CI am constructing three truss bridges. Two are 21-1/2 inches long and the center one is 54 inches long. I am going to build the two small bridges first. I am using Plastruct H and I beams. The main columns are 7/16 inch H beams. The diagonal bracing is 7/16" I beams. The top cross beams are 1 inch I beams and the bottom cross beams are 1-1/8 inch I beams. The top beams are 7/16 inch H beams constructed from ABS sheet. Each beam is three pieces.
Here are some pictures.