. In a seismic event, the ground acceleration acts upon this mass to generate inertial forces
If you forget to define the mass source, ETABS uses the default (self-weight only). You will underestimate mass by 30–50%, leading to an unconservative design.
You defined mass source from load patterns but forgot to assign any load pattern to the roof diaphragm. Or, you used a rigid diaphragm but the beams are not connected to the diaphragm constraint. etabs mass summary by story
: The rotational mass about the global Z-axis. This value is critical for determining the torsional response of the building. 3. The Role of the Mass Source
For large models, raw data is overwhelming. Export the Mass Summary to Excel and run these checks: You defined mass source from load patterns but
The specific floor level designated in your story data grid.
You have rigid floor diaphragms, but your walls are modeled with "Pier" sections that are not tied to the diaphragm, or you have a "soft" story with no diaphragm assignment. This value is critical for determining the torsional
This article will dissect the ETABS Mass Summary by Story. We will cover what it is, how ETABS calculates it, how to interpret every column of the output, common sources of error, and how to troubleshoot discrepancies.
: By default, ETABS lumps mass at story levels by taking half the weight from the story above and half from the story below. You can explicitly enable "Lump Lateral Mass at Story Levels" in the Mass Source definition to simplify the model into a multiple degree-of-freedom system.
When you open the table, you will see several key headings. Understanding these is vital for an accurate peer review or design check: The specific level of the building.