Etabs Mass Summary By - Story Better
ETABS displays mass in ton (metric) or kip-s²/in (imperial). Do not confuse mass (force/g) with weight (force). Seismic weight in kips is Mass (kip-s²/in) * 386.4 in/s².
Even with a perfect mass source, you may see strange results. Here is how to get better by debugging.
| Symptom | Likely Cause | Fix |
| :--- | :--- | :--- |
| Mass at roof is too high | Penthouses, water tanks not modeled as separate stories | Assign point mass or model as a dummy story |
| Mass is negative or zero | Invalid constraint or bi-axial hinge | Check Display > Show Undeformed > Joints for errors |
| Cumulative mass changes after analysis | Mass source relies on load-dependent deflection (e.g., nonlinear) | Use linear static for mass source |
| Mass differs between ETABS and Excel takeoff | Line loads, area loads not converted to mass | Convert all Uniform Loads to Mass via Assign > Area Loads > To Mass |
The ETABS Mass Summary by Story is not just a diagnostic tool—it is the foundation of your seismic safety. To make it better, you must:
The next time you run ETABS, spend 15 minutes on the Mass Summary. Compare it, question it, and improve it. Your future self—and the building's occupants—will thank you.
About the Author / Further Resources: For a deep dive, download the official CSI Analysis Reference Manual (Chapter 3: Mass and Weight). Or, check your local building code (ASCE 7, IBC, Eurocode 8, IS 1893) for specific mass combination factors—they vary, but the principles above remain universal. etabs mass summary by story better
In ETABS, the Mass Summary by Story is a critical diagnostic table used to verify the seismic weight and mass distribution of a structure. This data is essential for calculating the base shear during seismic analysis. Key Components of Story Mass
ETABS calculates story mass based on your defined Mass Source.
Element Self-Mass: Automatically calculated from the volume and material density of structural members.
Additional Mass: Manual assignments for non-structural elements like cladding or partitions.
Specified Load Patterns: Mass derived from weight (typically Dead Load + a percentage of Live Load). How ETABS Lumps Mass ETABS displays mass in ton (metric) or kip-s²/in
The software follows a specific logic to distribute mass at each level:
Tributary Distribution: ETABS "lumps" mass at the center of each floor by taking half the weight of the story below and half from the story above.
Master Node: Translational and rotational masses are concentrated at a master node, usually at the center of mass, to capture dynamic effects.
Mesh Sensitivity: Improper auto-meshing of walls or slabs can lead to incorrect mass calculations. Manual meshing is often recommended for higher accuracy. Accessing and Exporting the Report To generate and review this data for your project report: Additional Mass Frame Objects
As you build upward, the people get lighter. The next time you run ETABS, spend 15
For response spectrum or time history analysis, the Mass Summary is your truth serum. Compute the participating mass ratio from the modal analysis. The Mass Summary gives you the total mass of the building. The modal output gives you the effective modal mass.
Better Practice: Ensure that the sum of the effective modal masses in the X and Y directions equals at least 90% of the total mass listed in your Story Mass Summary. If they don’t match, your structure is either too flexible (need more modes) or your constraints (rigid diaphragms) are preventing proper mass participation.
Finally, a practical hallmark of a better engineer: unit discipline. The Mass Summary typically reports in kg-s^2/m (metric) or kip-s^2/in (imperial). This is mass, not weight. To get weight (force) for base shear:
$$Weight = Mass \times g$$
Before issuing a report, convert these summary values into kips or kN for the client or reviewer. Presenting mass in force units prevents confusion and demonstrates technical maturity.
