Etabs Mass Summary By Story May 2026

In structural engineering, the Mass Summary by Story in ETABS is the definitive record of your building's weight. It is essential for calculating seismic forces and verifying that your model reflects the actual design. 🏗️ Why Mass Summary Matters Seismic Design: Directly influences the Base Shear (

P-Delta Effects: Determines the stability and secondary moments of the frame.

Error Checking: Identifies accidental "double counting" or missing loads.

Dynamic Analysis: Impacts the natural periods and mode shapes. 📍 Where to Find It

To access the report, navigate to:Analysis Results > Structure Results > Mass Summary > Mass Summary by Story 📊 Understanding the Data Columns Story: The specific floor level. Mass X / Mass Y: Total mass in horizontal directions.

Mass Center (X, Y): The geographic "balance point" of the floor's weight. etabs mass summary by story

Cumulative Mass: The total weight from the roof down to that level. 💡 Pro-Tips for Accuracy

Check the Mass Source: Ensure "Self-Weight" and "Specified Load Patterns" are correctly toggled.

Lateral Mass vs. Vertical Mass: ETABS usually focuses on horizontal inertia for seismic runs.

Compare to Hand Calcs: Always do a "back-of-the-envelope" check on a typical floor.

Diaphragms: Ensure all slabs are constrained to a diaphragm to get a clean story-by-story readout. In structural engineering, the Mass Summary by Story

⭐ Key Takeaway: The Mass Summary is your model's "scale." If the numbers look off, your seismic results will be too. If you'd like to refine this for a specific audience: Tell me your target reader (student vs. senior engineer) Specify if you want a step-by-step tutorial Provide a preferred word count

Part 9: Best Practices for Defining Mass Source

To ensure your Mass Summary by Story is flawless, follow these rules:

  1. Never include transient loads at 100%. Use code-reduced live loads (0.25 for residential, 0.5 for storage).
  2. Separate loads by type. Do not combine dead and live in one pattern.
  3. Cladding/Curtain walls: Add these as line loads on perimeter beams, then include that load pattern in the mass source.
  4. Water tanks/equipment: Model as point masses (Assign > Mass), not just loads. They will appear correctly in U1/U2.
  5. Run a zero-analysis first. Before full analysis, run Analyze > Run Static Analysis Only. Then check the Mass Summary. It requires no dynamic solution and runs instantly.

6. Case Study: 5-Story Steel Frame

Objective: Verify that the total seismic mass matches the design calculation.

Input Data:

  • Dead Load (DL) = 500 kN per floor (including self-weight)
  • Live Load (LL) = 200 kN per floor (reducible)
  • Mass source: 100% DL + 50% LL = $500 + 0.5 \times 200 = 600 kN$ per floor (weight).
  • Mass per floor = $600 / 9.81 = 61.16 \text N·s²/m$.

ETABS Mass Summary Output:

  • Story 1 to 5: U1 Mass = 61.2 each.
  • Total mass = $5 \times 61.2 = 306.0 \text N·s²/m$.
  • Total seismic weight = $306.0 \times 9.81 = 3002 \text kN$.

Check: Expected $5 \times 600 = 3000 \text kN$. Error = 0.07% → Acceptable.

Conclusion: Mass summary matches hand calculation. Modal analysis can proceed.

Final Checklist for the Practicing Engineer

Before you finalize your ETABS model for seismic design:

  • [ ] Defined Mass Source (not default).
  • [ ] Summed total weight matches hand calculation within 2%.
  • [ ] No negative masses (indicates unstable geometry).
  • [ ] Diaphragms assigned to generate rotational mass.
  • [ ] Mass exists only at stories (not floating in mid-air).

5. Common Errors and Troubleshooting

| Observed Problem | Likely Cause | Solution | |----------------|--------------|----------| | Zero mass on a story | No assigned area/shell objects or missing mass source | Check Assign > Area Loads or revise mass source definition | | Asymmetric U1 vs U2 mass in symmetric building | Incorrect diaphragm assignment or accidental eccentricity | Verify rigid diaphragm assignments | | Unrealistically high mass | Duplicate loads or self-weight included twice | Review load patterns and mass source (exclude self-weight if already in dead load) | | Mass moment of inertia = 0 | No diaphragm or mass assigned | Assign a rigid diaphragm or specify rotational mass |


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