Form: Mass Source
Use the Define menu > Mass Source command to access the Define Mass Source form. Choose to define the source of the mass of a building as from self mass and additional mass, from loads, or from a combination of self mass, additional mass, and loads. Also specify how lateral mass is to be considered and located.
The Mass Source area has the following options:
Element Self 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. Also, if the link properties have been assigned to a joint or frame object, ETABS also includes the mass and weight as specified in the link property definition (see Link Properties)
Additional Mass option. It is also possible to assign additional mass to account for partitions, cladding, and so forth. (See Additional Area Mass, Additional Line Mass, and Additional Point Mass.) The additional 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.
Specified Loads Patterns option. Specify a load pattern 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 on a joint is upward, the mass at that joint is set to zero. Negative mass is not allowed 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: Negative mass is not allowed in ETABS in the final assembly. After masses are combined, e.g., Element Self Mass + Additional mass + Specified Load Patterns, if the net mass at a joint is negative, ETABS sets the mass at that joint to zero.
Adjust Diaphragm lateral Mass to Move Mass Centroid option. Use this option to move the mass centroid associated with the lateral diaphragm.
Move Direction (counterclockwise from +X): Specify the angle of the axis along which the mass centroid should be moved. The angle is measured in a counterclockwise direction from the positive global X axis.
Move (ratio to diaphragm dimension in move direction): Specify the distance the mass centroid should be moved along the axis defined above in the Move Direction. The distance is specified as a fraction of the total diaphragm dimension in the direction that the mass centroid is being moved.
Note: For flexible diaphragms with distributed mass, the "Adjust Diaphragm Lateral Mass to Move Mass Centroid" option moves the distributed masses so that the resulting mass centroid will move as specified.
Mass Multipliers for Loads Patterns area. This area is active when the Specified Loads Patterns option is selected. The mass source load combination is created by specifying one or more load patterns, each with an associated multiplier.
Add Add a load pattern to the mass source load combination definition as follows.
Select the load pattern name from the Load drop-down list.
Type in an appropriate multiplier in the Multiplier edit box.
Click the Add button.
Modify Modify the mass source load combination definition as follows.
Highlight the load pattern name. Note that the load pattern name and associated multiplier appear in the Load Pattern drop-down list and Multiplier edit box.
Select a different load, or type in the revised scale factor in the Multiplier edit box.
Click the Modify button.
Delete Delete a load pattern from the mass source load combination definition as follows.
Highlight the load pattern name.
Click the Delete button.
Include Lateral Mass check box. If this check box is checked, 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.
Include Vertical Mass check box. If this check box is checked, assigned translational mass in the global Z axis direction and assigned rotational mass moments of inertia about the global X and Y axes are considered in the analysis. Leave this box unchecked if you do not want to consider vertical dynamics.
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., Element Self 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 option.