Mass Summary By Story — Etabs

The "Mass Summary by Story" in ETABS is a critical output table used to verify the mass distribution of a structural model, ensuring that seismic and dynamic analyses are based on accurate data. Overview of Mass Summary by Story

This table provides a condensed look at the total mass assigned to each story level. It is typically found under Analysis Results > Structure Results > Mass Summary > Mass Summary by Story. It allows engineers to:

Verify Mass Sources: Ensure that the Mass Source (e.g., self-weight, specified dead loads, and a percentage of live loads) is correctly calculated by the software.

Check Mass Distribution: Confirm that the mass decreases or changes logically from the roof down to the base.

Identify Modeling Errors: Detect anomalies, such as a story having zero mass or an unusually high mass, which often indicates missing load assignments or incorrect material properties. Key Data Fields etabs mass summary by story

The table generally includes the following columns for each story:

UX, UY, UZ: The translational mass along the X, Y, and Z axes. In most lateral analyses, the vertical mass (UZ) may be ignored unless vertical seismic effects are being considered.

RX, RY, RZ: The mass moments of inertia about the X, Y, and Z axes. RZ (rotational mass) is particularly vital for evaluating the torsional response of the building.

Cumulative Mass: Some versions provide the total mass from the top down to the current story, useful for checking base shear calculations manually. Importance in Seismic Analysis The "Mass Summary by Story" in ETABS is

ETABS uses these values to determine the Center of Mass (CM) for each diaphragm. The relationship between the CM and the Center of Rigidity (CR) determines the building's eccentricity and its susceptibility to torsion during an earthquake.

If you are just getting started with these summaries, you can find introductory structural analysis tutorials on YouTube to walk through the basic table generation. Mass Source

Pitfall 1: Overlooking the Mass Source Definition

Many users assume ETABS uses the same load combinations for mass. It does not. By default, ETABS uses Dead Load (1.0) and Live Load (0.0). For seismic design, you must define a proper mass source via: Define > Mass Source > Add New Mass Source.

For most codes, set:

Dead Load: 1.0
Superimposed Dead: 1.0
Live Load: 25% to 50% (depending on occupancy).

A. Story and Output Case

Story: The name of the specific level (e.g., Roof, Story 2, Story 1).
Output Case: Usually specified as Mass or the specific load combination used for modal analysis.

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.

C. Total Mass (The Output)

Mass: This is the summation of the component loads divided by gravity ($g$).
- Formula: $\textMass = \frac\textDL + \textSDL + (\textLive Load Factor \times \textLL)g$
Weight: This is the Total Mass converted back into force units (Weight = Mass × $g$). This is the number to check against your manual dead load take-offs.

4. Troubleshooting Common Issues

If your Mass Summary looks incorrect, check these common modeling errors:

| Issue | Possible Cause | Solution | | :--- | :--- | :--- | | Mass is Zero | 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 $kN$). |