N Scale Workshop

Introduction

In the previous post of the Prairie River Bridge construction series, I had finished adding the remaining details to the end of the bridge. With the main structure completed, I will now be building the wooden deck and walkways that sit on top. The original bridge had ties bolted directly to the floor joists and would have had plank walkways of some sort. At some point, the ties were replaced with a wooden ballasted deck with metal walkways and hand railings on both sides. Ballasted decks are often added to make it easier to transition to approach tracks and minimize impulse loads on the structure caused by flat spots on metal wheels.

The prototype ballasted deck was created by bolting 12-foot lengths of 12" x 6" planks side-by-side to the floor joists between the floor beams. Layers of stringers were then bolted to the planks to create an 18"-deep "tub" to hold the ballast. Longer 20-foot planks were spaced roughly six feet apart to support the walkways and hand railings.

Decking

One scale foot-wide planks are stripped from 1/32" basswood. After lightly sanding the strips, 106 pieces of scale 12-foot lengths and 22 pieces of scale 20-foot lengths are cut from the strips using a "Chop-It" from Micro-Mark as shown in Figure 1. Additional pieces are cut to fir from the strips later for the stringers and supports for the walkways.

Before assembling the deck, the wood pieces are stained with thin washes of DecoArt burnt umber and raw umber acrylic followed with a wash of charcoal gray to give it a weathered creosote tint as shown in Figure 2. The stained pieces are allowed to dry thoroughly between coats to give them a random look. It is important to stain wood before assembly since stain will not soak into any areas covered with glue.

The deck is laid out on top of a CAD drawing created using QCAD Community Edition Placement of the long planks was estimated based on photographs of the prototype. Separating the long planks by either five or six of the shorter planks allows the 22 long planks to be distributed evenly over the length of the main bridge span.

Deck planks are glued to stringers using Zap canopy glue as shown in Figure 3. Gaps in the decking allow space for the deck to fit over the floor beams. The deck is test fit onto the bridge during assembly to allow for slight differences in the floor beam spacing. The deck should fit snuggly but not too tight to prevent chipping the paint on the floor beams.

After the basic decking is assembled, an additional two layers of stringers and scale four-foot lengths of strips are added on top of the existing stringers and the ends of the long planks. The pieces are alternated and overlap each other to create a scale 18" deep "tub" for the track ballast as shown in Figure 4. The top of the short pieces should be one layer lower than the tops of the stringers to allow the tops of the walkways to be level with the tops of the stringers.

Walkways

Graco 60-mesh paint sprayer filters from Home Depot like the one shown in Figure 5 were purchased to provide the material for the walkways. Fine mesh screen can also be purchased online from various industrial and scientific suppliers. Two of the paint sprayer filters provide enough material for both sets of walkways. The screen is separated from the filter tube and cut into scale 18" strips using an old pair of scissors.

0.020" strips are cut from 0.020" styrene sheet from Evergreen Scale Models to create stringers for the walkways. The strips are glued to the bottom sides of the screen using canopy glue, being careful not to clog the holes in the screen with too much glue. The strips allow the individual pieces of screen to be assembled into a continuous piece the length of the deck. Four lengths of walkway are assembled, two lengths for each side of the deck.

The styrene strips on the bottom of the walkways are given several thin coats of Mithral Silver and a drybrush of Boltgun Metal from Citadel Colour. Note that these colors are no longer available. Equivalent colors would probably be Grey Knights Steel and Leadbelcher. Most silver/grey metallic acrylics are suitable. After they are dry, the lengths of walkway are glued to the supports on one side of the deck using canopy glue as shown in Figure 7.

Conclusion

The remaining walkways are attached to the supports on the other side of the deck as shown in Figure 8, completing the wooden deck. In the next post, I should be able to wrap up the Prairie River Bridge construction series by adding the handrails and concrete support piers just in time for AP evaluation at the NMRA regional convention next month.