Building a Large Curved Trestle on a Grade – Part 5

Part 5 – Building the Beam Assemblies

The Beam Assemblies

There are two beam assemblies each made of eight 3/8” x 1½” beams spaced equally with angled braces as shown in Figure 1. These rest on short bents attached to a full-height bent. Bents #8 and #12 rest on these beams. Two additional beams run between the main assemblies, running through bent #10.

Figure 1 Beam Assembly

Figure 1 Beam Assembly

As there are only two of these I will not build a jig. Each beam has a 7.5-degree angle at each end, matching the bent angle. A full-size paper plan was glued to acrylic with a washable glue stick to keep it flat. The cuts are measured directly from the plan by putting a beam in place and marking the location. I also used some ½” square stock cut at the same angle for blocks to reinforce the beams and provide attachment points to the support bents. Photo 1 shows beams and blocks on the plan.

Photo 1 Beams and Blocks

Photo 1 Beams and Blocks

The beams and blocks are first glued in pairs, then the pairs are glued and finally the two halves are joined as shown in Photo 2.

The beam assemblies were then glued and nailed to the support bents. Note that the beams are NOT square to the bents – the bents are square to the workbench. Finally, the bent resting on the beams (also not square to the beams) is glued.

Photo 2 Beam Assembly

Photo 2 Beam Assembly

Adding Girts

Photo 3 Bent 7, 8 and 9 Assembly

Photo 3 Bent 7, 8 and 9 Assembly

Following RGS practice (on this bridge), there is 1 girt in the center and 1 near each end of every sill except the mud sills. So there will be 3 girts per story between each pair of bents. Because this bridge is curved, every girt will have angled ends (3.75 degrees), and mostly different lengths. The ends of each girt is notched a bit to better hold the bent in position. To accurately measure the length, a 3/8” square girt is positioned completely covering one sill, and marked where it crosses the other sill. I used a mini-hacksaw to cut a slit 5/16” from each end and a hobby knife to make a vertical cut. A quick pass with sandpaper finishes the notch. In theory, I should be able to mass-produce each size I need, but in practice I’ll not be surprised if each one is unique.

Figure 2 Beam Detail

Figure 2 Beam Detail

This assembly is fairly rigid. The bent 7, 8 and 9 assembly is shown in Photo 3.

Cement Board Base

Figure 3 Base Layout

Figure 3 Base Layout

Transfer plan Figure 3 to 2’ x 3’ cement boards. Use rip

ped Trex 2×6 for footings glued down with Gorilla Glue. Before gluing footings, use the lines to locate and drill holes from the top for screws driven from the bottom to secure the footings. Drill holes in the mud sill and the Trex for brass pins. Photo 4 shows the base after gluing and screwing the Trex footings.

Beam Assemblies

Photo 5 shows the two complete beam sections ready for assembly in the railway. Photo 6 shows the prototype bridge beam sections.

Photo 5 Beam Sections

Photo 5 Beam Sections

Photo 6 Prototype

Photo 6 Prototype

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