Etabs Mass Summary By Story May 2026

Understanding the ETABS Mass Summary by Story: A Guide for Structural Engineers

In structural analysis, verifying the mass of your building is just as critical as verifying its stiffness. An incorrect mass distribution leads to incorrect period calculations, base shears, and seismic forces.

ETABS provides a powerful output table known as the "Mass Summary by Story," which allows engineers to audit the gravity load distribution throughout the model. This article explains where to find this table, how to interpret the columns, and how to troubleshoot common discrepancies.

7. Conclusion

The ETABS Mass Summary by Story is not merely an output table—it is a diagnostic tool. Before interpreting any period, mode shape, or base shear, engineers must:

Verify the mass source definition.
Confirm that each story’s translational mass is positive and reasonable.
Ensure total seismic weight matches code-based hand calculations.
Check that mass moment of inertia is present for torsional response.

Failure to audit the mass summary is a common source of hidden errors in performance-based design.

Final Checklist for the Practicing Engineer

Before you finalize your ETABS model for seismic design:

[ ] Defined Mass Source (not default).
[ ] Summed total weight matches hand calculation within 2%.
[ ] No negative masses (indicates unstable geometry).
[ ] Diaphragms assigned to generate rotational mass.
[ ] Mass exists only at stories (not floating in mid-air).

Breaking Down the Table Columns

A typical Mass Summary by Story table includes the following columns:

Story: The label of the floor (e.g., Base, Story1, Story2, Roof).
UDL (Uniform Dead Load): Mass from self-weight of structural and non-structural components.
SDL (Superimposed Dead Load): Mass from finishes, partitions, MEP, etc.
Total Dead Load: Sum of UDL and SDL.
Live Load (Mass Source Factor): ETABS allows a fraction of live load (e.g., 25% or 50%) to contribute to seismic mass. This column shows that reduced value.
Total Mass (Force/Acceleration units): Typically in kN*s²/m or kip*s²/in (slugs).
Total Weight (Force units): Typically in kN or kip (equals Mass × Gravity).
Percentage of Total: How much each story contributes to the building's total mass.

Common Pitfalls and Fixes

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:

[ ] Defined Mass Source per code (DL + LL*reducer).
[ ] No mass at the base level.
[ ] U1 and U2 masses match expected distribution.
[ ] Rotational mass (R3) is realistic for floor plan.
[ ] Total mass * g equals hand-calculated weight.

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.

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 etabs mass summary by story

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 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).

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 Understanding the ETABS Mass Summary by Story: A

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:

Mass: The total mass of the structure, including the mass of beams, columns, slabs, walls, and any other assigned masses.
Mass per Unit Area: The mass per unit area of each story, which can be useful for checking the mass distribution.
Center of Mass: The location of the center of mass for each story, which is essential for understanding the building's dynamic behavior.

Why is Mass Summary by Story useful?

This feature is useful for:

Verifying mass distribution: Engineers can verify that the mass is distributed reasonably and uniformly across the building, which is essential for achieving good seismic performance.
Identifying irregularities: By reviewing the mass summary, engineers can identify any irregularities in the mass distribution, which may require special attention during design.
Dynamic analysis: The mass summary is used as input for dynamic analysis, such as modal analysis, response spectrum analysis, or time-history analysis.

How to access Mass Summary by Story in ETABS?

To access this feature in ETABS, follow these steps:

Open your ETABS model.
Go to the "Analyze" menu.
Select "Mass Summary" from the drop-down menu.
In the "Mass Summary" dialog box, select the "By Story" tab.

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?

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.

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 Verify the mass source definition

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

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

You can use this as a template or a direct submission for a course or technical report.

Error 4: Negative Mass (Rare but possible)

Symptom: Negative values in mass summary. Problem: Load combinations that subtract loads (e.g., 0.9DL + 1.0WL) are incorrectly assigned to the mass source, or you have overturning with tension elements. Fix: Never use load combinations in the Mass Source definition. Use only primary load patterns.

Decoding the ETABS Mass Summary by Story: The Foundation of Seismic Design

In the realm of structural engineering, few outputs are as deceptively simple yet critically important as the ETABS Mass Summary by Story. While novice users might scroll past this table on their way to flashy deflection diagrams or rebar contours, experienced engineers know that this summary is the first checkpoint for structural sanity.

If your dynamic analysis, base shear calculations, or period calculations are wrong, the root cause is almost always found here.