: Floating or unattached elements will still contribute to the story mass if they are assigned to that story level, even if they do not transfer load structurally.
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:
The Mass Summary by Story report in ETABS summarizes the mass properties of a building model, including: etabs mass summary by story
After running modal analysis, ETABS provides "Modal Participating Mass Ratios." For a valid model, the sum of modal mass ratios in the X and Y directions must exceed 90% of the total mass from the Mass Summary. If not, the analyst must add more modes.
): The core of any equivalent static lateral force procedure relies on the total seismic weight of the structure. By summing the weights of all stories, you get the total base shear weight, which must comply with building codes (e.g., ASCE 7, UBC). : Floating or unattached elements will still contribute
Occasionally, loads are applied to elements that are not properly connected to the diaphragm (e.g., a beam modeled without a support connection or a slab not meshed with walls).
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. ): The core of any equivalent static lateral
This is the critical part. ETABS calculates mass in three translational directions:
ETABS determines story mass based on your definition ( Define > Mass Source ). Typical Mass Source Configurations