For Time History analysis, the sum of inertial forces at any time step should equal the base shear. This only holds true if the mass per story is correctly defined. The Mass Summary serves as the baseline for debugging dynamic responses.
Before you finalize your ETABS model for seismic design:
In the realm of structural engineering software, CSI ETABS (Extended Three-dimensional Analysis of Building Systems) stands as a cornerstone for the analysis and design of building systems. Among the myriad of output tables and log files it generates, one of the most critical—yet often overlooked—is the Mass Summary by Story. etabs mass summary by story
Whether you are performing a seismic analysis (Response Spectrum or Time History) or simply checking vertical load distribution, understanding the Mass Summary is non-negotiable. This article breaks down what this summary tells you, where to find it, and how to use it to validate your model.
Before running response spectrum or time history analysis, check the Participating Mass Ratio (found in the Modal Analysis Tables). If your U1 and U2 participating mass ratios do not sum to at least 90% (per most codes), your building has insufficient modes captured. For Time History analysis, the sum of inertial
Often, the culprit is a massive story mass that ETABS cannot move due to stiff supports. Go back to the Mass Summary, verify the U3 values, and ensure your vertical elements (columns/walls) are not artificially restraining movement.
A typical Mass Summary by Story table includes the following columns: Before you finalize your ETABS model for seismic
You can access this summary in three ways:
These represent the mass contributing to inertia in the global X (U1), Y (U2), and Z (U3) directions.
After running modal analysis, ETABS provides "Modal Participating Mass Ratios." For a valid model, the sum of modal mass ratios in the X and Y directions must exceed 90% of the total mass from the Mass Summary. If not, the analyst must add more modes.