Etabs Mass Summary By Story
The accuracy of dynamic analysis in structural engineering is highly dependent on the correct definition and distribution of mass. ETABS (Extended Three-dimensional Analysis of Building Systems) provides a critical output called the "Mass Summary by Story." This paper explains the theoretical basis of mass summation in ETABS, differentiates between mass and weight, interprets the tabular output, and demonstrates how this summary is used for seismic load calculations (base shear), center of mass determination, and dynamic property verification (modal analysis).
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Once you have the summary, perform these three critical checks:
1. The "Missing Mass" Check Compare the total weight (multiply mass by ( g )) against your hand calculation. If the ETABS total is 5,000 kN but your Excel sheet says 7,500 kN, you likely forgot to include cladding, finishes, or the live load percentage in the Mass Source.
2. The Sudden Jump Test Scan down the U3 column. Mass should change gradually. A massive jump at one story (e.g., a 200% increase from Story 2 to Story 3) usually indicates a modeling error—such as a duplicate floor slab, an overly thick concrete mat, or an accidental 10 kPa load.
3. The Torsional Warning If ( UX ) or ( UY ) mass moment of inertia is zero, ETABS cannot calculate torsion correctly. This happens when you model all mass as point masses at the center of mass instead of using a diaphragm. Ensure you have assigned a Semi-Rigid or Rigid Diaphragm to every floor.
The ETABS Mass Summary by Story is far more than a routine output table. It is the structural engineer’s first line of defense against incorrect seismic modeling. By understanding how mass is calculated—from the mass source definition to the lumping onto diaphragms—you can diagnose hidden errors, validate your model’s behavior, and confidently proceed to response spectrum analysis.
Final checklist before running seismic design:
Master the mass summary, and you master the seismic response of your structure in ETABS.
This article is intended for educational purposes. Always verify with the latest CSI ETABS documentation and your local building code.
A very specific topic!
In ETABS, a popular software for structural analysis and design, the "Mass Summary by Story" feature provides a detailed report of the mass distribution of a building model, broken down by story (or floor).
Here's what it entails:
What is Mass Summary by Story?
The Mass Summary by Story report in ETABS summarizes the mass properties of a building model, including:
Why is Mass Summary by Story useful?
This feature is useful for:
How to access Mass Summary by Story in ETABS?
To access this feature in ETABS, follow these steps:
The Mass Summary by Story report will display, showing the mass properties for each story in your building model.
Do you have any specific questions about using this feature or interpreting the results?
Understanding the ETABS Mass Summary by Story In structural engineering, the Mass Summary by Story
in ETABS is a critical diagnostic output used to verify the mathematical model of a building. It provides a breakdown of the translational mass
assigned to each floor level, serving as the foundation for all subsequent seismic and dynamic analyses. 1. Why Mass Distribution Matters Structural mass is the "M" in the fundamental equation
. In a seismic event, the ground acceleration acts upon this mass to generate inertial forces
. If the mass is incorrectly defined—perhaps due to doubled-up dead loads or missing partitions—the calculated base shear and lateral forces will be fundamentally flawed. ETABS calculates this by aggregating the self-weight of elements and any additional mass sources defined in the Mass Source 2. Center of Mass (CM) vs. Center of Rigidity (CR)
The summary report doesn't just list weight; it identifies the Center of Mass (CM) coordinates (
) for every story. This is vital because the distance between the CM and the Center of Rigidity (CR) torsional eccentricity
. A significant gap between these two points suggests the building will "twist" during an earthquake, requiring engineers to either adjust the layout of shear walls or strengthen the perimeter frames to resist rotation. 3. Data Verification and Dynamic Analysis
Engineers use the Mass Summary to perform "sanity checks." By comparing the ETABS total mass against manual tributary area
calculations, one can ensure that the software is correctly interpreting the load patterns. Furthermore, this data is the primary input for determining the building’s natural periods mode shapes
. An error in story mass directly shifts the frequency of the building, potentially leading to an inaccurate assessment of how the structure resonates with ground motion. 4. Practical Application
When reviewing the table, engineers look for consistency. Sudden spikes or drops in mass between identical floors usually indicate a modeling error, such as a missed slab or an accidental "null" property. Ensuring a clean Mass Summary is the first step toward a reliable Seismic Response Spectrum Time History Analysis Should we look into how to troubleshoot discrepancies
between your manual load take-offs and the ETABS mass output?
In structural engineering, the Mass Summary by Story in ETABS is the definitive record of your building's weight. It is essential for calculating seismic forces and verifying that your model reflects the actual design. 🏗️ Why Mass Summary Matters Seismic Design: Directly influences the Base Shear (
P-Delta Effects: Determines the stability and secondary moments of the frame. etabs mass summary by story
Error Checking: Identifies accidental "double counting" or missing loads.
