Mass FAQ

On this page:

What constitutes building mass, and does it include SDL?

Answer: Building mass always refers to the definitions specified in the Define > Mass Sources menu. To account for superimposed dead load (SDL) in building mass, users should add a SDL case to the Load List and select its definitions from the Loads options.

Is it possible to obtain the mass moment of inertia for a semi-rigid diaphragm?

Answer: A rigid diaphragm is necessary to obtain the center of mass (CM) and center of rigidity (CR). Users may obtain these values by analyzing the model with rigid diaphragms, then for final analysis and design, convert to semi-rigid diaphragms.

Please note that a rigid diaphragm must be assigned to an area object, and not to a point, so that the internal points created during area meshing may be lumped to the story-level. The Edit > Auto Relabel command may be used for point labeling, and to avoid auto area meshing for shear walls, since masses are not lumped to the story-level when auto area meshing is applied to walls.

Is it possible to obtain building mass per story?

Extended Question: The output building mass, calculated as the sum of individual story masses, is not consistent with the reaction forces and the assembled point masses. Can this please be clarified?

Answer: This can be the case when elements are meshed between stories,  ETABS implements two rules when lumping masses to the story-level. They are listed as follows:

1. The point at the story-level and point(s) in between stories must have identical coordinates.
2. The starting-point label between each story must be identical to the point label at the story-level.