Here is an approximation of the bridge that appears on the cover of the catalog pictured above. Unlike the version in the picture, this model uses bracket pieces to attach the arched section to the rail bed. All the pieces in this build were available prior to 1994.
The arches are decorative and do not really contribute to the strength of the bridge. This ...
The compression member in your design looks thin to me, and the tension member oversize (assuming they are the top and bottom respectively). I would use string for tension and the technic beams for compression as the first step.
The low-part-count solution that occurs to me is going to kill the clearance under your bridge, but if that's acceptable a single ...
Based on the layouts from this answer, a half-bridge seems to be 2 straight tracks + 1 quarter-circle (split mid-track at the end, because the middle of the bridge is centered with the crossing track below it).
From this guy's measurements, a quarter-circle of the newer DUPLO tracks is about 18x18 studs. Straights are 8 studs long.
So, a half-bridge should ...
I just built the bridge yesterday. I didn't have all the parts so had to be creative at some points.
I'll improve some joints once I picked up more of my Lego at my dads' place.
Very funny project!
Since you mention an engineering class, I assume you probably mean a functional bridge that can be tested for capacity.
This bridge may not withstand a great deal of weight, but it appears in a 1964 LEGO Idea Book and is created only from basic bricks available at that time. (LEGO did issue a more detailed Tower Bridge set a few years ago)
I've used white nylon rope / string for my Lego bridges. When I think of twine I think of the tannish, brownish, rough, fraying type of string. The nylon stuff is cleaner and in my humble opinion appears sharper. A Lego purist would not allow any non-Lego element in a creation, but when it comes to suspension bridges, there are very few options. I've ...
Thanks for your help with this construction. I have made a faithful copy of this 1968 catalog bridge, pics:-
also a list of components. Hope that's helpful. I have a 12 image photo diary of the build ...