Section Definition
A section is a cut in the structure where forces and moments will be computed and stored in output files.
It is defined by:

Figure 1. Definition of a Section for an Oriented Solid
- A cutting plane
- A reference point to compute forces
- A direction of the section.

In Radioss the cutting plane is defined by a group of elements and
its orientation by a group of nodes as shown in Figure 2.

Figure 2. Definition of a Section for a Shell Mesh

Then, a point is defined for the center of rotation of the section and a reference frame is
attached to this point to compute the internal efforts.

Figure 3. Center of Rotation and its Associated Frame for a Section

The resultant of all forces applied to the elements and its application point are computed by:
F=∑fi
M=∑mi+∑ONi×fi
