This table can be viewed on-screen or printed as part of a comprehensive structural calculation document.
: This includes the rotational inertia about the X, Y, and Z axes for each story. It represents the resistance of the mass to changes in its rotation.
To ensure reliable results, follow these best practice recommendations:
To ensure your Mass Summary by Story is flawless, follow these rules:
After analysis, open the .log file or the .f2k text file. Search for STORY MASS . This raw data is useful for scripting or batch verification. etabs mass summary by story
The running total of the building's mass from the topmost story down to the current story. The bottom story row displays the total mass of the structure.
If the Mass Summary shows Story 1 with only 600 kN, you may have forgotten finishes or a transfer slab. If Story 5 shows 1200 kN (equal to lower floors), your model likely has an incorrect load pattern on the roof—unrealistic for most structures.
Why do engineers look at this table? Here are three primary reasons:
Before designing lateral systems, you must calculate the Seismic Response Coefficient ($C_s$). This requires the total seismic weight ($W$). This table can be viewed on-screen or printed
Building codes (ASCE 7, IBC, Eurocode 8, IS 1893) require the seismic weight (W) to calculate the base shear ( V = Cs * W ). If your Mass Summary shows an inconsistent total weight—say, you forgot to include partition loads or a heavy mechanical unit on the roof—your seismic forces will be dangerously wrong.
A quick glance at the cumulative mass helps verify if you are working in kips, tons, or kilograms. C. Calculating Eccentricity
If these rules are not respected, mass may not be properly aggregated to the correct story level, and the Mass Summary table will show incorrect distribution.
A missing or incorrectly defined Mass Source will produce completely wrong base shear, and you may never realize it unless you check the Mass Summary table. To ensure reliable results, follow these best practice
: Some versions provide the total mass from the top down to the current story, useful for checking base shear calculations manually. Importance in Seismic Analysis
: User-assigned point, line, or area masses to account for non-structural components like cladding or partitions. Specified Load Patterns : Often used for seismic calculations (e.g., Dead Load + Live Load) as per codes like 2. The Lumping Process
: Understanding mass distribution can provide insights into a structure's expected performance under lateral loads.
Occasionally, loads are applied to elements that are not properly connected to the diaphragm (e.g., a beam modeled without a support connection or a slab not meshed with walls).
option if you want the mass concentrated at the floor levels for simplified lateral analysis. Include Lateral Mass