A Nodal Weight (or lumped mass) is a load that can be applied to a node in most linear and nonlinear structural analyses. It is used to represent the mass of an attached part or subassembly that is not included in the actual model geometry.
A Nodal Weight can be used to resist the translation or rotation of a node. In order to resist rotation, the Nodal Weight must be attached to an element that supports rotational degrees of freedom (such as beams, plates, and shells). For elements with five DOF, such as plates (which lack the rotational DOF about the axis normal to the element), a Nodal Weight will only effectively provide rotation resistance about the other two axes.
Apply Nodal Weights
If you have nodes selected, you can right-click in the display area and select the Add pull-out menu. Select the Nodal Weight command. You can also access this load via the ribbon command, Setup 
Loads Weight.
Select the appropriate radio button in the Mass Input section to determine if the Nodal Weight input values are defined in units of force or mass (mass = weight/gravity). For nonlinear analyses, only mass units are permitted (the Units of force selection is grayed-out).
If the Nodal Weight is equally effective in all translational directions, activate the Uniform check box and specify the magnitude of the mass in the X Direction field of the Mass/Weight section. Typically, a mass will be assumed to act equally in all directions for the majority of applications. If the mass/weight is assumed to have different magnitudes along the three translational directions, deactivate the Uniform check box and specify the appropriate values in the X Direction, Y Direction, and Z Direction fields in the Mass/Weight section. For example, if a mass (like a car) is sitting on wheels on a ship deck, you may want to assume that deck motion along the direction of travel does not effectively accelerate the mass of the car in that direction (because the wheels rotate easily). In such cases, enter zero or a reduced magnitude for that direction.
If the Nodal Weight is to be effective in rotational directions, specify the appropriate values in the X Direction, Y Direction, and Z Direction fields in the Mass Moment of Inertia section. Here, with the exception of spherical objects or regular cubes, the mass moment of inertia will normally vary considerably about the three axes.
How the weight/mass and mass moment of inertia behave in the various linear analysis types, for global or local coordinate systems, and when negative values are specified is summarized in the following table:
| Analysis Type | Mass Weight | Mass Moment of Inertia | |
|---|---|---|---|
| Static Stress with Linear Materials | Global Coordinates |
|
With centrifugal acceleration, inertias are converted to torques by T i =I i x α i , where i is the X, Y, and Z directions and α is the angular acceleration. |
| Local Coordinates | Mass behaves as if input is in global coordinates, not local coordinates. | With centrifugal acceleration, inertias are converted to torques by T i =I i x α i , where i is the appropriate direction and α is the angular acceleration. | |
| Negative Mass or Weight Input | Negative mass/weight and mass moment of inertia input values are converted to positive values during the solution. Therefore, results are identical for positive and negative values. A warning message appears in the analysis log for each value converted. | ||
| Linear Natural Frequency (Modal) | Global Coordinates | Masses follow global coordinate system and effect the vibration in the corresponding direction. | Inertias follow global coordinate system and effect the vibration in the corresponding direction. |
| Local Coordinates | Masses follow local coordinate system and effect the vibration in the corresponding direction. | Inertias follow local coordinate system and effect the vibration in the corresponding direction. | |
| Negative Mass or Weight Input | Negative mass/weight and mass moment of inertia input values are converted to positive values during the solution. Therefore, results are identical for positive and negative values. A warning message appears in the analysis log for each value converted. | ||
| Linear Natural Frequency (Modal) with Load Stiffening | Global Coordinates |
|
Inertias follow global coordinate system and effect the vibration in the corresponding direction. |
| Local Coordinates | Local coordinate systems not supported. | Local coordinate systems not supported. | |
| Negative Mass or Weight Input |
Negative mass/weight and mass moment of inertia input values are interpreted as positive FORCE values of the same magnitude, regardless of the Mass/Force Units selection. No warning messages are produced in the analysis summary or log files. In other words, if you specify a mass of -500, the result will be the same as for a positive force of 500. The resultant nodal load will differ by a factor of g. Recommendation: To avoid confusion and potential model setup errors, do not enter negative input values. |
||
| Critical Buckling Load | Nodal Weights are not supported for this analysis type (neither masses/weights nor mass moments of inertia). | ||
| Transient Stress (Direct Integration) | Global Coordinates | Inertial effects follow the global coordinate system. | Inertias follow global coordinate system and effect the motion in the corresponding direction. |
| Local Coordinates | Inertial effects follow the local coordinate system. | Inertias follow local coordinate system and effect the motion in the corresponding direction. | |
| Negative Mass or Weight Input | Negative mass/weight and mass moment of inertia input values are converted to positive values during the solution. Therefore, results are identical for positive and negative values. A warning message appears in the analysis log for each value converted. | ||
| Nonlinear Analyses | Global Coordinates | Inertial effects follow the global coordinate system. | Inertias follow global coordinate system and effect the motion in the corresponding direction. |
| Local Coordinates | Inertial effects follow the local coordinate system. | Inertias follow local coordinate system and effect the motion in the corresponding direction. | |
| Negative Mass or Weight Input 1 | Negative masses/weights produce reaction forces in the opposite direction relative to positive input values. For example, a force in the direction of the gravity vector acts on a positive nodal mass when gravity is applied to the model. For negative values, the force will act in the opposite direction from the gravity vector. | Negative mass moments of inertia produce reaction moments acting in the opposite direction relative to positive input values. For example, a positive inertia produces a torque that opposes the rotational motion of the mass. For a negative value, the torque acts in the same direction, assisting the rotational motion. | |
1 Note: While negative masses, weights, or mass moments of inertia have theoretical significance, and their effects can be quantified in a nonlinear analysis, there really are no examples of this behavior in nature. Therefore, you will only need to use positive nodal weights when modeling real-world phenomena.
Comments Regarding Linear Dynamics Restart Analyses:
Frequency Response, Random Vibration, Response Spectrum, Transient Stress (Modal Superposition), and DDAM analyses are all based on a prerequisite modal analysis. The un-scaled response of the structure (vibration mode shape results) are scaled according to the specified excitation (ground motion) and other applicable loads. Even though nodal weights cannot be applied in these analysis types, their effects can be included in the initial modal analysis. Therefore, nodal weights affect the restart analysis results, because the natural frequency results on which they are based are affected by the nodal weights.
Note that load stiffening effects due to gravity are not calculated for nodal weights in a Natural Frequency (Modal) with Load Stiffening analysis. Therefore, you will have to apply a nodal force in the direction of gravity in addition to each nodal weight to account for this effect. Be sure to run a Natural Frequency (Modal) with Load Stiffening analysis first, if you want load stiffening effects to be represented in your restart analysis results.