Etabs Mass Summary By Story [SAFE]

Rotational inertia around the Z-axis (important for torsional effects). The cumulative weight of the story ( 3. How ETABS Calculates Mass (Mass Source)

| Issue | Possible Cause | Solution | | :--- | :--- | :--- | | | Mass Source defined incorrectly. | Go to Define > Mass Source . Ensure "Include Lateral Mass only" is not checked if you need gravity mass, or check that loads are included in the definition. | | Mass is Double | Self-weight counted twice. | In Mass Source, check if "Element Self Mass" is checked AND you have also applied a Dead Load case that includes self-weight. Usually, let ETABS calculate self-weight automatically. | | Inconsistent Story Mass | Unmeshed slabs/walls. | Ensure slabs and walls are meshed ( Assign > Shell > Auto Mesh ). Unmeshed areas may not transfer loads correctly to the frame/diaphragm. | | Units Confusion | Mass vs. Weight. | Check your display units. Remember that Mass is force/gravity ($kips \cdot s^2/in$ or $ton$), while Weight is force ($kips$ or $kN$). |

Why do engineers look at this table? Here are three primary reasons:

Maya scrolled to the modal mass participation. The first few modes captured most of the mass in the X and Y directions—good, the primary modes were behaving. But when she examined torsional participation, a small but stubborn 6% torsion appeared in the first mode. That matched the slight eccentricity the mass summary hinted at; the stair core’s offset was real, and under seismic shaking the building would twist. etabs mass summary by story

Here is a detailed guide on how to extract, interpret, and troubleshoot the Mass Summary by Story in ETABS. Why Story Mass Matters in Seismic Design

: Using "Auto Mesh" on walls can sometimes lead to incorrect mass distribution. Manual meshing of shells and walls is often recommended to ensure all elements are properly connected and their masses correctly attributed to the nearest story.

Engineers check this table to ensure the Total Weight matches their manual calculations for base shear. If the total mass is too low, the calculated seismic forces will be underestimated, leading to unsafe design. B. Identifying Torsional Issues | Go to Define > Mass Source

If you see a massive drop in weight from one story to the one below it, check for missing elements or incorrectly assigned material properties in the lower story.

Indicates a transition in building structure (e.g., changing from concrete to steel, or a setback). High Torsional Mass (RZ): High RZcap R cap Z values, even with low

You can view the story mass breakdown after running the structural analysis. Step-by-Step Navigation Analyze your model (). Go to the Model Explorer window on the left. Expand Tables > Analysis Results > Structure Results . Double-click Mass Summary by Story . | In Mass Source, check if "Element Self

: This includes the rotational inertia about the X, Y, and Z axes for each story. It represents the resistance of the mass to changes in its rotation.

. In a seismic event, the ground acceleration acts upon this mass to generate inertial forces

If your dynamic analysis, base shear calculations, or period calculations are wrong, the root cause is almost always found here.