Etabs Mass Summary By Story

To get a clear picture of how your building’s weight is distributed, follow these steps in ETABS. This summary is essential for verifying your seismic mass and ensuring your model’s gravity loads make sense. How to Find the Mass Summary

Run the Analysis: You must have a completed analysis run (the lock icon should be closed). Navigate the Menu: Go to Display > Show Tables. Find the Specific Table: Expand Analysis Results. Expand Structure Results.

Select Center of Mass and Rigidity. (This is the most common way to see story-by-story mass).

Alternatively: Check Model Definition > Structural Data > Mass Summary by Story for the raw input mass before analysis. What to Look For

Mass X / Mass Y: These should generally be equal unless you have strange property modifiers. This represents the total translational mass per floor.

Cumulative Mass: Check the bottom story to see the total mass of the entire building. Compare this to your hand calculations (Area × Dead Load) to ensure you haven't missed any loads.

Center of Mass (XCCM, YCCM): These coordinates tell you where the "weight" of the floor is centered. If these are far from the geometric center, you’ll have high accidental torsion.

Check your Mass Source: Go to Define > Mass Source. Ensure you’ve included 100% of Dead Loads and the required percentage of Live Loads (usually 25% for storage or as per your local code).

Diaphragms: To get a "per story" breakdown in the Analysis tables, you usually need to have diaphragms assigned to your floor shells. If you want to verify the numbers, let me know: The building's total area Your typical dead load (PSF or kN/m²)

Which building code (ASCE 7, Eurocode, etc.) you're following

Mass Summary by Story in ETABS provides a high-level overview of a structure's distribution of mass, which is critical for seismic and dynamic analysis. To extract this, navigate to

Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story

Below is a drafted report structure for presenting this data. Structural Mass Summary Report 1. Project Overview ETABS 2026 Model Status: Analysis Completed Mass Source: Defined as [e.g., Self Mass + 25% Live Load] 2. Mass Source Definition

The mass used for dynamic analysis was derived from the following criteria: Element Self Mass: Additional Mass: [e.g., Cladding or Floor Finishes] Lumped Mass:

Lateral mass lumped at story levels for accurate seismic response. 3. Mass Summary Table etabs mass summary by story

The following table summarizes the mass distributed at each floor level. Mass X (kg) Mass Y (kg) Cum. Mass X (kg) Cum. Mass Y (kg) 4. Observations & Verification Mass Summary for ETABS Stories | PDF - Scribd

Column 2: Unique Name

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

2. Modal Analysis Check

The distribution of mass across stories directly dictates mode shapes and periods. A top-heavy model will produce longer periods and higher overturning moments. By reviewing the summary, you can quickly spot if a penthouse or water tank mass was accidentally omitted.

Conclusion

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.

The Mass Summary by Story in ETABS is a standard output table that aggregates the distributed and point masses assigned to a building model at each floor level. It serves as a primary verification tool for structural engineers to ensure that the total seismic weight used for base shear calculations is accurate and adheres to building codes. 1. Accessing the Table

To view the report after running an analysis, navigate to:Display > Show Tables > Analysis > Results > Structure Results > Mass Summary by Story. 2. Core Components and Columns

The table typically displays the following values for each defined story: Story: The specific level of the building being reported.

Mass X and Mass Y: The translational mass along the global X and Y axes. These values are used to calculate the inertial forces during seismic events.

Mass Z: The vertical mass component. While often excluded for standard lateral analysis, it is necessary for vertical seismic effects or special design requirements.

Mass Moment of Inertia (MMI): The rotational mass about the global Z-axis. This value is critical for determining the torsional response of the building. 3. The Role of the Mass Source

The values in the summary table are derived directly from the Mass Source definition (Define > Mass Source). ETABS calculates mass based on three primary settings: To get a clear picture of how your

Element Self Mass: Calculates mass based on the volume and material density of beams, columns, walls, and slabs.

Additional Mass: Includes user-assigned point, line, or area masses for non-structural elements like cladding or partitions.

Specified Load Patterns: Converts assigned loads (e.g., Dead Load + a percentage of Live Load) into mass by dividing the weight by the gravitational multiplier ( ). For example, codes like IS 1893:2016 often require Dead Load plus Live Load depending on load intensity. 4. Mass Lumping Mechanics

ETABS generally "lumps" masses at the floor levels to simplify the analysis into a Multiple Degree of Freedom (MDOF) system. Mass - CSI Knowledge Base

The ETABS Mass Summary by Story is a crucial output used to verify the seismic weight and inertia of a structure. It provides a detailed breakdown of the mass and weight assigned to each level, which is essential for calculating base shear and checking for irregularities. Key Components of the Mass Summary Report

The standard report typically includes the following columns and data points:

Story Name: Identifies each floor level defined in the model.

Mass X and Mass Y: The translational mass assigned to each story in the global X and Y directions, respectively.

Mass Moment of Inertia (MMI): The rotational mass calculated about the center of mass, used for dynamic and seismic analysis.

Cumulative Mass: The total mass from the top of the building down to the current story. Story Weight: The vertical weight (

) calculated for each floor, often verified against manual calculations. Proper Content for Documentation

When presenting a "Mass Summary by Story" in a structural report, you should include:

Mass Source Definition: Explicitly state how the mass was derived (e.g., Element Self Mass, Additional Mass, or Specified Load Patterns like

Summary Table: The direct export from the Display > Show Tables > Structure Output > Other Output Items > Mass Summary by Story. Column 2: Unique Name The object ID of

Center of Mass (CM) and Center of Rigidity (CR): Documentation of these coordinates to demonstrate any inherent torsion or eccentricity in the building.

Verification: A brief comparison between ETABS results and a manual "check" of the total building weight to ensure no loads were missed or double-counted. How ETABS Lumps Mass

Mass is typically calculated based on material density and element volume. It is then "lumped" at story levels based on: Mass Summary for ETABS Stories | PDF - Scribd

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