The section of the Third Avenue Bridge that was within my layout perimeter was completed a long time ago. I could have left it with that, a girder bridge ending on the eastern rest pier of the swing bridge. Modeling the complex web of a truss bridge was out of reach for me.
After my recent work with a scratch-built structure using laser cutting, I gave a second look at the swing span. And when my wife Karen encouraged me to build it, and the Covid-19 lockdown in spring 2020 gave me plenty of time, I decided to give it a shot.
Which Bridge Configuration?
I hit the first roadblock when I realized that I’d have to design the bridge based on photos only. Fortunately, I found a sketch of the old neighboring Willis Avenue Bridge replaced in 2010. The swing span section of the bridge has a very similar quadrangular truss design with a smoothly curved top chord. It is only 304 ft long which is a bit shorter than my prototype (364 ft). That’s even better for me. My compressed N scale version of the Third Avenue Bridge would still be almost 23″ long. What about the width? The original bridge from 1898 consisted of four trusses enclosing three sections that carried two roadways and two trolley tracks in the middle section. When the tracks were removed in the mid 1950’s, the bridge was reconfigured to only two sections carried by three trusses.
Since I included the tracks on my approach bridge on the Bronx side, I decided to go for the original four truss version. The swing span section would be wider than the girder bridge, and in the design I’d also have to include the sidewalks outside of the trusses. I’d also make the two roadway sections slightly smaller than the one in the center with the tracks to make them fit into the rest pier I already had (though I would revise that in a later version in favor of three equally wide sections).
To keep the proportions, I’d stick to the sketch of the Willis Avenue Bridge even though the Third Avenue Bridge seemed to be slightly higher. Finally, I didn’t have to worry about the weight: The Third Avenue Bridge must have been the heaviest bridge in the world when it opened in 1898. In contrast, my bridge would be built entirely of 1/8″ MDF, laser board, and Styrene.
Truss Design
My first laser cut job for the bridge gave me a good idea how far I could go with details. The 1/8″ truss frame was sturdy enough to hold the members which are cut out of .023″ laser board. But the resulting truss looked very flimsy and incomplete and would not be rigid enough to carry the operator’s house that sat in the top center of the bridge. Also, the truss members were unrealistically flat, and I wanted a more 3-dimentional look that was closer to the laced struts of the prototype. So I went back to the drawing board and redesigned the members.
First, I added an outer layer of laser board that would cover the truss frame but that also would support the web. Visually, it would provide the gusset plates which cover the connections of the members to the truss. Furthermore, the covers’ connections aligned with the diagonal member beams would create U-channels. These looked now much more prototypical like the struts of the bridge. I could tab both the cover and the diagonal member so that both would form a properly functional laced strut. Also, my design required to glue the cover layer firmly to the MDF truss frame. Additionally, I’d glue the diagonal members down to the few diagonal connections of the cover. The cover layers on the front and back then work as an envelope, tightly enclosing the struts. This makes the construction very rigid.
An Improved Design
Batch 2 of laser cut components for my bridge definitely proved the concept. Gluing the member struts into the frame required patience but was rewarding. I literally had to weave the positively sloped into the negatively sloped members. Based on my first test design, I chose to tighten the interlocking of those two member types. The result was a tighter fit with no loose members. At the same time, it made the construction a little more complicated as I had to follow a specific order when adding the members.
Visually, the resulting truss looks now more appealing. It has depth, appears realistic and closer to the prototype even if it is not a perfect match. There is certainly room for more added details once all trusses have been assembled. In addition, the glued and painted frame is now sufficiently strong for this project.
Building the Trusses
With the trusses at the center of the bridge construction, I’d spend countless hours fitting and gluing the truss members into the frame. Half frame by half frame, eight times until I had all four trusses completed. I’d start by gluing the bottom side of the laser board envelope to the frame. Then, slowly working from the end to the center, I’d add first the enclosing member struts. Once I had these glued to the frame, I’d go back to the end and add the enclosed members. I’d have to carefully insert them through the interlocking hole, then turn them until perpendicular to the envelope. When in place, I’d hear when the tabs snapped into their corresponding notch in the envelope.
Once verified that they were sitting properly in their position, I’d glue them down. With all members glued to the frame, I then could glue the edges of the member struts to the envelope. Additionally, I would also secure the interlocking between the diagonal members with glue. To finish, I’d add the top envelope to the frame and its members.
Having repeated the process of building half a truss with its members eight times and giving them a first coating of primer, I now had complete raw structure. The result looked impressive and much better than I expected. I was very happy with level of detail and the proportions achieved with my laser-cut design.
Next Steps
The bridge, however, is far from being complete. I yet have to add the operator’s house, the driveways on the decks, and the sidewalks with railings. The bridge also misses important details, like lateral bracing on all three segments. And I also intend giving the trusses more details and depth by adding separately applied gusset plates and an additional layer on the top chord.