- Add a straight truss in your project - select this truss - goto properties of the selected item - below the list you can set circle angle and circle diameter This way you can crreate any curved truss type.

Dear Alfred,

Although you can do what Luc suggested please be aware that the truss pieces actually already exist in the library. These are the actual pieces that Eurotruss sells and can build up different circles. Please make a search for "FD34" and you will see them by having a number for the degrees and also the diameter of the circle they create.
For example: "FD34 Rectangular section, 90° / 4m (FD-34-C0400P4)"