Dynamic Analysis: Impacts the natural periods and mode shapes. 📍 Where to Find It
To access the report, navigate to:Analysis Results > Structure Results > Mass Summary > Mass Summary by Story 📊 Understanding the Data Columns Story: The specific floor level. Mass X / Mass Y: Total mass in horizontal directions.
Mass Center (X, Y): The geographic "balance point" of the floor's weight.
Cumulative Mass: The total weight from the roof down to that level. 💡 Pro-Tips for Accuracy
Check the Mass Source: Ensure "Self-Weight" and "Specified Load Patterns" are correctly toggled.
Lateral Mass vs. Vertical Mass: ETABS usually focuses on horizontal inertia for seismic runs.
Compare to Hand Calcs: Always do a "back-of-the-envelope" check on a typical floor.
Diaphragms: Ensure all slabs are constrained to a diaphragm to get a clean story-by-story readout.
⭐ Key Takeaway: The Mass Summary is your model's "scale." If the numbers look off, your seismic results will be too. If you'd like to refine this for a specific audience: Tell me your target reader (student vs. senior engineer) Specify if you want a step-by-step tutorial Provide a preferred word count
Before you finalize your ETABS model for seismic design:
When reviewing the Mass Summary by Story, ask yourself these three questions:
By mastering the Mass Summary table, you move from simply "running the model" to truly understanding the behavior of your structure.
In ETABS, the Mass Summary by Story is one of the most critical outputs for structural engineers. It provides a concise breakdown of how weight is distributed across the height of a building—a factor that directly dictates seismic forces and dynamic behavior.
Whether you are checking for modeling errors or prepping data for base shear calculations, here is everything you need to know about the Mass Summary by Story table. 1. How to Access the Mass Summary Table
Once you have run your analysis, follow this path to find the data: Path: Display > Show Tables
Table Location: Analysis Results > Structure Results > Mass Summary > Mass Summary by Story 2. Understanding the Columns
When you open the table, you will see several key headings. Understanding these is vital for an accurate peer review or design check: Story: The specific level of the building.
Mass X & Mass Y: These represent the total mass considered for horizontal motion in the X and Y directions. In most cases, these values are identical unless you have assigned specific directional mass properties.
Cumulative Mass: The sum of the mass of the current story plus all stories above it. This is useful for checking the total "weight" of the building at any level.
Center of Mass (XCM, YCM): The geometric coordinates where the story mass is concentrated.
Cumulative Center of Mass: The center of mass for the entire structure from that story upward. 3. Why This Data Matters A. Seismic Base Shear Verification
Seismic codes (like ASCE 7 or Eurocode 8) calculate the base shear based on the total "Seismic Weight" (
, the total mass in your ETABS summary (multiplied by gravity) should match your hand-calculated dead loads and applicable live load percentages. B. Spotting Modeling Errors
If a story shows a mass significantly higher or lower than expected, it’s a red flag. Common culprits include:
Duplicate Members: Accidental double-modeling of slabs or beams.
Incorrect Mass Source: If your "Mass Source" (Define > Mass Source) is set to include "Element Self Mass" AND "Loads," but you’ve also manually assigned mass, you might be double-counting.
Unit Errors: A quick glance at the cumulative mass helps verify if you are working in kips, tons, or kilograms. C. Calculating Eccentricity
By comparing the Center of Mass (CM) from this table to the Center of Rigidity (CR) (found in a separate table), you can determine the structural eccentricity. High eccentricity leads to significant torsional (twisting) effects during an earthquake. 4. Tips for Accurate Mass Results
Diaphragms: Ensure you have assigned diaphragms to your floor shells. ETABS calculates story mass properties most effectively when floors are constrained as diaphragms. Mass Source: Most modern codes require Mass = StorageLiveLoadcap S t o r a g e cap L i v e cap L o a d
. Double-check that your Mass Source definition reflects your local code.
Self-Weight Multiplier: Ensure your Dead Load pattern has a self-weight multiplier of 1, so the program automatically includes the weight of beams, columns, and slabs. 5. Exporting for Documentation
For professional design reports, you can right-click the table and select Export to Excel. This allows you to create charts showing the mass distribution over the building height, which is a great visual aid for client presentations or calculation packages.
Understanding the ETABS Mass Summary by Story In structural engineering, specifically when performing seismic or dynamic analysis, the distribution of mass is just as critical as the stiffness of the members. ETABS provides a dedicated output called the Mass Summary by Story to help engineers verify that their model’s inertia is correctly accounted for.
