Here are photos for the kit and the instructions.
Total of five pieces.
Two pieces doe side walkways and fillers between the rails.
Two pieces with main girder, internal crossmembers, and walkway supports and railings.
Support brace with concrete pads.
Looking at the parts and the instructions there is no fixed place to position the support. It does not need to be in the center or even present. The parts are nicely detailed. The stiffeners and angle iron riveted to together and to the steel plate web.
Jan