Etabs Mass Summary By Story Better [better]

In ETABS, the Mass Summary by Story table is a critical output that aggregates the total mass and weight at each level, forming the basis for seismic and dynamic analysis. Overview of Mass Summary

The Mass Summary by Story table provides the cumulative and individual mass for each story based on the Mass Source defined in your model. It typically includes: UX, UY, UZ: Total mass in the X, Y, and Z directions.

Sum UX, Sum UY: Cumulative mass from the top story down to the current level. XCM, YCM: Coordinates of the Center of Mass for each story. How to Access the Table To view the mass summary after running an analysis: Navigate to the Display menu. Select Show Tables.

Expand Analysis Results > Structure Results > Mass Summary > Mass Summary by Story. Improving Mass Accuracy ("Better" Mass)

To ensure your mass summary is "better" (more accurate), verify your Mass Source definition:

Self-Mass: Ensure "Element Self-Mass" is checked if you want ETABS to automatically calculate mass from material density and volumes. etabs mass summary by story better

Additional Mass: Use "Additional Mass" to account for non-structural elements like partitions or cladding that aren't modeled as physical objects.

Specified Load Patterns: For seismic mass, it is common practice to include of Dead Load and a percentage (e.g.,

) of Live Load by selecting "Specified Load Patterns" in the Mass Source dialog. Key Applications

Seismic Base Shear: The total weight (W) used to calculate base shear ( ) is derived from this table.

Drift Checks: Code-compliant drift checks must be based on the Center of Mass coordinates provided in this summary. In ETABS, the Mass Summary by Story table

Soft Story Check: Mass distribution across stories is a factor in identifying structural irregularities like soft stories.

2. Live Load Clauses Are Ignored

ASCE 7 allows reduced live loads for seismic mass (e.g., 25% of floor live load in storage areas). The default summary does not apply these reductions automatically. You need a better mass source.

✅ Step 2 – Validate Story Mass Value

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%. where ( g ) = 9

Why it matters

• Ensures correct mass distribution for earthquake response.
• Validates model-loading assumptions (e.g., whether live load is included).
• Helps identify modeling errors (missing masses, discontinuities).
• Provides inputs for code checks (e.g., ASCE 7: base shear and modal mass participation).

6. Automating a Better Workflow: Reporting and QA/QC

A "better" Mass Summary is not just a number—it is a deliverable. You should export the table and compare it to your design basis.

Step-by-step: Generate Mass Summary by Story in ETABS

1. Open your ETABS model and ensure all loads and load cases are defined and assigned (including gravity load patterns used for mass).
2. Run a modal analysis (Response Spectrum/Modal time history) or at least a mass-consistent modal extraction.
3. From the menu: Display → Show Tables → Masses → Story Mass Summary (menu names may vary slightly by ETABS version).
4. Select the load cases/patterns to include (e.g., Dead + portion of Live if required by code).
5. Export the table to CSV or spreadsheet for reporting.

1. Why the Default Mass Summary is Dangerous

The default ETABS Mass Summary is only as good as your assumptions. Many engineers open the table, see a number, and immediately proceed to design. This is a mistake.

The Problem: 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:

• 25% of live load (required by ASCE 7 / Eurocode 8 for seismic mass)
• Partition loads
• Cladding and facade masses
• Equipment live loads

The Consequence: Underestimated mass leads to underestimated base shear, leading to an unsafe design.

The Goal: A "Better" Mass Summary means matching three things:

1. Code-based seismic weight (W)
2. Gravity design reactions (for foundation checks)
3. Dynamic properties (periods and mode shapes)

Part 5: Interpreting Your Mass Summary – A Diagnostic Checklist

You’ve built a better ETABS mass summary by story. Now, what does it tell you?

| Observation | Diagnosis | Fix | |-------------|-----------|-----| | Mass on roof > floor below | Penthouse mass merged into roof | Split story definition | | Mass at basement is huge | Soil interaction mass included | Check mass source: exclude soil | | Mass not integer (weird decimals) | Shell meshing causing partial masses | Remesh with regular grid | | Lateral stiffness unreasonably low | Missing diaphragm constraint | Assign Rigid Diaphragm | | Mass changes between Ultimate & Service | Different load multipliers | Set both to seismic combo |