Calculate the dead load of a typical bay by hand. Multiply it by the floor area, and compare it directly to the UX/UY values in the Mass Summary table. A variance of less than 5% is generally acceptable.
Click the button (F5) to solve the model. ETABS calculates the mass distribution based on the physical properties and applied loads. Step 3: Extract the Story Mass Table
definition. To align with most seismic codes (like ASCE 7-16), ensure you include 100% of Dead Loads and a percentage of Live Loads (typically 25% for storage or specific occupancy). Unit Conversion
The translational mass calculated for horizontal movements. These values are typically identical unless unique directional properties are applied. etabs mass summary by story better
💡 Always compare the Total Mass in the summary table to the Base Reaction for a dead load case. They should be identical. If they aren't, your mass source is likely missing a specific load pattern.
This method calculates mass based on the loads defined in your model, which is the . It correlates directly with the building's actual weight. A key tip: the load combination 1.0DL + 0.5LL (Dead Load + 50% Live Load) is used in many codes, such as ASCE 7, to represent the effective seismic weight. Crucially, the 'From Loads' definition with DL+0.5LL was found to produce results closest to manually calculated values, accurately aligning with code recommendations (ASCE 7–10).
Whether your model uses .
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Run Display > Show Mass Summary and toggle “Show as Table” . Then click “Export to Excel.” Plot story mass vs. height. Any anomalies will jump out visually.
Ensure you are checking the "Include Lateral Mass" option for seismic analysis. Calculate the dead load of a typical bay by hand
The ETABS Mass Summary by Story provides the (typically in ton-sec²/mm or kip-sec²/in) used for modal analysis (eigenvalue extraction) and equivalent lateral force (ELF) procedures. Understanding it correctly prevents underpredicting seismic demands.
After defining your mass source and running the Modal analysis, you must check the table. This table tells you how much of the total building mass (as defined in your mass source) is "activated" or participating in each vibration mode.
If your Mass Source includes "Element Self-Weight" checked AND you explicitly included the Dead Load case (which already has a self-weight multiplier of 1.0) in the specified load patterns, ETABS will double-count the structural weight. Fix: Use "Specified Load Patterns" and set the Dead Load multiplier to 1.0, keeping "Element Self-Weight" unchecked. Click the button (F5) to solve the model
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).
The "Mass Summary by Story" table provides a fundamental breakdown of a building's seismic weight. In ETABS, it is the cornerstone for seismic analysis, enabling: