Relander Modelworks

Introduction

In this article, I will be assembling a test girder for the Prairie River Bridge project. This girder was originally going to be a proof of concept, but it might make it into the final model. Everything is fabricated from 0.010" Evergreen styrene strips and sheet. I tried to use commercially available pieces as much as possible, but I had to build up custom channels from strips to get the correct dimensions.

The bridge was originally built in 1899 and was strengthened in 1929 with the addition of a third truss line down the center. Although each girder was constructed using standard methods at the time, they have an odd mix of designs. Some use zigzag lacing, some use standard cross lacing, and other use overlapping laces. I was originally thinking about using the N scale girders available from Central Valley Model Works but decided to scratch build the girders to match the unique lacing and plating patterns on the prototype as closely as possible.

Side Channels

The webs of the side channels are cut from scale 9" wide strips of 0.010" styrene using a CAD drawing of the side view of the girder created in QCAD. These need to be stripped from sheet styrene since they are not commercially available. The angles on each end of the web have different angles and must be cut precisely since they are a critical part of the trusses. Side channel flanges are cut from 0.030" strips of 0.010" styrene. Each flange is cut longer than necessary and then trimmed to fit after gluing them to the edges of the side channel webs as shown in Figure 1. The angle of the cut must match the angle of the ends of the webs. The flanges should be parallel to each other and perpendicular to the webs.

Back Plate

Back plates are cut from scale two-foot-wide strips of 0.010" styrene. Two feet in N scale is 0.150" which may be available commercially, but they are wide enough that they are easily stripped from 0.010" sheet material. The back plates are cut longer than necessary and trimmed to fit after attaching the side channels. The flanges of the side channels are glued to the flat side of the back plate along the edges as shown in Figure 2. The side channels should be parallel to each other and perpendicular to each other, After the side channels are attached, the back plate is trimmed to match the angles on the ends of the side channels.

Laced Face

Rather than add lacing and other details to the remaining side channel flanges as would have been done on the prototype, the flanges are removed from the side channel webs and new ones are cut from 0.030" x 0.010" styrene strip. Future side channels will be constructed with the flanges already removed.

The new side channel flanges are taped over a CAD drawing of the laced face of the girder created using QCAD. Stay plates are cut from scale two-foot-square pieces of 0.010" sheet styrene scrap and glued to the side channel flanges over the CAD drawing. Lacing is fabricated from strips of 0.020" x 0.010" styrene which is glued to the flanges and cut to length as it is added. The bottom lacing is added in one direction and then top lacing is added in the other direction, attaching to the ends of the first lacing as shown in figure 3.

The tape holding the laced face to the CAD drawing is removed and the assembly is carefully removed from the paper. The flanges may stick to the paper if excess glue from attaching the lacing gets between the paper and the flanges. It may help to use a layer of wax paper over the CAD drawing during assembly. The lacing is trimmed to length and cleaned up as shown in Figure 4. The entire assembly is lightly sanded on both sides, taking care not to damage any of the delicate lacing or bend the flanges.

Conclusion

I fastened the flanges on the laced face to the edges of the webs, making sure that everything was square and that the side channel webs were parallel as shown in Figure 5. It was noted in a Facebook discussion that I should have painted the inside of the girder during assembly. I was hoping to use this girder in the final project but will probably just use this one as a learning lesson and paint the insides on the new girders. Rivet decals from Archer Fine Transfers and several thin layers of base coat will be added later to prevent accidental damage during truss assembly. Next, I will be building more girders before starting to assemble them into trusses.