Use the Fltprops.inp file, located in the \profil folder under the environment folder ..\ProgramData\Trimble\TeklaStructures\<version>\environments\, to define flat bar thickness, width and material. The exact location may vary depending on your environment.

The first row in the file contains flat bar material definitions (enclosed in quotes " ", and without spaces) followed by plate thicknesses. If you do not define a material, you can use all materials for all flat bars. The following rows define the widths of available flat bars.

The units are millimeters.