ETABS calculates the coordinate where the floor's mass is concentrated. Comparing the CM to the Center of Rigidity (CR) allows engineers to determine the "eccentricity," which induces torsion during lateral loading. 3. Engineering Significance Base Shear Verification:
This feature is useful for:
The total mass assigned to that story acting in the global X and Y horizontal directions. In most standard buildings, UX and UY will be identical.
To access the summary:
). During an earthquake, the inertial forces generated at each floor level depend entirely on the mass assigned to that specific story. Engineers review the story mass summary to ensure:
Many users assume ETABS uses the same load combinations for mass. 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 .
Always double-check your current unit system in the bottom right corner of the ETABS window. Mass is reported in mass units (e.g., Toncap T o n ), not force units ( etabs mass summary by story
Modern codes often require the consideration of rotational mass moment of inertia ($J_mass$), particularly for flexible or semi-rigid diaphragms. The Mass Summary table provides the rotational mass about the vertical axis passing through the center of mass.
The center of mass for the entire structure from that story upward. 3. Why This Data Matters A. Seismic Base Shear Verification
To obtain a Mass Summary by Story in ETABS, you must first define your mass source and then run the analysis to view the generated tables. This summary is essential for verifying the seismic weight and ensuring the structure's center of mass is correctly captured for lateral analysis. 1. Define the Mass Source ETABS calculates the coordinate where the floor's mass
If your building weighs significantly less than expected, check Define > Material Properties to ensure your concrete or steel densities are correctly defined with non-zero values. Check the Units: Ensure your output units (e.g.,
Accidental double-modeling of slabs or beams.
The Mass Summary by Story is a report table that lists the translational seismic mass (UX, UY, and UZ) for each story in your building model. It is calculated based on the Mass Source you defined in your model, making it the authoritative report for seismic weight distribution. During an earthquake, the inertial forces generated at