Etabs Mass Summary By Story Better Exclusive

Understanding the mass distribution of a building is the foundation of accurate seismic analysis. In ETABS, the "Mass Summary by Story" table is your primary diagnostic tool for verifying that your mathematical model reflects physical reality.

When you use ETABS for high-rise or complex structures, simply looking at the total mass isn't enough. You need to ensure the distribution aligns with the structural intent. Why Mass Summary by Story Matters

Seismic forces are calculated based on mass. If your story mass is off, your base shear and lateral forces will be incorrect. This table allows you to: Verify the self-weight of slabs, beams, and columns.

Check if superimposed dead loads (SIDL) are applied correctly.

Confirm the inclusion of the appropriate percentage of live loads.

Detect "ghost mass" from accidental double-counting or modeling errors. Accessing the Data To find this specific table after running an analysis: Navigate to the Model Explorer. Expand Tables > Analysis Results > Structure Results. Right-click Mass Summary by Story and select Show Table. Key Columns to Analyze Significance UX / UY The total translational mass in the X and Y directions. Sum UX / Sum UY

Cumulative mass from the top down; used to check total building weight. RX / RY / RZ

Mass moments of inertia; critical for capturing torsional effects. XCM / YCM

The Center of Mass coordinates; compare these to the Center of Rigidity. How to Improve Your Mass Source Setup

For better results, your "Mass Source" definition should follow these industry standards:

Include Lateral Mass Only: Ensure you are checking the "Include Lateral Mass" option for seismic analysis.

Weight Multipliers: Typically, use 1.0 for Dead/SIDL and 0.25 for Live Load (depending on your local building code).

Lumped vs. Distributed: ETABS automatically lumps mass at the story levels. If you have significant mass between floors, consider adding dummy levels or intermediate joints. Troubleshooting Common Discrepancies

If your Mass Summary by Story looks "wrong" compared to your manual hand calculations, check these three areas:

Object Modifiers: Check if you applied property modifiers (like 0.0 weight) to certain elements.

Overlapping Areas: Ensure you haven't modeled a slab over a slab, which doubles the mass in that specific story.

Cladding Loads: Architects often provide cladding as a line load. Ensure these are applied to the perimeter beams and that the mass source is set to include "Specified Load Patterns."

💡 Pro Tip: Always compare the Total Mass in the summary table to the Base Reaction for a dead load case. They should be identical. If they aren't, your mass source is likely missing a specific load pattern.

If you'd like to dive deeper into the technical side, I can help you with: The exact IBC or Eurocode formulas for mass source. How to export this data to Excel for faster reporting.

Interpreting eccentricity between the Center of Mass and Center of Rigidity.

The Mass Summary by Story table in ETABS is often considered "better" or more reliable than other mass tables because it provides a comprehensive accounting of all structural mass assigned to a level, regardless of whether a diaphragm is present. Why "Mass Summary by Story" is Often Preferred

Completeness: Unlike the Mass Summary by Diaphragm table, which only includes mass associated with specific diaphragm assignments, the story summary captures every ounce of mass (columns, walls, beams, and slabs) assigned to that story level.

Total Seismic Weight: It is the standard reference for verifying the total seismic weight of the structure against manual calculations.

Modal Analysis: It helps you verify which parts of the structure (e.g., superstructure vs. substructure) are contributing to specific Modal Mass Participation Ratios. Where to Find It You can access this table by following this path in ETABS: Go to the Display menu. Select Show Tables. etabs mass summary by story better

Navigate to Analysis Results > Structural Results > Other Definitions > Mass Data > 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 in global directions.

Mass Moment of Inertia: The rotational mass about the Z-axis.

Centers of Mass (XCM, YCM): The geometric center where the story mass is concentrated. Common Issues to Watch For Center of rigidity - ETABS - CSI Knowledge Base

To generate a professional Mass Summary by Story report in ETABS that is "better" than a basic data dump, you should focus on verifying the Mass Source and providing clear context for seismic design Draft Report Structure 1. Project Overview & Calculation Basis Mass Source Definition: Explicitly state the load patterns used (e.g., Dead Load + Live Load) Lumped Mass Logic:

Confirm that "Lump Lateral Mass at Story Levels" is enabled to ensure masses are correctly aggregated at each floor for seismic analysis Coordinate System:

Note the global origin used for Center of Mass (CM) and Center of Rigidity (CR) calculations CSI Knowledge Base 2. Story Mass Summary Table Extract this table by navigating to

Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story Cumulative Mass X (Sum to N) Total Mass

Check that the mass values are in the correct units (e.g., kilograms or tonnes); ETABS may require manual conversion (dividing by gravity if using weight units) 3. Torsional Sensitivity (CM vs. CR) For a superior report, include the Centers of Mass and Rigidity table to identify eccentricity Table Path:

Display > Show Tables > Analysis Results > Structure Results > Centers of Mass and Rigidity CSI Knowledge Base

Explain that the distance between CM and CR (eccentricity) directly influences the torsional shear on the building Fawad Najam 4. Quality Control & Verification Mesh Verification:

Mention if manual meshing was used for walls and slabs, as auto-meshing can sometimes lead to lumping errors CSI Knowledge Base Diaphragm Check: Confirm that Rigid Diaphragms

are assigned to all floors to ensure the story mass is reported accurately CSI Knowledge Base

In ETABS, tracking and verifying building mass is a critical step for seismic and dynamic analysis

. Users frequently encounter two different tables to evaluate this: Mass Summary by Story Diaphragm Mass Summary Mass Summary by Story

table is generally the superior and more reliable choice for extracting the total seismic weight of a structure. Direct Comparison: Story Mass vs. Diaphragm Mass Mass Summary by Story Diaphragm Mass Summary Scope of Mass mass assigned to the story (slabs, beams, columns, walls).

Only captures mass associated with nodes assigned to a specific diaphragm. Best Used For

Total seismic base shear verification & manual weight checks.

Evaluating torsional eccentricities (Center of Mass vs. Center of Rigidity). Handling of Off-Diaphragm Mass Includes it flawlessly. Ignores it, leading to underreported building mass. Sensitivity to Meshing High; requires proper node alignment to lump correctly.

High; unmeshed or improperly assigned shells will be ignored. Why "Mass Summary by Story" is Usually Better Mass Summary by Story

table provides a much more holistic view of your structure's mass. The primary reasons it is considered the superior reference for total building weight include:

Mass Summary by Story is not the same as Mass Summary by Diaphragm ETABS 2016

In ETABS, the Mass Summary by Story table is a critical output that aggregates the total mass and weight at each level, forming the basis for seismic and dynamic analysis. Overview of Mass Summary Understanding the mass distribution of a building is

The Mass Summary by Story table provides the cumulative and individual mass for each story based on the Mass Source defined in your model. It typically includes: UX, UY, UZ: Total mass in the X, Y, and Z directions.

Sum UX, Sum UY: Cumulative mass from the top story down to the current level. XCM, YCM: Coordinates of the Center of Mass for each story. How to Access the Table To view the mass summary after running an analysis: Navigate to the Display menu. Select Show Tables.

Expand Analysis Results > Structure Results > Mass Summary > Mass Summary by Story. Improving Mass Accuracy ("Better" Mass)

To ensure your mass summary is "better" (more accurate), verify your Mass Source definition:

Self-Mass: Ensure "Element Self-Mass" is checked if you want ETABS to automatically calculate mass from material density and volumes.

Additional Mass: Use "Additional Mass" to account for non-structural elements like partitions or cladding that aren't modeled as physical objects.

Specified Load Patterns: For seismic mass, it is common practice to include of Dead Load and a percentage (e.g.,

) of Live Load by selecting "Specified Load Patterns" in the Mass Source dialog. Key Applications

Seismic Base Shear: The total weight (W) used to calculate base shear ( ) is derived from this table.

Drift Checks: Code-compliant drift checks must be based on the Center of Mass coordinates provided in this summary.

Soft Story Check: Mass distribution across stories is a factor in identifying structural irregularities like soft stories.

In ETABS, the Mass Summary by Story table is a critical output for verifying your building's seismic weight and ensuring your Mass Source is defined correctly. It provides a lumped mass per level, which is more intuitive for global checks than viewing individual joint or element masses. How to Access the Table

You can find this table after running an analysis by navigating to: Display > Show Tables

Analysis > Results > Structure Results > Mass Summary by Story Why This Table is "Better"

Seismic Weight Verification: It allows for a quick manual comparison against estimated building weights (e.g., Dead Load + % Live Load) to ensure no significant loads were missed.

