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 ...
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 ...
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!
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 ...