Etabs Mass Summary By Story Better
: Mass distribution across stories is a factor in identifying structural irregularities like soft stories.
The total mass participating in the X or Y direction. Xcmcap X sub c m end-sub Ycmcap Y sub c m end-sub : The location of the Center of Mass. Xcm,cumcap X sub c m comma c u m end-sub Ycm,cumcap Y sub c m comma c u m end-sub : The cumulative center of mass up to that story. 4. Strategies for Better Seismic Modeling A. Identify Mass Irregularities Early
Seismic codes require a strict understanding of the Center of Mass versus the Center of Rigidity (COR) to calculate accidental eccentricity.
A sample Python snippet to get a better mass summary: etabs mass summary by story better
By comparing the Center of Mass (Location X and Y) with the Center of Rigidity (CR), you can easily calculate the building's eccentricity. Large gaps between the CM and CR reveal a high risk of structural twisting during an earthquake. Tips for Better Mass Management
When you run Analyze > Run Analysis > Display > Show Tables > Analysis Results > Structure Results > Mass Summary by Story , ETABS reports:
Interpreting between the Center of Mass and Center of Rigidity. : Mass distribution across stories is a factor
| Component | Description | Key Details & Potential Pitfalls | | :--- | :--- | :--- | | | The total mass in translational and rotational directions per story. | Mass is "lumped" to the defined story levels following specific rules (e.g., identical point coordinates for meshed elements). | | Mass Summary vs. Mass Summary by Diaphragm | Different methods can yield different values. | Discrepancies often arise if mass is assigned outside a defined diaphragm. Ensure all mass-contributing elements are connected to a diaphragm. | | Mass vs. Weight | Weight = Mass × g . | ETABS uses mass for dynamic analysis ( m = W/g ) but may use weight directly for static base shear calculations per some codes. | | Story Mass vs. Group Total Mass | The sum of story masses often differs from the total of all structural elements' mass. | Hypothesis: In many cases, Story mass is less than Group All mass. Root Cause: This signals a modeling error. A common reason is surfaces with more than 4 sides that are not auto-meshed; the program may effectively ignore them. Resolution: Edit the geometry to use only 3- or 4-sided polygons. | | Reactions vs. Mass Summary | Reactions from a DEAD load case may not match the weight in the mass summary. | Hypothesis: The reactions and mass summary are not matching. Root Cause: This mismatch is possible when elements are meshed across story levels. Check diaphragm assignments and ensure points at floor levels are not shared with other stories. |
Notice: The roof mass dropped to realistic levels because the penthouse was separated. Cumulative seismic weight is now correct for story shear calculation.
A typical story summary provides (translational mass) and R1, R2, R3 (rotational mass moment of inertia). Better interpretation requires you to ask specific questions: Xcm,cumcap X sub c m comma c u
Understanding the ETABS Mass Summary by Story Engineers must verify the structural mass distribution to ensure accurate seismic and lateral load analysis. ETABS provides this critical data through the table. This article explains how to access, interpret, and utilize this report to optimize your structural models. How to Access the Mass Summary Table
By following these guidelines and using the Mass Summary by Story feature in ETABS, structural engineers can gain valuable insights into the mass properties of their building designs, ultimately leading to safer and more efficient structures.
). Cross-reference these coordinates with the geometric center of your floor plan. Large, unexplained offsets indicate missing elements, unintended concentrated loads, or modeling geometry errors. Troubleshooting Common Mass Discrepancies