Identifying Modeling Errors: If the "Total Mass" does not match your base reactions, it often indicates elements meshed between stories or incorrect labeling of points.

Superstructure vs. Substructure: It helps you isolate the mass of specific parts of the building to calculate precise mass participation factors for complex structures. Key Columns & Interpretation Description Story The name of the specific floor level. Mass X / Mass Y

The lumped translational mass in the global X and Y directions. Weight The total vertical weight lumped at that story level. Cumulative Mass The total mass of that story plus all stories above it. Important Tips for Accuracy Mass Source

The structural weight and mass distribution are the core pillars of seismic and dynamic analysis in ETABS. Understanding the Mass Summary by Story table is not just a matter of pulling software data—it is a critical step in verifying that your building will behave exactly as intended during a lateral event.

The following breakdown details how to access this data, why it matters, and how to optimize your modeling to ensure your mass summaries are as accurate as possible. 📥 How to Access the Mass Summary by Story in ETABS

To locate the data after running an analysis, follow this simple path in the software menu: Go to Display on the top menu bar. Select Show Tables. Expand the Analysis Results folder. Expand Structure Results.

Click on Centers of Mass and Rigidity or look for specific mass tables depending on your software version.

This will generate a clear grid displaying your stories, their respective masses in orthogonal directions, and the mass moment of inertia ( MMIcap M cap M cap I 🔍 Why the Mass Summary by Story is So Critical Title: Stop Guessing: How to Actually Use the

The mass of your building is the primary driver of inertial forces during an earthquake. The Mass Summary by Story report serves several high-stakes engineering purposes:

Here’s a professional, engaging post you can use on LinkedIn, a blog, or an engineering forum like Eng-Tips.

Title: Stop Guessing: How to Actually Use the ETABS Mass Summary by Story

Post:

We’ve all been there. You run an ETABS model, check the modal participating mass ratio… and something’s off. The period doesn’t match your hand calcs. The base shear feels wrong.

The culprit? The Mass Summary by Story.

Many engineers just glance at the total mass and move on. But if you want a better, more reliable dynamic analysis, you need to read this table like a detective.

Here’s how to master the ETABS Mass Summary:

1️⃣ Understand what “Mass” ETABS is actually using

2️⃣ Check the distribution, not just the total

3️⃣ Compare UX, UY, and RZ mass values

4️⃣ Use the story mass to validate your center of mass (CM)

Pro tip:
Run Display > Show Mass Summary and toggle “Show as Table”. Then click “Export to Excel.” Plot story mass vs. height. Any anomalies will jump out visually.

Better ETABS mass summary = Better seismic design. Period.

Save this post for your next model check. ✅

#ETABS #StructuralEngineering #SeismicDesign #Dynamics #StructuralAnalysis #CSIsoftware

✅ Step 5: Live Load Reduction for Mass (Do Not)

Engineers often apply live load reduction factors (for gravity design) to the mass source. Stop. For seismic mass, use unreduced live loads multiplied by the code factor (e.g., 0.25). Never apply area reduction to mass.

Effective Use and Interpretation of the ETABS Mass Summary by Story

✅ Step 1 – Check Mass Source Definition

✅ Step 2: Diaphragm Rigidity

If your mass summary shows different masses in U1 vs U2 for a symmetric plan, your diaphragms are likely semi-rigid or not assigned correctly. Better approach: Use Rigid Diaphragm for story mass checks.

Conclusion

The ETABS Mass Summary by Story is an indispensable tool in structural engineering, providing critical insights into a building's mass distribution. By effectively interpreting and utilizing this information, engineers can design safer, more efficient structures that meet or exceed code requirements. Whether you're a seasoned professional or a student, understanding this aspect of ETABS can significantly enhance your structural analysis and design skills.

Method B: Mass from Reactions (The Verifier’s Method)

A foolproof way to cross-check your "better" mass summary is to look at the base reactions from a static Load Case that includes the mass source loads with g = 386.4 in/s².

Run a DEAD load case and a MASS load case.

Compare this to the reported mass summary total. They must match within 1%. If not, your mass source is misapplied.

Part 4: Advanced Techniques for an Even Better Summary

If the above is still not meeting your needs, use these power-user methods.

Part 7: Summary Checklist for a Better ETABS Mass Summary

Before you submit your next seismic design, verify these items:

Educational Renaissance