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?
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 etabs mass summary by story
. 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?
Understanding Mass Summary by Story in ETABS In structural engineering, the Mass Summary by Story is a critical output in
used to verify a building's seismic weight and dynamic characteristics. It provides a tabulated breakdown of the total mass attributed to each floor level, which is essential for calculating seismic base shear and ensuring the accuracy of the structural model. 1. Definition and Source of Mass The values in the story mass summary are derived from the Mass Source defined by the user. Element Self-Mass
: Automatically calculated from material density and member volume. Additional Mass
: User-assigned point, line, or area masses to account for non-structural components like cladding or partitions. Specified Load Patterns : Often used for seismic calculations (e.g., Dead Load + Live Load) as per codes like 2. The Lumping Process
ETABS typically "lumps" mass at the story levels to simplify analysis. To obtain a Mass Summary by Story in
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
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
You can use this as a template or a direct submission for a course or technical report.
At its core, the Mass Summary by Story is a tabulated report that calculates and displays the total mass and weight (W) for each floor level of your model. ETABS automatically computes these values based on the assigned dead loads, live loads, superimposed dead loads, and mass source definitions.
Unlike simple gravity takeaway, this summary specifically focuses on mass (force/acceleration), which is the primary driver of inertial forces during an earthquake.
The ETABS Mass Summary by Story is not just a table—it is the DNA of your structural model. Garbage mass leads to garbage periods, garbage base shear, and ultimately, an unsafe design.
Take five minutes to review this summary before every analysis. It will save you weeks of debugging hidden errors and ensure your building stands strong when the ground shakes.
By understanding where your mass comes from and how ETABS organizes it by story, you move from being a software operator to a true structural analyst.
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 ( Method 1: From the Display Menu (Visual)
). 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
These columns represent the summation of forces acting on the individual diaphragms.
Assign > Joint/Frame/Shell Mass.Occasionally, loads are applied to elements that are not properly connected to the diaphragm (e.g., a beam modeled without a support connection or a slab not meshed with walls).
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).
Perhaps the most critical column in the Mass Summary table is the Center of Mass (CM) location relative to the Center of Rigidity (CR). While the CR is often calculated separately, the Mass Summary provides the precise location of the resultant inertial force for each floor.
The offset between the CM and CR creates the eccentricity that drives torsional forces during an earthquake. A deep analysis of the Mass Summary tracks the trajectory of the CM coordinates ($X_m, Y_m$) up the building height.