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I am always on the look out for good software to help design model bridges. My latest find, Dr. Software, is a series of programs that allow you to customize a design, put a load on it, and see what happens. Dr. Truss comes in 2D and 3D options. You can also get Dr. Beam, which allows you to word with a beam and not a truss. The pictures in this post are from Dr. Truss 2D.
The Dr. Software is very comprehensive and in-depth. You can do a lot of different kinds of analysis on your truss design. One of the coolest features is that all your modifications are shown in real time. Even while you are dragging a joint to a new location the program shows you the changes in the forces and moments of inertia. I think that the Dr. Software can be a serious help to the serious model builder who wants to better understand the physics of truss design. Check out their website: