Here are a few options for a positive review of the ETABS Mass Summary by Story feature or report, ranging from professional to more enthusiastic. Professional & Precise
"The ETABS Mass Summary by Story provides an exceptionally clear and detailed breakdown of building mass at every level. It has become an essential part of our seismic weight verification process, making it easy to cross-check load pattern multipliers and ensure code compliance for lateral load analysis. The ability to see exactly how mass is lumped at each story level significantly simplifies our quality control workflows." Efficiency-Focused ETABS Mass Summary by Story Data | PDF | Physics - Scribd
Unlocking Precision: Why "Mass Summary by Story" is Better for ETABS Analysis
As a structural engineer, you know that the soul of a seismic analysis lies in how you define your building's mass. While ETABS offers various ways to view your model's weight, the Mass Summary by Story report is often the gold standard for high-accuracy design.
Here is why this specific report is better for your workflow and how to master it. 1. Superior Accuracy in Seismic Weight Verification
The primary benefit of the Story Mass Summary is its role in "Mass calculated manual" verification. By breaking down the weight floor-by-floor, you can easily compare ETABS’ output against manual calculations for:
Slab Weight: Effective area (total minus openings) times thickness and unit weight.
Vertical Components: The summary accounts for half the mass of columns/walls above and below the floor, ensuring vertical continuity.
Adjusted Top Stories: It correctly identifies that the top story has no vertical components above it, preventing the overestimation of roof mass. 2. Streamlined Diaphragm and Lateral Load Analysis
The Story Mass Summary is essentially the "heartbeat" of your lateral force distribution.
Lumped Mass Efficiency: ETABS can lump the mass of all nodes on a story to a "master node" at the center of mass. This simplifies the mass matrix and significantly reduces analysis time for high-rise buildings without sacrificing accuracy.
Drift Control: Since story drift is computed at the center of mass, having a clean, story-by-story mass breakdown is critical for verifying that your building stays within the strict 0.020h0.020 h 0.025h0.025 h seismic limits. 3. Better Troubleshooting of Mass Discrepancies
Have you ever noticed your base reactions don't match your mass? The Story Summary helps you pinpoint the culprit: etabs mass summary by story better
Element Meshing: It reveals if elements meshed between stories are lumping mass to the wrong level.
Null Areas: It helps identify if Superimposed Dead Loads (SDL) on null areas are failing to transfer to the seismic mass because a diaphragm wasn't assigned. How to Access the Report To get this better view of your structure’s data: Navigate to the Display menu. Select Show Tables. Go to Output > Other Output Items > Mass Summary by Story. Pro-Tip: The "Mass Source" Secret
For the best results, ensure your Mass Source is defined via Define > Mass Source. To avoid double-counting, don't check "Element Self Mass" if you are also including a Dead Load pattern with a 1.0 multiplier in your load list.
How are you currently verifying your building's seismic weight in ETABS? Let’s discuss your favorite verification techniques in the comments!
Explore the CSI Knowledge Base for detailed FAQs on mass application. Mass Source
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 Here are a few options for a positive
Let’s compare a 6-story concrete building example.
Seismic Design: For seismic design, understanding the mass distribution is key. The mass of a structure directly influences its seismic response. A well-distributed mass can lead to a more stable structure under earthquake loads.
Torsional Effects: The center of mass and the moments of inertia help engineers assess the torsional effects on the building. Torsion can significantly impact the structural response, especially in irregular buildings.
Structural Stability: By analyzing the mass distribution, engineers can identify potential stability issues early in the design process, allowing for timely modifications.
Load Calculation: Accurate mass calculation is essential for determining the loads on the structure, including gravity loads, seismic loads, and wind loads.
| Story | UTotal (ton-s²/m) | RTotal (ton-s²-m) | Verification Check | |-------|------------------|-------------------|--------------------| | Roof | 125.4 | 8,420 | ✅ Cladding + roof live 20% | | 5 | 210.2 | 12,150 | ✅ Partition added | | 4 | 210.5 | 12,140 | ✅ Uniform | | 3 | 210.3 | 12,160 | ✅ Uniform | | 2 | 211.0 | 12,170 | ✅ Slight increase from stair infill | | Base | 0.0 | 0.0 | ✅ No mass at fixity |
Analysis > Results > Structure Output > Mass Summary by Story.Seismic Weight (kN) = Mass (ton) * 9.81.A typical story summary provides U1, U2, U3 (translational mass) and R1, R2, R3 (rotational mass moment of inertia). Better interpretation requires you to ask specific questions:
U3 (Vertical Mass): This is the most reliable number. It should equal the total dead load plus applicable live load on that floor. Check: Does the mass at the roof match the cladding, roofing, and mechanical units? A discrepancy here means your diaphragm constraints or area loads are incorrectly assigned.
U1 & U2 (Lateral Mass): In a rigid diaphragm, these should be nearly identical to U3. If they are significantly less, you have likely assigned a semi-rigid diaphragm without meshing your slabs or you have gaps in your lateral load-resisting system. If they are more, you may have duplicate mass assignments (e.g., self-weight counted twice).
R3 (Mass Moment of Inertia about Vertical Axis): This is the most overlooked and most critical value for torsional irregularities. A better engineer checks whether R3 is physically plausible. For a rectangular floor, $R3 \approx \fracMass12(a^2 + b^2)$. If your ETABS R3 is off by an order of magnitude, your torsional response (and consequently, accidental torsion) will be wildly inaccurate.
The ETABS Mass Summary by Story is not just a diagnostic tool—it is the foundation of your seismic safety. To make it better, you must:
The next time you run ETABS, spend 15 minutes on the Mass Summary. Compare it, question it, and improve it. Your future self—and the building's occupants—will thank you. Part 6: Comparison – Default vs
About the Author / Further Resources: For a deep dive, download the official CSI Analysis Reference Manual (Chapter 3: Mass and Weight). Or, check your local building code (ASCE 7, IBC, Eurocode 8, IS 1893) for specific mass combination factors—they vary, but the principles above remain universal.
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
Understanding ETABS Mass Summary by Story: A Comprehensive Guide
When working with ETABS, a powerful software for structural analysis and design, understanding the mass distribution of a building is crucial for ensuring its stability and performance under various loads, including seismic and wind loads. The Mass Summary by Story report in ETABS provides engineers with essential data to assess how mass is distributed across different stories of a building. In this post, we'll delve into the significance of the ETABS Mass Summary by Story, how to interpret it, and why it's vital for structural engineering projects.
You’ve built a better ETABS mass summary by story. Now, what does it tell you?
| Observation | Diagnosis | Fix | |-------------|-----------|-----| | Mass on roof > floor below | Penthouse mass merged into roof | Split story definition | | Mass at basement is huge | Soil interaction mass included | Check mass source: exclude soil | | Mass not integer (weird decimals) | Shell meshing causing partial masses | Remesh with regular grid | | Lateral stiffness unreasonably low | Missing diaphragm constraint | Assign Rigid Diaphragm | | Mass changes between Ultimate & Service | Different load multipliers | Set both to seismic combo |