Etabs Mass Summary: By Story

. If the mass is incorrectly defined—perhaps due to doubled-up dead loads or missing partitions—the calculated base shear and lateral forces will be fundamentally flawed. ETABS calculates this by aggregating the self-weight of elements and any additional mass sources defined in the Mass Source 2. Center of Mass (CM) vs. Center of Rigidity (CR)

Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story etabs mass summary by story

| Story | U1 Mass (X-dir) | U2 Mass (Y-dir) | U3 Mass (Vertical) | Mass Moment of Inertia about Z | |-------|----------------|----------------|--------------------|--------------------------------| | Roof | 45,200 | 45,200 | 0 | 1,820,000 | | 3rd | 78,500 | 78,500 | 0 | 3,410,000 | | 2nd | 78,500 | 78,500 | 0 | 3,410,000 | | Base | 0 | 0 | 0 | 0 | Center of Mass (CM) vs

A deep read of the Mass Summary looks for . A sudden, unexplainable increase in mass at a specific story is rarely a structural requirement; it is almost always a modeling error. Common culprits include: Common culprits include: Finally, the summation at the

Finally, the summation at the base of the table acts as the ground truth for the . The total building mass reported in the summary must match the "Total Weight" used in the calculation of the base shear ($V$).

ETABS provides a powerful output table known as the which allows engineers to audit the gravity load distribution throughout the model. This article explains where to find this table, how to interpret the columns, and how to troubleshoot common discrepancies.

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