Milan Branch of the Iowa Interstate RR

Category: Planning

Lattice girder development

Work is progressing with the lattice  truss bridges. My friend, Clark C., who’s helping me with the bridge design and fabrication is doing a fantastic job. I really appreciate his talent.

We decided the best approach would be to design and laser the girder structures for the two spans first so that I can put them in place to get the track operating. Clark has designed the bridge so that the side trusses can be fabricated separately and then attached to the girders at a later time.

Of course, once one of the trusses is complete to our satisfaction, the other three will be easy to create.

I’m very excited about these bridges because they will be my signature scene on the layout. Lattice truss bridges are not modeled very often, so they will be a truly unique feature.

Lattice truss bridge spans

Below is a picture of two scale cardboard mockups showing the lattice truss bridges which will span the Rock River.

Lattice truss bridges are not modeled very often, even though there are many examples of that type of bridge throughout the country.

The photo also shows the location of a speaker which will be embedded into a small river island and will play moving water and bird sounds. Attached is a small snippet of the sound which will loop once installed.

 

A friend, Clark C., is helping me create the custom bridge spans with his expertise using lasers. He first mocked up a schematic for each of the different lattice configurations based on an Illustrator file I sent him (below). The spans have several distinct horizontal lattice designs which include angle iron and I-beams in two different crisscrossing reinforcing lattice patterns. In addition, those designs alternate between the inside and outside of the main truss. This creates a very interesting and somewhat complex fabrication challenge.

We will be developing the depth of each side span in “layers” similar to 3D printing. This will allow the unique open areas (see image below) between the main lattice members to be rendered accurately. At least, that’s the idea. 🙂

Clark mocked up a test section to see how the laser would render a portion of the truss and supporting lattice.

Below is what the computer rendering looks like. Each tone represents a different depth of cut, and the laser can be sped up (more intense ablation) or slowed down (multiple passes for better detail) to achieve different quality levels.

By their design nature, turn-of-the-century lattice truss bridges are delicate looking which makes them unique but also challenging to construct. We’re hopeful that the “layered” approach will allow those delicate cross members to be modeled faithfully without the potential for bending or warping along their length.

Clark discovered that the laser cuts did warp the material as shown below. But by applying oven heat to slightly ease the material and then applying weights as it cooled resulted in flat, uniform pieces.

Below you can see some of the cross members attached to the main truss and weathered for effect.

Clark will be creating some additional cuts to see if the material can be thinned down even more. So far, it looks very promising for these unique spans. Stay tuned!

 

Layout plan

The layout room is a modest 16 x 19 feet with the layout running along the walls – 24 inches off the wall. (Move the cursor over the plan to zoom in.)

The Turnout runaround is in Rock Island, IL, and acts as fiddle staging. Moving south the line services three different industries before crossing the Rock River and entering Milan, IL.

In Milan, the branch splits at the town’s namesake depot with part of the line moving east (Eagle Sub) to service a food distributor and a lumber dealer. The other extension continues south (Andalusia Sub), under Interstate 280, and then turns west to service a number of different companies.

Below is a Google aerial view showing the modeled and staging segments of the layout.

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