# 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:

- 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.

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.

The resultant of all forces applied to the elements and its application point are computed by:

$$F={\displaystyle \sum {f}_{i}}$$

$$M={\displaystyle \sum {m}_{i}}+{\displaystyle \sum O{N}_{i}}\times {f}_{i}$$