Etabs Mass Summary By Story ((new)) May 2026

In ETABS, the Mass Summary by Story is a fundamental output table used to verify the "Seismic Weight" (

) of a structure, which directly dictates the base shear calculation (

). This table consolidates all mass defined in your Mass Source and attributes it to specific floor levels. How to Access the Table To view the data after running an analysis: Go to the Display menu. Select Show Tables.

Navigate through: Other Definitions > Mass Data > Mass Summary by Story. Key Components of the Summary

The table typically includes the following columns for each story: Story: The specific floor level.

UX, UY, UZ: Translational masses in the global X, Y, and Z directions. UX and UY represent the lateral mass used for seismic forces. Mass: The total mass at that level. Weight: The gravitational force ( ) calculated for that story. Crucial Concepts

Mass Lumping: By default, ETABS lumps mass at story levels by taking half the weight from the story above and half from the story below. You can explicitly enable "Lump Lateral Mass at Story Levels" in the Mass Source definition to simplify the model into a multiple degree-of-freedom system.

Mass Source Definition: The values in this table are entirely dependent on how you define your Mass Source. Common practice includes 100% of Dead Loads and a percentage of Live Loads (e.g., 25% or 50% depending on occupancy/intensity) as per codes like ASCE 7 or IS 1893.

Manual Verification: If your ETABS mass summary doesn't match manual calculations, check if you have "Element Self Mass" checked while also including dead load patterns with a multiplier of 1.0; this can double-count the self-weight. Common Pitfalls

Meshing Issues: Using "Auto Mesh" on walls can sometimes lead to incorrect mass distribution. Manual meshing of shells and walls is often recommended to ensure all elements are properly connected and their masses correctly attributed to the nearest story.

Missing Diaphragms: Mass defined on shells or points might not appear correctly in story summaries if they aren't properly associated with a floor diaphragm. TECH TIPS: Mass Source Definition in ETABS

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

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. etabs mass summary by story

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.

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 In ETABS, the Mass Summary by Story is

. 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?

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: Mass Moment of Inertia (Ix, Iy, Iz) Center

  1. Dynamic Analysis: During dynamic analysis, such as earthquake or wind load analysis, the mass of the building plays a critical role in determining the structure's response. The mass summary by story helps engineers to identify the mass distribution, which is crucial for accurate dynamic analysis.
  2. Structural Stability: The center of mass and center of rigidity are essential for evaluating the structural stability of the building. A significant difference between the center of mass and center of rigidity can indicate potential stability issues.
  3. Load Distribution: The mass summary by story helps engineers to understand how loads are distributed throughout the building. This information is critical for designing structural elements, such as foundations, beams, and columns.

How to Generate ETABS Mass Summary by Story

Generating the mass summary by story in ETABS is a straightforward process:

  1. Open the ETABS Model: Open the ETABS model of the building for which you want to generate the mass summary.
  2. Go to the "Analyze" Menu: Click on the "Analyze" menu and select "Mass Summary" from the drop-down menu.
  3. Select the Story: In the "Mass Summary" dialog box, select the story or floor for which you want to generate the report.
  4. Choose the Mass Properties: Select the mass properties you want to include in the report, such as mass, moment of inertia, center of mass, and center of rigidity.
  5. Run the Analysis: Click "OK" to generate the mass summary report.

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:

  1. Mass: The mass value represents the total mass of the building at each story. A higher mass value indicates a greater amount of material or load at that story.
  2. Moment of Inertia: The moment of inertia (Ix, Iy, Iz) values represent the resistance of the building to bending and torsion. Higher moment of inertia values indicate greater stiffness.
  3. Center of Mass: The center of mass (X, Y, Z) values represent the location of the building's center of mass at each story. The center of mass should ideally coincide with the center of rigidity.
  4. Center of Rigidity: The center of rigidity (X, Y, Z) values represent the location of the building's center of rigidity at each story. The center of rigidity should ideally coincide with the center of mass.

Common Issues and Troubleshooting

When generating and interpreting the ETABS mass summary by story, engineers may encounter some common issues:

  1. Inconsistent Mass Values: Inconsistent mass values may indicate errors in the structural model or loading.
  2. Center of Mass and Center of Rigidity Mismatch: A significant difference between the center of mass and center of rigidity may indicate potential stability issues.
  3. Missing or Incorrect Data: Missing or incorrect data in the mass summary report may indicate errors in the structural model or loading.

Best Practices

To ensure accurate and reliable results from the ETABS mass summary by story, follow these best practices:

  1. Verify the Structural Model: Verify the structural model for accuracy and completeness.
  2. Check Loading: Check loading for accuracy and completeness.
  3. Use Consistent Units: Use consistent units throughout the analysis.
  4. Review Results Carefully: Review results carefully to identify potential issues.

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.

2.2 Mass Source Definition

Engineers must define a Mass Source via Define > Mass Source:

Incorrect mass source is a leading cause of erroneous dynamic results.

Scenario B: "The roof mass is zero."

Cause: You defined mass source from load patterns but forgot to assign any load pattern to the roof diaphragm. Or, you used a rigid diaphragm but the beams are not connected to the diaphragm constraint.

Key Distinctions: Mass vs. Weight

If the mass summary is wrong, the building’s period (T) and modal participation ratios will be wrong.

1. How to Access the Table

Before you can view the mass summary, you must ensure you have defined your mass source correctly (via Define > Mass Source). Once the model is analyzed, follow these steps:

  1. Go to the top menu bar and select Display > Show Tables.
  2. In the dialog box that appears, expand the Analysis Results folder.
  3. Expand the Building Output sub-folder.
  4. Check the box labeled Mass Summary by Story.
  5. Click Apply (or OK).

3. Practical Applications

Why do engineers look at this table? Here are three primary reasons:

Column 2: Unique Name

The object ID of the diaphragm constraint or the story's default diaphragm.