass (COM) Tracking
Seismic design is a battle against torsion. The Mass Summary report typically includes the coordinates for the Center of Mass
. When viewed floor-by-floor, this allows the engineer to track the alignment of the building’s inertial center. If the COM shifts drastically from one floor to the next, it indicates a potential for high eccentricity and torsional shear, prompting a redesign of the lateral force-resisting system before the project advances too far. 4. Efficiency in Base Shear Calibration
Code-based seismic design often requires a comparison between the Static Base Shear Dynamic Base Shear
. The story mass summary provides the total "W" (effective seismic weight) needed for the Equivalent Lateral Force Procedure. Having this data consolidated by story makes it effortless to calculate the vertical distribution of forces ( cap F sub x
) and ensures that the model’s total weight matches the design assumptions. Conclusion Mass Summary by Story
is not just another table; it is a quality control powerhouse. By condensing thousands of elements into a story-by-story narrative, it gives engineers the bird’s-eye view necessary to ensure the model is accurate, the seismic weight is compliant, and the building's dynamic behavior is predictable. Are you looking to troubleshoot
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:
Here are a few options for a positive review of the ETABS Mass Summary by Story feature or report, ranging from professional to more enthusiastic. Professional & Precise
"The ETABS Mass Summary by Story provides an exceptionally clear and detailed breakdown of building mass at every level. It has become an essential part of our seismic weight verification process, making it easy to cross-check load pattern multipliers and ensure code compliance for lateral load analysis. The ability to see exactly how mass is lumped at each story level significantly simplifies our quality control workflows." Efficiency-Focused ETABS Mass Summary by Story Data | PDF | Physics - Scribd
Understanding the mass distribution of a building is the foundation of accurate seismic analysis. In ETABS, the "Mass Summary by Story" table is your primary diagnostic tool for verifying that your mathematical model reflects physical reality. The next time you run ETABS, spend 15
When you use ETABS for high-rise or complex structures, simply looking at the total mass isn't enough. You need to ensure the distribution aligns with the structural intent. Why Mass Summary by Story Matters
Seismic forces are calculated based on mass. If your story mass is off, your base shear and lateral forces will be incorrect. This table allows you to: Verify the self-weight of slabs, beams, and columns.
Check if superimposed dead loads (SIDL) are applied correctly.
Confirm the inclusion of the appropriate percentage of live loads.
Detect "ghost mass" from accidental double-counting or modeling errors. Accessing the Data To find this specific table after running an analysis: Navigate to the Model Explorer. Expand Tables > Analysis Results > Structure Results. Right-click Mass Summary by Story and select Show Table. Key Columns to Analyze Significance UX / UY The total translational mass in the X and Y directions. Sum UX / Sum UY
Cumulative mass from the top down; used to check total building weight. RX / RY / RZ
Mass moments of inertia; critical for capturing torsional effects. XCM / YCM
The Center of Mass coordinates; compare these to the Center of Rigidity. How to Improve Your Mass Source Setup
For better results, your "Mass Source" definition should follow these industry standards:
Include Lateral Mass Only: Ensure you are checking the "Include Lateral Mass" option for seismic analysis.
Weight Multipliers: Typically, use 1.0 for Dead/SIDL and 0.25 for Live Load (depending on your local building code).
Lumped vs. Distributed: ETABS automatically lumps mass at the story levels. If you have significant mass between floors, consider adding dummy levels or intermediate joints. Troubleshooting Common Discrepancies
If your Mass Summary by Story looks "wrong" compared to your manual hand calculations, check these three areas:
Object Modifiers: Check if you applied property modifiers (like 0.0 weight) to certain elements.
Overlapping Areas: Ensure you haven't modeled a slab over a slab, which doubles the mass in that specific story.
Cladding Loads: Architects often provide cladding as a line load. Ensure these are applied to the perimeter beams and that the mass source is set to include "Specified Load Patterns."
💡 Pro Tip: Always compare the Total Mass in the summary table to the Base Reaction for a dead load case. They should be identical. If they aren't, your mass source is likely missing a specific load pattern.
If you'd like to dive deeper into the technical side, I can help you with: The exact IBC or Eurocode formulas for mass source. How to export this data to Excel for faster reporting.
Interpreting eccentricity between the Center of Mass and Center of Rigidity.
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
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
Understanding ETABS Mass Summary by Story: A Comprehensive Guide
When working with ETABS, a powerful software for structural analysis and design, understanding the mass distribution of a building is crucial for ensuring its stability and performance under various loads, including seismic and wind loads. The Mass Summary by Story report in ETABS provides engineers with essential data to assess how mass is distributed across different stories of a building. In this post, we'll delve into the significance of the ETABS Mass Summary by Story, how to interpret it, and why it's vital for structural engineering projects.
Let’s fix the default output. Follow these steps to generate an accurate, plan-check-ready mass summary.
You reach the roof. Here stands a tiny, lightweight acrobat.
ETABS calculates story mass based on:
Critical Warning: By default, ETABS does not automatically include all gravity loads. It uses the Mass Source defined in Define > Mass Source. If you skip this, the program uses only the self-mass of elements—which will dramatically undervalue your seismic weight.
Engineers often apply live load reduction factors (for gravity design) to the mass source. Stop. For seismic mass, use unreduced live loads multiplied by the code factor (e.g., 0.25). Never apply area reduction to mass.