Modern building codes (e.g., ASCE 7, IBC, Eurocode 8) require dynamic analysis for irregular or tall structures. Before running modal or response spectrum analysis, engineers must verify that the mass model is correct. The ETABS Mass Summary by Story serves as the primary quality control tool to ensure that the total seismic weight ($W$) matches code expectations and that mass is distributed logically over the building height.
ETABS uses these values to determine the 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.
Double-click to open the interactive data table. etabs mass summary by story
These coordinates tell you where the "weight" of the floor is centered. If these are far from the geometric center, you’ll have high accidental torsion.
: Identifies each floor level defined in the model. Modern building codes (e
: 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.
If your table displays zeros or unexpectedly low values, check your Mass Source definition (). Ensure you have selected Specified Load Patterns and assigned appropriate multipliers (e.g., 1.0 for Dead Load, 0.25 or 0.50 for Live Load depending on your local building code). 2. Diaphragm Settings ETABS uses these values to determine the for each diaphragm
| Story | U1 Mass (kN-s²/m) | U2 Mass (kN-s²/m) | R3 Mass (kN-s²/m-rad) | | :--- | :--- | :--- | :--- | | Roof | 85 | 85 | 12,500 | | Story4 | 110 | 110 | 18,200 | | Story3 | 110 | 110 | 18,200 | | Story2 | 110 | 110 | 18,200 | | Story1 | 115 | 115 | 19,000 | | Base | 0 | 0 | 0 |