Part 1: http://www.trainsanddioramas.com/ultimate-scrap-building-bascule-bridge-introduction/
Part 2: http://www.trainsanddioramas.com/scratch-building-a-lift-bridge-in-n-scale-design/
Part 3: http://www.trainsanddioramas.com/bascule-lift-bridge-the-concept-of-scrap-building/
The Rocking Truss
This is a little more complicated than the leaf as there is no straight, square angle in this structure, and it is highly important to use the reference drawing to cut each section. Photos 9 and 10 show the starting points for building the central base. Notice that that the longer arms on the side have two holes drilled in them. These will be linked (not riveted/joined like we did for the leaf), with the tower to create the pivot on which the truss will move: more on this later. You can also see the top and bottom end-posts cut to size in Photo 10.
The next step is to create the triangular parts of the rocking truss. In Photos 11 and 12 you see how each side is fabricated by cutting each beam precisely as dictated by the design. One of the best practices that I discovered while building truss bridges is, once you lock in the perfect dimensions for one assembly, use that as a template for all similar assemblies. In this case, once one triangle was fixed, I used the first two beams as templates to make the beams of the other three triangles (Photo 12).
In Photo 13, you see how one side of the base structure of the rocking truss is made. The same method is used to create the triangular sides on the other end of the central base; I then used crossbeams to strengthen the structure and bring it to completion (Photo 14).
At this point, the overall structure has started to take shape, and you can start finding similarities between the components with the front view in Sketch 3.