Editing in 3D - Autodesk AutoCAD 2013 Practical 3D Drafting and Design - page 176

1. Open the drawing A3D_06_02.DWG . This drawing contains two beams,
one with a rectangular section and the other with a circular section. From
these we are going to create the structure of a roof.
2. These elements were created on the plane. First, rotate them so that the
rectangular beam has its lower section on the plane. Apply ROTATE3D ,
and as the axis of rotation, specify the lower vertices of the section and
the angle is 90 degrees. Notice that the lower face of the rectangular
beam must be on the XY plane.
3. To complete the first beam structure, apply the MIRROR3D command
three times. For the first application, select the beam, define the mirror
plane by the three top vertices, and do not delete the source object. Then,
apply the command two more times, selecting one or two beam elements
and defining the mirror plane by three endpoints or three midpoints.