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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.
Mastering the ETABS Mass Summary by Story: How to Get Better, Safer Results
If you have ever run a seismic analysis in ETABS, you have undoubtedly stared at the Mass Summary by Story table. It appears deceptively simple: a list of stories, masses in X, Y, Z, and a summation. Yet, for many engineers, this table is a source of quiet anxiety.
Is the mass correct? Did the software double-count the dead load? Why is the seismic weight different from the gravity takeoff? And most importantly—how can you get a better, more reliable Mass Summary? etabs mass summary by story better
In this article, we will move beyond the default settings. We will explore how to extract, validate, and optimize the ETABS Mass Summary by Story to ensure your dynamic analysis (Response Spectrum, Time History) and modal results are physically accurate.
✅ Step 1 – Check Mass Source Definition
- Go to
Define > Mass Source. - Recommended for seismic design:
Dead Load (1.0) + Specified Live Load (e.g., 0.25 or 0.5) + Partition Load (if any) - Avoid including “Additional Self Mass” twice.
What is "Mass Summary by Story"?
Mass summary by story aggregates the tributary mass (including all modeled masses from dead load, live load participation, and assigned masses) for each floor level. It’s essential for seismic base shear calculations, modal mass participation checks, and dynamic analysis validation.
✅ 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. Understanding ETABS Mass Summary by Story: A Comprehensive
5. Sample Summary Table (Better Format)
| 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 |
Introduction
A concise guide on generating and interpreting mass summaries by story in ETABS, useful for structural engineers performing seismic and dynamic analysis.
1. The Golden Rule: Mass Source vs. Load Patterns
The most common mistake is neglecting how ETABS calculates mass. By default, ETABS does not automatically know what to weigh. You must define a Mass Source (Define > Mass Source). Go to Define > Mass Source
A better approach involves two distinct definitions:
- Mass Source for Modal Analysis: This should include
1.0 Dead Load + Specified Live Load (e.g., 0.25 for floors, 0.50 for roofs). Do not include self-weight manually if it is already part of the DL. - Mass Source for Seismic Weight (per ASCE 7): This is used for base shear calculations, not modal analysis.
Better Practice: Create a Mass Source that references load patterns (DEAD, SUPER_DEAD, LIVE_REDUCED). Never use the "From Self & Specified Mass" option unless you fully understand the lumped mass elements you've added. A clean mass source leads to a clean summary.
