Effective Use and Interpretation of the ETABS Mass Summary by Story

1. Purpose

The ETABS Mass Summary by Story provides the lateral seismic mass (typically in ton-sec²/mm or kip-sec²/in) used for modal analysis (eigenvalue extraction) and equivalent lateral force (ELF) procedures. Understanding it correctly prevents underpredicting seismic demands.

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.

Have a specific mass summary bug? Describe your model in the comments.

How are you currently verifying your building's seismic weight in ETABS? Let’s discuss your favorite verification techniques in the comments!

Method C: Python Scripting via ETABS API

For large projects, manual checking is error-prone. Use the ETABS API (C# or Python) to extract:

  1. Seismic design: The report helps engineers understand the distribution of mass in the building, which is essential for seismic design. By analyzing the mass summary, engineers can identify potential issues with the building's seismic resistance.
  2. Wind design: The report provides valuable information for wind design, as the mass of the building affects its wind resistance.
  3. Structural optimization: By analyzing the mass summary, engineers can identify opportunities to optimize the structural system, reducing the overall mass of the building while maintaining its stability.

4. Common Red Flags (What “Better” Avoids)

| Red Flag | Fix | |----------|-----| | Mass at base (Foundation) | Exclude from lateral mass unless basement walls provide restraint | | Sudden drop in mass at top story | Missing penthouse/cladding mass | | RTotal zero for all stories | Assign rigid diaphragm – mass moment computed from floor plan | | Identical mass for Ux and Uy but building is asymmetric | Add point masses to reflect actual distribution |

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.