Rigid Body (/RBODY)

A rigid body is defined by a set of secondary nodes and a main node. It can be compared to a part with an infinite stiffness. No relative displacement is allowed between secondary nodes, and the general motion of the rigid body manages the main node.

As a kinematic condition is applied on each secondary nodes and for all directions, no other nodal constraint is allowed. However, in the case of the Lagrange Multiplier method, the solution can be found if no incompatible kinematic conditions are applied.

Four options are available to position the center of gravity of the rigid body:
Case 1:
COG is computed with secondary and main nodes mass and coordinates
Rigid body mass is the sum of the added mass and secondary node mass
Case 2:
COG is computed with secondary nodes mass and coordinates only
Rigid body mass is the sum of the added mass and secondary node mass
Case 3:
COG is set at main node's coordinates
Rigid body mass is the sum of the added mass and secondary node mass
Case 4:
COG is set at main node's coordinates
Rigid body mass is equal to the added mass

It is strongly recommended to use an artificial node (not part of an element) as a main node, since Radioss Starter is likely to move the main node. The main node is moved by Radioss Starter to the center of gravity; unless the ICoG flag is set to 3 or 4. It is advised to set ICoG to 2 to get the most realistic behavior; the center of gravity is then computed taking into account only the secondary nodes. If ICoG is set to 1, the main node, with its own mass, is included to compute the center of gravity.

If the main node is initially set to the center of gravity, the behavior using ICoG = 1 or 2 is similar.

Figure 1. Center of Gravity Computation
In a car crash simulation, rigid bodies are largely used and three typical uses of rigid bodies can be distinguished:
  • Rigid body covering a part of a finite element model, including shells, solids or other elements: In this case, the mass of secondary nodes gives the total rigid body mass and no added mass is needed. The main node can be located anywhere, and it will be moved to the center of mass. This kind of rigid body saves a large amount of CPU time.
  • Rigid body representing a non-modeled component connected on some structural nodes: In this case, only a few number of secondary nodes are used to connect the rigid body to the finite element model. The mass and inertia are added and the main node is located at the component center of mass. The main node will only move a little, taking into account the mass of the secondary nodes. In some cases, a dummy mesh is used to visualize the rigid body or to simulate the contacts, but if the dummy elements have a small mass, the previous remarks are still true.
  • Rigid body used to connect two or more parts together: For these rigid bodies no added mass is needed and the main node can be located anywhere. A spherical inertia must be used for these rigid bodies; as these rigid bodies are usually very small (4 to 8 nodes), the inertia is often very small in one direction and very large for one specific direction. This may lead to instability; therefore, through the use of spherical inertia, inertia will be identical for any direction.


Rigid bodies can be activated or deactivated with /SENSOR or using the /RBODY/ON or /RBODY/OFF Engine options.

To activate two rigid bodies with the main node 1 and 2, add the following option in Rootname_0001.rad:
1 2
All elements included in both rigid body are deactivated.
To deactivate one rigid body whose main node ID is 3, add:
All elements are reactivated, strains and stresses are those from previous deactivation time.

Rigid Body Motions

One of the main uses of rigid body activation and deactivation concerns the roll-over motions, as in roll-over simulations. During the free flight of the car, the elements deformation can be neglected. A large part of CPU time can be saved if the whole structure is replaced with a temporary rigid body during the fly. Before the impact on the ground, this rigid body is deactivated and eventually activated again after rebound.


Figure 2. Activation - Deactivation of Rigid Bodies in a Roll-Over Example

Initial Static Equilibrium

Another use of rigid body activation and deactivation is for initial static equilibrium, where only gravity is applied. In a crash analysis, it may be of interest to set the whole car in equilibrium over the suspension, and the dummy on the seat. For an explicit analysis, it takes a long time to reach a static equilibrium; consequently using a large rigid body allows a solution to be obtained faster. However, in this case the deformation in a car or in a dummy under gravity is neglected and validity of this assumption needs to be carefully watched.

Figure 3. Static Equilibrium Examples

Merge Rigid Bodies

The /MERGE/RBODY option can be used to merge rigid bodies together or add secondary nodes to an existing rigid body. The secondary entities defined become secondary to a main rigid body. The secondary entities can be rigid bodies, single nodes, or node sets. Multiple secondary entities can be merged into one main rigid body by defining multiple lines in /MERGE/RBODY.

Some use cases include merging two different assemblies defined in two separate include files by defining the merge in the main input file. This can also be useful when separate parts defined as rigid need to be merged into one rigid body to model a complex component like an engine.

The main rigid body defined in /MERGE/RBODY can be defined as a secondary rigid body in another /MERGE/RBODY. However, complex hierarchies should be avoided as they can become difficult to debug. The secondary entities can only be defined in one /MERGE/RBODY and not in any /RBODY; otherwise, incompatible kinematic conditions occur.

Before merging, the inertia, mass, and center of gravity of each the secondary and main rigid body is calculated based on their /RBODY properties. Next, the secondary entities are merged to the main rigid body and new rigid body properties are calculated based on the /MERGE/RBODY Iflag option.