Etabs Mass Summary By Story

On a rainy Tuesday, Maya, a young structural engineer, opened ETABS to check the mass summary of a 12‑story apartment building she’d modeled. The model had been quiet for days, but tonight the mass summary promised answers.

When the report loaded, the first line showed the total seismic weight per story. Maya read it like a heartbeat: lower floors carried more—concrete podiums, parking slabs, and heavy mechanical rooms—while the upper residential stories were lighter, filled with drywall and furniture assumptions. The variation was clear in the table: Story 1 held 1,200 kN, Story 2 had 1,000 kN, and by Story 12 it tapered to 650 kN. The building’s center of mass whispered its position: slightly offset toward the west because of the heavier stair core.

Maya scrolled to the modal mass participation. The first few modes captured most of the mass in the X and Y directions—good, the primary modes were behaving. But when she examined torsional participation, a small but stubborn 6% torsion appeared in the first mode. That matched the slight eccentricity the mass summary hinted at; the stair core’s offset was real, and under seismic shaking the building would twist.

She checked vertical mass contribution: slabs and partitions dominated, and the elevators concentrated mass at the core. The rigid diaphragm assumption held—floor masses were lumped at the diaphragm level—so any local stiffness irregularity could amplify torsion. Maya made notes: reduce eccentricity by rebalancing the core layout, or add a small shear wall on the east face to shift mass center.

Next she compared imposed loads used to compute masses. The mass summary reminded her of the live load reduction she’d applied per code: residential floors used a reduced live load factor, but the rooftop mechanical area still used full live load. That explained a spike at Story 11 where the plant room sat. She adjusted the live load assignment—moving some equipment to a lower floor—and watched the mass numbers settle into a more uniform gradient.

Finally she exported the mass summary and sketched a quick mass map: heavier blocks shaded near the core and podium, lighter tones toward the top and corners. The map translated numbers into strategy: redistribute heavy equipment, tweak diaphragm rigidity, and consider small torsional-resisting elements.

By midnight the model’s mass summary told a clearer story: the building would likely behave well laterally if Maya corrected the modest eccentricity and redistributed concentrated masses. She closed ETABS satisfied—mass was not just numbers on a table; it was the building’s voice, telling her where to reinforce, where to simplify, and how to keep the structure honest when the ground moved.

If you want, I can produce:

The Mass Summary by Story in ETABS is a standard output table that aggregates the distributed and point masses assigned to a building model at each floor level. It serves as a primary verification tool for structural engineers to ensure that the total seismic weight used for base shear calculations is accurate and adheres to building codes. 1. Accessing the Table etabs mass summary by story

To view the report after running an analysis, navigate to:Display > Show Tables > Analysis > Results > Structure Results > Mass Summary by Story. 2. Core Components and Columns

The table typically displays the following values for each defined story: Story: The specific level of the building being reported.

Mass X and Mass Y: The translational mass along the global X and Y axes. These values are used to calculate the inertial forces during seismic events.

Mass Z: The vertical mass component. While often excluded for standard lateral analysis, it is necessary for vertical seismic effects or special design requirements.

Mass Moment of Inertia (MMI): The rotational mass about the global Z-axis. This value is critical for determining the torsional response of the building. 3. The Role of the Mass Source

The values in the summary table are derived directly from the Mass Source definition (Define > Mass Source). ETABS calculates mass based on three primary settings:

Element Self Mass: Calculates mass based on the volume and material density of beams, columns, walls, and slabs.

Additional Mass: Includes user-assigned point, line, or area masses for non-structural elements like cladding or partitions. On a rainy Tuesday, Maya, a young structural

Specified Load Patterns: Converts assigned loads (e.g., Dead Load + a percentage of Live Load) into mass by dividing the weight by the gravitational multiplier ( ). For example, codes like IS 1893:2016 often require Dead Load plus Live Load depending on load intensity. 4. Mass Lumping Mechanics

ETABS generally "lumps" masses at the floor levels to simplify the analysis into a Multiple Degree of Freedom (MDOF) system. Mass - CSI Knowledge Base

To get a clear picture of how your building’s weight is distributed, follow these steps in ETABS. This summary is essential for verifying your seismic mass and ensuring your model’s gravity loads make sense. How to Find the Mass Summary

Run the Analysis: You must have a completed analysis run (the lock icon should be closed). Navigate the Menu: Go to Display > Show Tables. Find the Specific Table: Expand Analysis Results. Expand Structure Results.

Select Center of Mass and Rigidity. (This is the most common way to see story-by-story mass).

Alternatively: Check Model Definition > Structural Data > Mass Summary by Story for the raw input mass before analysis. What to Look For

Mass X / Mass Y: These should generally be equal unless you have strange property modifiers. This represents the total translational mass per floor.

Cumulative Mass: Check the bottom story to see the total mass of the entire building. Compare this to your hand calculations (Area × Dead Load) to ensure you haven't missed any loads. The Mass Summary by Story in ETABS is

Center of Mass (XCCM, YCCM): These coordinates tell you where the "weight" of the floor is centered. If these are far from the geometric center, you’ll have high accidental torsion.

Check your Mass Source: Go to Define > Mass Source. Ensure you’ve included 100% of Dead Loads and the required percentage of Live Loads (usually 25% for storage or as per your local code).

Diaphragms: To get a "per story" breakdown in the Analysis tables, you usually need to have diaphragms assigned to your floor shells. If you want to verify the numbers, let me know: The building's total area Your typical dead load (PSF or kN/m²)

Which building code (ASCE 7, Eurocode, etc.) you're following

Many users assume ETABS uses the same load combinations for mass. It does not. By default, ETABS uses Dead Load (1.0) and Live Load (0.0). For seismic design, you must define a proper mass source via: Define > Mass Source > Add New Mass Source.

For most codes, set:

Your base (foundation level) often has a mass value if you have a basement slab or ground floor. In elevated structures (e.g., podium slabs), ensure mass is not double-counted between the base and first floor.