Forms: Mass Source, Mass Source Data
Use the Advanced > Define > Mass Source command to access the Mass Source form to specify how CSiBridge should calculate mass for the model. In CSiBridge, mass and weight serve different functions. Mass is used for the inertia in dynamic analyses, and for calculating the built-in acceleration loads. Weight is a load that you define and assign in one or more load patterns (see the topics in the Assign - Area Loads, Frame Loads, Cable Loads, Tendon Loads, Joint Loads, Link Loads, and Solid Loads books of this Help), which can then be applied in one or more static or dynamic load cases. Weight can also be used to define mass, which is the purpose of the mass source.
Mass Source is a named definition. Most models have a single mass source that applies to all load cases, and this is the default. Additional mass sources can be created to handle special situations, such as the dynamics of a structure supporting different configurations of heavy equipment, or to explicitly consider the effect of different eccentricities of story mass on the mode shapes.
On the Mass Source form:
Use the Add New Mass Source, Add Copy of Mass Source, Modify/Show Mass Source, and Delete Mass Source buttons to create/modify/delete mass source definitions.
Use the Default Mass Source option to select one mass source as the default to be used for all load cases unless specified otherwise in the load case definition.
The default Mass Source cannot be deleted.
Again, for most models, only a single Mass Source is needed.
Using the Add New, Add Copy, or Modify/Show buttons on the Mass Source form opens the Mass Source Data form for a given mass source definition. Use this form to specify how mass is to be computed for all load cases use this mass source:
Mass Source Name: Used for reference when defining load cases
Mass Source. Choose how to define the mass of the structure using either or both of the following options:
Element Self Mass and Additional Mass option. This is the default. With this option, mass is calculated from the following specifications:
Mass density specified for materials.
Mass specified for link properties.
Additional mass assigned directly to the joints.
Additional area mass assigned to area objects.
Specified Load Patterns option. With this option, mass is calculated from a scaled combination of load patterns (see Define Mass Multiplier for Loads, below). The net load acting downward (in the negative global Z direction) on each element is divided by the acceleration due to gravity, in the current units, and is used for the mass in the three translational directions. See "Detailed Calculations" below. Load patterns should generally represent weight, and should not contain self-equilibrating loads, such as temperature and prestress.
If both options are selected, the total mass is the sum of the mass from each option. Be careful not to include load patterns containing self-weight when using the Element Self Mass, or the mass may be double-counted.
Mass Multipliers for Load Patterns. If the Specified Load Patterns option was selected for Mass Source, enter a combination of one or more load patterns and corresponding multipliers in the table using the Add, Modify, and Delete buttons. These load patterns will be scaled by the specified factors and then added together to determine the mass. Do not include the acceleration due to gravity in the multipliers; the load patterns should represent weight, and the gravity constant will be applied internally.
Detailed Calculations
The mass used by analysis for a given mass source definition is computed as follows:
If the option Specified Load Patterns is selected, the loads acting at every joint are computed from the scaled combination of the load patterns. Only the component acting in the global Z direction at each joint is of interest. This result is then divided by the gravitational constant and applied as equal mass to the three translational degrees of freedom in the joint. Downward loads generate positive mass and upward loads generate negative mass.
If the option Element Self Mass and Additional Mass is selected, the mass from materials, link properties, joint mass, line mass, and area mass are added to the results from step 1. Except for joint mass, these contributions are always positive and equal for the three translational degrees of freedom at each joint. Joint mass can be positive or negative and may be different at each of the joint's six degrees of freedom.
The net result after combining steps 1 and 2 for any degree of freedom may be positive or negative and will be reported as such in the "Assembled Joint Mass" table.
During the equation solution, any negative mass values that remain after combining mass for constraints are set to zero and a warning is issued to the analysis log file.
Using Mass Source
A defined Mass Source can be selected for the following types of load cases:
Nonlinear static.
Nonlinear staged construction
Nonlinear direct-integration time history
If you do not select a specified Mass Source for these types of load cases, the Mass Source used will be the default Mass Source if the load case starts from zero initial conditions, or the Mass Source used in the previous load case if continuing from another one of these same three types of load cases.
For all other load cases, a similar rule applies. The default Mass Source will be used if the load case uses the stiffness from zero load conditions. Otherwise, the Mass Source used will be the same as for the nonlinear static, staged-construction, or direct-integration time-history load case used to calculate the stiffness. Response-spectrum and modal time-history load cases use the Mass Source of their corresponding modal load case.