The Vertical Narrative: A Critical Analysis of the ETABS Mass Summary by Story
Before you finalize your ETABS model for seismic design:
Conclusion
Symptom:
The "Base" or "Story1" (below ground) shows U1/U2 mass > 0. Problem: You applied floor loads to the foundation level, or the base slab is modeled as a flexible diaphragm. Fix: Delete loads at base or assign the base points to a fixed support with no mass source assignment. etabs mass summary by story
Why this matters: You must compare the Center of Mass (CM) with the Center of Rigidity (CR). If they do not align, the building will experience torsion during a seismic event. Large eccentricities here indicate potential torsional irregularities.
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