These comands are used for the definition of the mass of a building from self mass, specified mass, from loads, or from a combination of self mass, specified mass, and loads. It also specifies how lateral mass is to be considered. Further explanation is as follow:-

1 From Self and Specified Mass option. Each structural element has a material property associated with it; one of the items specified in the material properties is a mass per unit volume. When this check box is checked, ETABS determines the building mass associated with the element mass by multiplying the volume of each structural element times its specified mass per unit volume. This is the default.

It is also possible to assign additional mass to account for partitions, cladding, and so forth. (See Additional Area Mass Assignments to Area Objects, Additional Line Mass Assignments to Line Objects, and Additional Point Mass Assignments to Point Objects.) ETABS adds any additional mass assignments to the element mass to derive a total mass.

Also, if link properties have been assigned to a point or line object, ETABS also include the mass and weight as specified in the link property definition (see Link Properties).

By default, the total mass is applied to each joint in the structure on a tributary area basis in all three translational directions. Note that additional masses can be input as negative, and that those additional masses are added to the masses determined from the material property masses. If, as a result of including negative additional mass the net mass at a joint is negative, ETABS sets the mass at that joint to zero.

2 From Loads option. Specify a load combination that essentially defines the mass of the structure. The mass is equal to the weight defined by the load combination divided by the gravitational multiplier, g. Only the global Z-direction loads are considered when calculating the mass. This mass is applied to each joint in the structure on a tributary area basis in all three translational directions. Net downward loads acting on a joint are considered a positive mass. If the net load acting up on a joint is upward, the mass at that joint is set to zero. You cannot have a negative mass in ETABS. By default, the mass is applied to each joint in the structure on a tributary area basis in all three translational directions.

Note that although it is possible to assign additional mass to account for partitions, cladding, and so forth (see hyperlinks above), when this option is selected, ETABS ignores any additional mass assignments, including link property assignment (see hyperlink above), in deriving the building mass.

o Define Mass Multiplier for Loads area. This area is active when the From Loads option is selected. The mass source load combination is created by specifying one or more load cases, each with an associated scale factor.

· Add Add a load case to the mass source load combination definition as follows.

1. Select the load case name from the Load drop-down list.

2. Type in an appropriate scale factor in the Multiplier edit box.

3. Click the Add button.

· Modify Modify the scale factor for a load case already specified as a part of the mass source load combination definition as follows.

1. Highlight the load case name. Note that the load case name and associated scale factor appear in the Load drop-down list and Multiplier edit box.

2. Select a different load, or type in the revised scale factor in the Multiplier edit box.

3. Click the Modify button.

· Delete Delete a load case from the mass source load combination definition as follows.

1. Highlight the load case name.

2. Click the Delete button.

3 From Self and Specific Mass and Loads option. Use this option to combine the other two options, allowing consideration of self-weight, specified mass, and loads in the same analysis. With this option, ETABS adds the following masses

a. Self Weight. Building mass associated with the element mass obtained by multiplying the volume of each structural element times its specified mass per unit volume.

b. Loads. Weight defined by the load combination divided by the gravitational multiplier, g. Only the global Z-direction loads are considered when calculating the mass.

c. Specified Mass. Possible additional mass assigned to account for partitions, cladding, and so forth as well as link property assignments (see hyperlinks above).

By default, the mass is applied to each joint in the structure on a tributary area basis in all three translational directions. Net downward loads acting on a joint are considered a positive mass. If the net load acting up on a joint is upward, the mass at that joint is set to zero. You cannot have a negative mass in ETABS. Therefore, although additional masses can be input as negative and the additional masses are added to the masses determined from the material property masses, if the net mass at a joint is negative, ETABS sets the mass at that joint to zero.

4 Include Lateral Mass Only check box. If this check box is checked, only assigned translational mass in the global X and Y axes directions and assigned rotational mass moments of inertia about the global Z-axis are considered in the analysis. All other assigned masses are ignored.

a. Check this box if you do not want to consider vertical dynamics in your model.

b. Leave this box unchecked if you do want to consider vertical dynamics.

5 Lump Lateral Mass at Story Levels check box. As the name suggests, this option "moves" lateral mass that may occur between story levels to the nearest story level during analysis. This option can be used in conjunction with the default (i.e., From Self and Specified Mass option for which the total mass is applied to each joint in the structure on a tributary area basis in all three translational directions) and with the Include Lateral Mass Only option

Note that the above was lifted from Etab help. If you find yourself in a similar situation, press F1 and you will get the relevant help that you needed.

Regards

Teddy