Etabs Mass Summary By Story Access

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

To access: Display > Show Tables > Analysis Results > Structure Output > Mass Summary by Story.

A typical output table includes the following columns (Units: kg, N-s²/m, or kip-s²/in): 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 |

These represent the mass contributing to inertia in the global X (U1), Y (U2), and Z (U3) directions. Why do engineers look at this table

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.

The accuracy of dynamic analysis in structural engineering is highly dependent on the correct definition and distribution of mass. ETABS (Extended Three-dimensional Analysis of Building Systems) provides a critical output called the "Mass Summary by Story." This paper explains the theoretical basis of mass summation in ETABS, differentiates between mass and weight, interprets the tabular output, and demonstrates how this summary is used for seismic load calculations (base shear), center of mass determination, and dynamic property verification (modal analysis). Modern building codes (e

These columns represent the summation of forces acting on the individual diaphragms.