Mass Source DefinitionIn ETABS, the user has the option of choosing one of three options for defining thesource of the mass of a structure. Click the Define menu -> Mass Source command tobring up the Define Mass Source form. The following options appear on the form:1. From Self and Specified Mass:Each structural element has a material property associated with it; one ofthe items specified in the material properties is a mass per unit volume.When the From Self and Specified Mass box is checked, ETABSdetermines the building mass associated with the element mass bymultiplying the volume of each structural element times its specifiedmass per unit volume.2. From Loads:This specifies a load combination that defines the mass of the structure.The mass is equal to the weight defined by the load combination dividedby the gravitational multiplier, g. This mass is applied to each joint in thestructure on a tributary area basis in all three translational directions.3. From Self and Specified Mass and Loads:This option combines the first two options, allowing you to consider selfweight,specified mass, and loads in the same analysis.It is important to remember when using the From Self and Specified Mass andLoads option, NOT to include the Dead Load Case in the Define Mass Multiplierfor Loads box. This will account for the dead load of the structure TWICE.