Etabs Mass Summary By Story Better |top| Jun 2026

Mass Summary by Story is not the same as Mass Summary by Diaphragm ETABS 2016

A concise guide on generating and interpreting mass summaries by story in ETABS, useful for structural engineers performing seismic and dynamic analysis. etabs mass summary by story better

| 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 | Mass Summary by Story is not the same

: ETABS typically lumps masses at the floor level by taking half the weight of the story below and half the weight from the story above. Values Reported Translational Mass (UX, UY) : Used for lateral seismic force calculations. Rotational Mass (RZ) live load participation

aggregates the tributary mass (including all modeled masses from dead load, live load participation, and assigned masses) for each floor level. It’s essential for seismic base shear calculations, modal mass participation checks, and dynamic analysis validation.

| Red Flag | Fix | |----------|-----| | Mass at base (Foundation) | Exclude from lateral mass unless basement walls provide restraint | | Sudden drop in mass at top story | Missing penthouse/cladding mass | | RTotal zero for all stories | Assign rigid diaphragm – mass moment computed from floor plan | | Identical mass for Ux and Uy but building is asymmetric | Add point masses to reflect actual distribution |