I had a similar situation when I tried to design dockside unloading cranes for my container ships. I work with sheet styrene and a digital cutter. I spent a week designing the cranes, and trying various combinations that might approximate L, I and box beams. When I built the first prototype, I quickly realized a crane from Plastruct shapes would be far easier and stronger, albeit more expensive, than constructing my own faux beams. I abandoned the project.
Plastruct (or Evergreen) X- or V-braced beams are very expensive. I think K&S used to make brass beams (one side X-braced and the other solid, so two were required for a beam) at a more reasonable cost, but I haven't seen them for a long time--well since I last visited my LHS in Albuquerque about five years ago. I think Traincat2 used to offer the makings for beams, but I haven't seen him for a few years.
I have been thinking of commissioning a run of brass girders of various sizes for a line of bridges. Like K&S, these would be flat and need to be bent lengthwise. Right now I need to recover some of my investment in N scale ships before launching anything new. (Hint: I included a few beams in my last photoetched brass order, and they came out well.)