Mass summary by story is a quick diagnostic and reporting tool that ensures your dynamic and seismic analyses are based on correct mass distribution. Regularly check this table during modeling and before final design checks.
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| Story | Mass DL | Mass 0.25LL | Total Mass | Seismic Weight (W) | Cumulative W | |-------|---------|-------------|------------|--------------------|---------------| | Roof | 850 | 50 | 900 | 8820 kN | 8820 kN | | PH | 300 | 10 | 310 | 3038 kN | 11858 kN | | 5 | 890 | 60 | 950 | 9310 kN | 21168 kN |
Notice: The roof mass dropped to realistic levels because the penthouse was separated. Cumulative seismic weight is now correct for story shear calculation.
For response spectrum or time history analysis, the Mass Summary is your truth serum. Compute the participating mass ratio from the modal analysis. The Mass Summary gives you the total mass of the building. The modal output gives you the effective modal mass.
Better Practice: Ensure that the sum of the effective modal masses in the X and Y directions equals at least 90% of the total mass listed in your Story Mass Summary. If they don’t match, your structure is either too flexible (need more modes) or your constraints (rigid diaphragms) are preventing proper mass participation.
The ETABS Mass Summary by Story is not a passive output; it is a sanity check, a diagnostic radiograph, and a dynamic Rosetta stone. To use it better:
Master the Mass Summary, and you will no longer wonder why your building twists unexpectedly or why your base shear seems low. You will know.
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. etabs mass summary by story better
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:
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
To create a professional and comprehensive ETABS story mass summary report, you should organize your findings into a clear structure that includes modeling assumptions, primary data tables, and verification checks. 1. Report Structure & Components
Project Overview: Briefly describe the structure, including the number of stories, heights, and general building type.
Mass Source Definition: Explicitly state how the mass was derived (e.g., element self-mass plus specified load patterns like 100% Dead Load and a percentage of Live Load). Mass summary by story is a quick diagnostic
Assumptions: Mention if masses were "Lumped at Story Levels" to ensure they are properly associated with floor diaphragms. 2. Primary Data Table: Story Mass Summary
The core of your report should be a table extracted from ETABS under Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story. Mass X (kg/kN) Mass Y (kg/kN) Cumulative Mass X Cumulative Mass Y
Note: If your model uses diaphragms, also include the Center of Mass and Rigidity table to show eccentricity. 3. Verification & Quality Checks
A "better" report includes validation to ensure the model is accurate:
Base Reaction vs. Total Mass: Compare the total vertical base reaction ( Fzcap F sub z
) under dead load cases to the total cumulative mass. These should align closely; a significant discrepancy often indicates unassigned masses.
Participating Mass Ratio: Include a check for Participating Mass Ratios (found under Analysis Results > Modal Results). Ensure that at least 90% of the mass is participating in the principal directions for seismic analysis. 4. How to Generate in ETABS
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. | Story | Mass DL | Mass 0
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
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
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:
Before we can make it better, we must understand what the software is doing under the hood.
When you run Analyze > Run Analysis > Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story, ETABS reports:
| 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 |