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Roof mass is 39% higher than typical floor due to penthouse and water tank weight being mis-assigned.
21 Feb 2025 — ETABS stands for Extended Three-Dimensional Analysis of Building Systems.
Based on seismic codes like IS 1893:2016, you should typically define your mass source using a load combination of 1.0 * (Dead Load) + (Applicable % of Live Load). For the "From Loads" option, a common combination is 1.0 DL + 0.5 LL (for residential buildings) or 1.0 DL + 0.25 LL (for others). .
Mastering this facet of ETABS not only ensures your seismic analyses are robust and compliant but also builds confidence in your model's overall integrity. As you continue to refine your skills, treat the Mass Summary by Story table not as a final answer, but as a critical checkpoint in your pursuit of accurate, reliable, and efficient structural design. etabs mass summary by story better
This option combines both approaches, using the self and additional mass and the specified load patterns. It is the most flexible but also the most complex, as you must ensure no double-counting occurs. Research shows it often yields values for time period, base shear, and story drift.
If your Mass Summary shows a sudden drop in mass at a residential floor, check if the "Live Load Reduction" was applied in the Mass Source definition. 3. How to Interpret and Utilize the Mass Summary Table
By default, ETABS calculates mass from element self-weight. If you modify the Mass Source to include specified load patterns (like 100% Dead + 25% Live) but forget to uncheck "Element Self-Weight," ETABS may double-count the structural self-weight. 2. Overlapping Slab Areas Roof mass is 39% higher than typical floor
Before checking the output, verify your input. Go to Define > Mass Source . For standard seismic design (e.g., ASCE 7), your mass source should typically include: of structural elements (Multiplier = 1.0). Superimposed dead loads (Multiplier = 1.0). Permanent partitions (Multiplier = 1.0).
ETABS calculates mass as (Load Cases assigned as mass source) / g (acceleration due to gravity). By default, ETABS may only include self-weight and superimposed dead loads. It often ignores :
If a slab or area element is missing a diaphragm assignment, or if it is disconnected due to a modeling tolerance issue, its mass might not attribute correctly to the story lateral force center. Comparing the Mass Summary table against the table helps ensure no mass has been "orphaned" by the software. Conceptual Unit Errors A mismatch in units (e.g., entering material density in For the "From Loads" option, a common combination is 1
The Mass Summary by Story report provides the following information:
The table typically presents data for each story, showing the total effective mass (often in or kN·s²/m ) participating in lateral (UX, UY) and vertical (UZ) directions, as well as rotational inertia (RX, RY, RZ) when applicable. The most critical columns for seismic analysis are UX and UY , which represent the mass that will generate seismic forces in those lateral directions.