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Etabs Mass Summary By Story -

To get a clear picture of how your building’s weight is distributed, follow these steps in ETABS. This summary is essential for verifying your seismic mass and ensuring your model’s gravity loads make sense. How to Find the Mass Summary

Run the Analysis: You must have a completed analysis run (the lock icon should be closed). Navigate the Menu: Go to Display > Show Tables. Find the Specific Table: Expand Analysis Results. Expand Structure Results.

Select Center of Mass and Rigidity. (This is the most common way to see story-by-story mass).

Alternatively: Check Model Definition > Structural Data > Mass Summary by Story for the raw input mass before analysis. What to Look For

Mass X / Mass Y: These should generally be equal unless you have strange property modifiers. This represents the total translational mass per floor. etabs mass summary by story

Cumulative Mass: Check the bottom story to see the total mass of the entire building. Compare this to your hand calculations (Area × Dead Load) to ensure you haven't missed any loads.

Center of Mass (XCCM, YCCM): These coordinates tell you where the "weight" of the floor is centered. If these are far from the geometric center, you’ll have high accidental torsion.

Check your Mass Source: Go to Define > Mass Source. Ensure you’ve included 100% of Dead Loads and the required percentage of Live Loads (usually 25% for storage or as per your local code).

Diaphragms: To get a "per story" breakdown in the Analysis tables, you usually need to have diaphragms assigned to your floor shells. If you want to verify the numbers, let me know: The building's total area Your typical dead load (PSF or kN/m²) To get a clear picture of how your

Which building code (ASCE 7, Eurocode, etc.) you're following

A. Story and Output Case

Part 5: Interpreting the Data – A Walkthrough Example

Assume a 5-story steel frame in a high-seismic zone.

| Story | U1 Mass (kN-s²/m) | U2 Mass (kN-s²/m) | R3 Mass (kN-s²/m-rad) | | :--- | :--- | :--- | :--- | | Roof | 85 | 85 | 12,500 | | Story4 | 110 | 110 | 18,200 | | Story3 | 110 | 110 | 18,200 | | Story2 | 110 | 110 | 18,200 | | Story1 | 115 | 115 | 19,000 | | Base | 0 | 0 | 0 |

What does this tell you?

  1. Uniform distribution: U1 and U2 are nearly equal (good for symmetrical building).
  2. Reduced roof mass: Roof has no imposed live load and lighter finishes. This is correct.
  3. Higher first story: Story1 has slightly more mass due to shorter columns and heavier spandrel beams. Realistic.
  4. Non-zero base: The base should be ZERO. If the base has mass, you forgot to assign supports as fixed or you modeled soil springs incorrectly. This would cause unrealistic base sliding.

4.2 Base Shear Verification ($V$)

Code base shear ($V = C_s \cdot W$) uses this $W$. If the Mass Summary shows incorrect $W$, the design base shear will be wrong, leading to unsafe or over-conservative designs.

Conclusion

The ETABS Mass Summary by Story is not just a table—it is the DNA of your structural model. Garbage mass leads to garbage periods, garbage base shear, and ultimately, an unsafe design.

Take five minutes to review this summary before every analysis. It will save you weeks of debugging hidden errors and ensure your building stands strong when the ground shakes.

By understanding where your mass comes from and how ETABS organizes it by story, you move from being a software operator to a true structural analyst. Story: The name of the specific level (e

You can use this as a template or a direct submission for a course or technical report.

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