This report is the primary tool for checking the "Mass Source" definition and ensuring that the dead loads, live load percentages, and self-weight are accurately represented at each floor level. How to Access the Mass Summary Table The accuracy of dynamic analysis in structural engineering
Once you have successfully run your analysis in ETABS, follow these steps to view the data: Navigate to the Model Explorer on the left-hand side. Go to Tables > Analysis > Results > Structure Results. Double-click on Mass Summary by Story.
Alternatively, you can go to the top menu: Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story. Breaking Down the Data Fields
When you open the table, you will see several columns. Understanding these is vital for a precise seismic analysis:
Story: The name of the floor level defined in your story data.
Mass X / Mass Y: This represents the translational mass in the X and Y directions. In most standard buildings, these values will be identical unless you have defined specific directional mass properties.
Mass Source: Indicates which "Mass Source" pattern (defined in Define > Mass Source) was used to calculate these values.
Center of Mass (X, Y): The coordinates representing the "centroid" of the mass for that specific story. This is used by ETABS to calculate accidental eccentricity and torsional effects.
Cumulative Mass: The total mass from the top of the building down to that specific story. Why the Mass Summary Matters 1. Seismic Base Shear Verification The total seismic weight (
) of the building is the sum of the masses across all stories multiplied by gravity (
). If your Mass Summary shows significantly lower values than your manual "take-off" calculations, your seismic base shear ( ) will be underestimated, leading to an unsafe design. 2. Identifying Modeling Errors
A common mistake in ETABS is "double-counting" mass—for example, including the self-weight in the Mass Source definition while also having a Dead Load pattern that includes self-weight. By checking the Mass Summary, you can see if the totals look abnormally high. 3. Torsional Sensitivity
By comparing the Center of Mass (CM) in this table to the Center of Rigidity (CR) (found in a separate table), engineers can determine the eccentricity of the building. If the CM and CR are far apart, the building will experience significant rotation during an earthquake. Tips for Accuracy
Include Lateral Mass: Ensure your Mass Source includes 100% of the Dead Load and the code-specified percentage of the Live Load (usually 25% for storage or per local IBC/ASCE codes).
Check the Units: Always verify if you are looking at Mass (e.g., ) or Weight ( ). ETABS reports Mass in this specific table.
Diaphragms: To get an accurate story-by-story breakdown, ensure you have assigned diaphragms to your floor slabs. Conclusion
The ETABS Mass Summary by Story is more than just a list of numbers; it is the "sanity check" for your building’s dynamic behavior. Before trusting your period of vibration or seismic drift results, always validate that the mass distribution matches your engineering intent.
To obtain a Mass Summary by Story in ETABS, you must first define your mass source and then run the analysis to view the generated tables. This summary is essential for verifying the seismic weight and ensuring the structure's center of mass is correctly captured for lateral analysis. 1. Define the Mass Source
Before running the analysis, you must tell ETABS how to calculate the building's mass. Computers and Structures, Inc. Define > Mass Source Specified Load Patterns
to include dead loads and a portion of live loads (standard seismic practice often uses 1.0 for Dead/Super Dead and 0.25 for Live Load). Lump Lateral Mass at Story Levels
option if you want the mass concentrated at the floor levels for simplified lateral analysis. Include Lateral Mass
is checked to account for translational mass in X and Y directions. 2. View the Mass Summary Table
Once the analysis is complete, you can access the tabular data: Display > Show Tables In the tree menu, navigate to: Analysis Results Structure Results Mass Summary Table: Mass Summary by Story to display the table. 3. Key Columns in the Table Mass Summary by Story table typically includes the following: : The name of each floor level.
: The translational mass in the X and Y directions (usually identical).
: Vertical mass (included if "Include Vertical Mass" was selected in the Mass Source). Sum UX / Sum UY : Cumulative mass from the top down to that specific story. : The equivalent weight of the story mass (Mass × Computers and Structures, Inc. 4. Verification Tips Manual Check
: ETABS typically lumps mass at a story by taking half the weight from the story below and half from the story above. Diaphragms : To see the Center of Mass (CM) coordinates, you must assign a Rigid Diaphragm
to each floor and check the "Calculate Diaphragm Center of Rigidity" option in the analysis settings.
: Always double-check your units (e.g., kg vs. tonf-s²/m) to ensure the values align with your manual seismic weight calculations. table directly to for further reporting?
In ETABS, the Mass Summary by Story is a critical output table used to verify the total weight and mass distribution of a building for seismic and dynamic analysis. How to Access the Mass Summary Table To view the story mass data after running your analysis: Navigate to the Display menu. Select Show Tables.
