The Center of Mass coordinates; compare these to the Center of Rigidity. How to Improve Your Mass Source Setup
Seismic design codes require you to calculate lateral forces based on individual floor masses and elevations. Reviewing the mass summary by story gives you clean, structured data for your external engineering worksheets. This simplifies your quality assurance checks for items like the Equivalent Lateral Force (ELF) procedure. How ETABS Assembles Story Mass
If your mass summary looks incorrect (e.g., lower than expected):
A "better" mass summary means one that matches manual calculations and aligns with seismic design intent. A. Avoid Double Counting etabs mass summary by story better
I can provide targeted troubleshooting steps for your exact situation. Share public link
A "Better" Mass Summary means matching three things:
ETABS provides several mass-related tables, including Assembled Mass Results and Center of Mass Determinations . However, the table is often the better choice for everyday verification for several reasons: The Center of Mass coordinates; compare these to
| Column | Description | Engineering Significance | |--------|-------------|--------------------------| | | Floor level | Check mass jumps between stories | | UTotal (or UX, UY, UZ) | Translational mass (same in X & Y for symmetric buildings) | Lateral seismic mass: dead + appropriate live load + partition + equipment | | RTotal (or RX, RY, RZ) | Rotational mass moment of inertia about vertical axis | Affects torsional response (accidental eccentricity) | | Self Mass | Mass from element self-weight | Based on assigned materials | | Additional Mass | User-defined mass (shell uniform load, point masses) | Often missed – check carefully |
To get the most accurate summary, you must first define the mass source correctly.
Getting this right is essential, as the mass source definition directly impacts the story shear, base shear, and structural demand calculation ( This simplifies your quality assurance checks for items
Why ETABS "Mass Summary by Story" is Superior for Seismic Design
is not just another table; it is a quality control powerhouse. By condensing thousands of elements into a story-by-story narrative, it gives engineers the bird’s-eye view necessary to ensure the model is accurate, the seismic weight is compliant, and the building's dynamic behavior is predictable. Are you looking to troubleshoot
Now we come to the most practical part of the guide: real-world problems that cause mass summaries to be inaccurate and how to solve them. The information below is derived directly from user forums and official knowledge bases and represents the most common pitfalls.
For the best results, ensure your 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.
Run the structural model to generate the results.