By Story — Etabs Mass Summary

ETABS Mass Summary — A Short Story

: User-assigned point, line, or area masses to account for non-structural components like cladding or partitions. Specified Load Patterns : Often used for seismic calculations (e.g., Dead Load + Live Load) as per codes like 2. The Lumping Process

Cause:

Mesh distortion or different orientation of shell elements. Check local axes of floors. Some minor asymmetry (openings, elevator shafts) is realistic. Difference < 5% is acceptable.

Warning:

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.

Part 7: The Mass Summary vs. Modal Analysis

Mass Moment of Inertia (MMI)

: 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

Symptom:

Negative values in mass summary. Problem: Load combinations that subtract loads (e.g., 0.9DL + 1.0WL) are incorrectly assigned to the mass source, or you have overturning with tension elements. Fix: Never use load combinations in the Mass Source definition. Use only primary load patterns.