I had to dremel the 5mm pegs on the girders to get them to fit correctly, but now I can try them out for the first time.  They fit well as I able to support a hex piece with just two girders (all that I have so far).

The girders have a 5mm peg on the bottom and top.  The top of the girder also has a lip to secure it to the inside edge of the hexagon piece.