In the tree view, go to Analysis Results > Structure Output > Other Output Items > Table: 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 of the floor in the global X and Y directions.
Mass Moment of Inertia: The rotational mass around the center of mass.
Center of Mass (X, Y): The coordinates where the floor's mass is concentrated.
Weight: The gravitational force associated with that story's mass. Critical Setup Requirements
To ensure your mass summary is accurate, check these settings:
Mass Source: Define this under Define > Mass Source. You must specify whether to use element self-mass, additional mass, or specific load patterns (like 100% Dead Load + 25% Live Load). Master the mass summary, and you master the
Lump Lateral Mass: In the Mass Source definition , ensure Lump Lateral Mass at Story Levels is checked to consolidate nodal masses into a single floor-level master node.
Rigid Diaphragms: Assigning rigid diaphragms to your floor slabs helps the software correctly identify and lump the center of mass for each story. Common Discrepancies
If the total mass seems incorrect compared to manual calculations:
Meshing Issues: Elements meshed between story levels might not have their mass lumped correctly to the nearest story.
Double Counting: Ensure you aren't selecting both "Element Self Mass" and a "Dead Load" pattern that already includes self-weight in your Mass Source. Mass FAQ - ETABS - CSI Knowledge Base
The "Mass Summary by Story" in ETABS is a critical output table used to verify the mass distribution of a structural model, ensuring that seismic and dynamic analyses are based on accurate data. Overview of Mass Summary by Story
This table provides a condensed look at the total mass assigned to each story level. It is typically found under Analysis Results > Structure Results > Mass Summary > Mass Summary by Story. It allows engineers to:
Verify Mass Sources: Ensure that the Mass Source (e.g., self-weight, specified dead loads, and a percentage of live loads) is correctly calculated by the software.
Check Mass Distribution: Confirm that the mass decreases or changes logically from the roof down to the base.
Identify Modeling Errors: Detect anomalies, such as a story having zero mass or an unusually high mass, which often indicates missing load assignments or incorrect material properties. Key Data Fields
The table generally includes the following columns for each story:
UX, UY, UZ: The translational mass along the X, Y, and Z axes. In most lateral analyses, the vertical mass (UZ) may be ignored unless vertical seismic effects are being considered.
RX, RY, RZ: The mass moments of inertia about the X, Y, and Z axes. RZ (rotational mass) is particularly vital for evaluating the torsional response of the building.
Cumulative Mass: Some versions provide the total mass from the top down to the current story, useful for checking base shear calculations manually. Importance in Seismic Analysis
ETABS uses these values to determine the Center of Mass (CM) for each diaphragm. The relationship between the CM and the Center of Rigidity (CR) determines the building's eccentricity and its susceptibility to torsion during an earthquake.
If you are just getting started with these summaries, you can find introductory structural analysis tutorials on YouTube to walk through the basic table generation. Mass Source
ETABS Mass Summary by Story: A Comprehensive Guide
ETABS, a popular structural analysis software, provides a powerful tool for engineers to analyze and design buildings. One of the essential features of ETABS is the ability to generate a mass summary by story, which is a crucial step in understanding the dynamic behavior of a building under various loads. In this article, we will delve into the concept of ETABS mass summary by story, its importance, and how to interpret the results.
What is ETABS Mass Summary by Story?
ETABS mass summary by story is a report that provides a detailed breakdown of the mass properties of a building, organized by each story or floor. The report includes various mass properties, such as:
These properties are calculated based on the structural elements, such as beams, columns, walls, and slabs, that make up the building.
Importance of ETABS Mass Summary by Story
The mass summary by story is a vital tool for structural engineers, as it helps them understand the distribution of mass within the building. This information is essential for several reasons:
How to Generate ETABS Mass Summary by Story
Generating the mass summary by story in ETABS is a straightforward process:
Interpreting ETABS Mass Summary by Story
The mass summary by story report provides a wealth of information about the mass properties of the building. Here's how to interpret the results:
Common Issues and Troubleshooting
When generating and interpreting the ETABS mass summary by story, engineers may encounter some common issues:
Best Practices
To ensure accurate and reliable results from the ETABS mass summary by story, follow these best practices:
Conclusion
The ETABS mass summary by story is a powerful tool for structural engineers to analyze and design buildings. By understanding the mass properties of a building, engineers can identify potential issues and optimize the structural design. By following the guidelines outlined in this article, engineers can generate and interpret the ETABS mass summary by story with confidence, ensuring accurate and reliable results.
To access the summary:
Alternatively, you can find it in the output text file (.OUT) generated after analysis.