Etabs | Mass Summary By Story Better
For a given story, manually compute:
Story Mass (UTotal) = Σ(Dead load + applicable live load) / g
where ( g ) = 9.81 m/s² (or 386.4 in/s² in imperial).
Compare with ETABS output – difference should be <2%.
The most common mistake is neglecting how ETABS calculates mass. By default, ETABS does not automatically know what to weigh. You must define a Mass Source (Define > Mass Source).
A better approach involves two distinct definitions:
Better Practice: Create a Mass Source that references load patterns (DEAD, SUPER_DEAD, LIVE_REDUCED). Never use the "From Self & Specified Mass" option unless you fully understand the lumped mass elements you've added. A clean mass source leads to a clean summary.
| Story | Mass DL | Mass 0.25LL | Total Mass | Seismic Weight (W) | Cumulative W | |-------|---------|-------------|------------|--------------------|---------------| | Roof | 850 | 50 | 900 | 8820 kN | 8820 kN | | PH | 300 | 10 | 310 | 3038 kN | 11858 kN | | 5 | 890 | 60 | 950 | 9310 kN | 21168 kN |
Notice: The roof mass dropped to realistic levels because the penthouse was separated. Cumulative seismic weight is now correct for story shear calculation.
Unlocking Precision: Why "Mass Summary by Story" is Better for ETABS Analysis
As a structural engineer, you know that the soul of a seismic analysis lies in how you define your building's mass. While ETABS offers various ways to view your model's weight, the Mass Summary by Story report is often the gold standard for high-accuracy design. etabs mass summary by story better
Here is why this specific report is better for your workflow and how to master it. 1. Superior Accuracy in Seismic Weight Verification
The primary benefit of the Story Mass Summary is its role in "Mass calculated manual" verification. By breaking down the weight floor-by-floor, you can easily compare ETABS’ output against manual calculations for:
Slab Weight: Effective area (total minus openings) times thickness and unit weight.
Vertical Components: The summary accounts for half the mass of columns/walls above and below the floor, ensuring vertical continuity.
Adjusted Top Stories: It correctly identifies that the top story has no vertical components above it, preventing the overestimation of roof mass. 2. Streamlined Diaphragm and Lateral Load Analysis
The Story Mass Summary is essentially the "heartbeat" of your lateral force distribution.
Lumped Mass Efficiency: ETABS can lump the mass of all nodes on a story to a "master node" at the center of mass. This simplifies the mass matrix and significantly reduces analysis time for high-rise buildings without sacrificing accuracy.
Drift Control: Since story drift is computed at the center of mass, having a clean, story-by-story mass breakdown is critical for verifying that your building stays within the strict 0.020h0.020 h 0.025h0.025 h seismic limits. 3. Better Troubleshooting of Mass Discrepancies
Have you ever noticed your base reactions don't match your mass? The Story Summary helps you pinpoint the culprit: For a given story, manually compute: Story Mass
Element Meshing: It reveals if elements meshed between stories are lumping mass to the wrong level.
Null Areas: It helps identify if Superimposed Dead Loads (SDL) on null areas are failing to transfer to the seismic mass because a diaphragm wasn't assigned. How to Access the Report To get this better view of your structure’s data: Navigate to the Display menu. Select Show Tables. Go to Output > Other Output Items > Mass Summary by Story. Pro-Tip: The "Mass Source" Secret
For the best results, ensure your Mass Source is defined via Define > Mass Source. To avoid double-counting, don't check "Element Self Mass" if you are also including a Dead Load pattern with a 1.0 multiplier in your load list.
How are you currently verifying your building's seismic weight in ETABS? Let’s discuss your favorite verification techniques in the comments!
Explore the CSI Knowledge Base for detailed FAQs on mass application. Mass Source
In ETABS, the Mass Summary by Story is a critical diagnostic table used to verify the seismic weight and mass distribution of a structure. This data is essential for calculating the base shear during seismic analysis. Key Components of Story Mass
ETABS calculates story mass based on your defined Mass Source.
Element Self-Mass: Automatically calculated from the volume and material density of structural members.
Additional Mass: Manual assignments for non-structural elements like cladding or partitions. where ( g ) = 9
Specified Load Patterns: Mass derived from weight (typically Dead Load + a percentage of Live Load). How ETABS Lumps Mass
The software follows a specific logic to distribute mass at each level:
Tributary Distribution: ETABS "lumps" mass at the center of each floor by taking half the weight of the story below and half from the story above.
Master Node: Translational and rotational masses are concentrated at a master node, usually at the center of mass, to capture dynamic effects.
Mesh Sensitivity: Improper auto-meshing of walls or slabs can lead to incorrect mass calculations. Manual meshing is often recommended for higher accuracy. Accessing and Exporting the Report To generate and review this data for your project report: Additional Mass Frame Objects
A foolproof way to cross-check your "better" mass summary is to look at the base reactions from a static Load Case that includes the mass source loads with g = 386.4 in/s².
Run a DEAD load case and a MASS load case.
Compare this to the reported mass summary total. They must match within 1%. If not, your mass source is misapplied.
