This document explains how to define masses in RAM Elements and what are the available options for the source of such masses.
1.1 Mass source
The mass source must be understood as the actual mass that will be used in the modal analysis. The modal analysis is the first step to perform a dynamic analysis.
The analysis dialog presents two options to define the masses source in the model:
- Node mass
- Element self-mass and node mass
Note. To display the analysis dialog go to the Process ribbon tab, then click on the Analyze model button. |
1.2 Node mass
This is the default option and by using it, the program will perform the modal analysis using only the information introduced by the user in the Masses spreadsheet:
Notes. - Both translational and rotational mass are considered. - By using this option, the user is responsible of entering the masses and apply them to the corresponding nodes. - The Masses spreadsheet has excellent tools that calculate the center of mass for one or several rigid floor diaphragms. |
1.3 Element self-mass and node mass
When this option is selected, RAM Elements calculates the self-mass of all members and shells in the whole model. This calculation is made internally at the time the analysis is run and only considers translational self-masses. However, this option also considers whatever masses are defined as explained in 1.2 above.
The following notes are important in order to know how this option works:
Notes. - The self-mass is calculated only and exclusively from the material density of the members and/or shells. - For members: the program calculates concentrated mass values at the member end points. - For shells: the program calculates concentrated mass values at all shells corner nodes. - In this option, there is no output for the values of the calculated masses. However the report will show mode frequencies, mass participation, spectral values, modal shapes (node displacements) and base shears. |
1.4 What option should I use?
For structures where rigid floor diaphragms can be defined, the first option is the best. However in many structures, rigid diaphragms cannot be modeled and thus, the second option is suitable (e.g. towers, warehouses/sheds, industrial structures, etc.).