This past weekend I built and installed a new timber trestle over Big Rock Creek on the line running through Red Rock Canyon. Until now, the track had been crossing the chasm on a temporary bridge. The temporary bridge was simply a continuation of the PVC ladder roadbed across the opening. It didn’t look very good, but it did allow me to get trains up and running on this section of the layout. Like many of the things on my “to-do” list, I kept putting off building the trestle. Here is what the temporary bridge looked like on Friday:
I won’t go into the details about how I built the new trestle in this post since I have previously written articles regarding trestle construction. This post will explain my bridge-building philosophy and only show the things I did differently on this bridge. Here are links to a couple of my previous articles on trestles.
model trestle article - http://www.rhyman.org/articles/new-bridge-44b-at-red-rock-creek
prototype trestle article - http://www.rhyman.org/articles/trestles-rgs-style
Garden railroading, by its very nature, is a mix of compromises. When I began building wooden trestles for my Fn3 (1:20.3 scale, 3-foot gauge) layout, I first tried to replicate the bridges I had previously done in the smaller indoor scales. I quickly learned that a few changes in modeling techniques were necessary to enable these scale structures to survive in the 1:1 scale environment. Building a model structure that is capable of withstanding the wrath of Mother Nature and still appear prototypically correct is not as easy as one might think. As I worked on methods to counter the combined forces of gravity, wind, blinding sun, rain, snow, sleet, hail, and even the occasional earthquake, I found myself moving ever closer to actual prototype materials and construction practices.
One of the most obvious compromises is in the selection of suitable timber sizes. For example, 12” square timbers are commonly used for posts and sills on prototype trestles. In F scale, this equates to 15mm or 0.591 inches. I attempt to build my F scale trestles using prototype dimensions and construction practices insofar as possible. I use western red cedar for trestle construction. As a general rule, I use metric measurements for posts, sills, and ties because these sizes are readily available commercially in F scale. For the lumber that I mill from cedar fence boards, such as stringers, guard timbers, girts, and sway braces, I like to use fractional measurements wherever possible for convenience. The following table shows the sizes I use to represent prototype bridge timbers in 1:20.3 scale.
The timber components in prototype trestles are connected with metal fasteners including:
- long nails (called boat spikes) - ⅜” dia. x 9” long
- steel rods (called drift pins) - ³⁄₄” dia. x 24” or 30” long
- lag screws - ⁵⁄₈” dia. x 12” long
- steel bolts - ½” dia. x 30” long with washers and nuts
I use a Grex 23 gauge pneumatic pin nailer to connect the model timber pieces together. These pins have a nominal diameter of 0.025 inches, which equates to ½” in F scale. Pin nails are available in various lengths ranging from ½” to 2”.
The one significant change I made on this bridge was to use strips of aluminum between the stringers. Each of the four strips used on this bridge is 1/8” thick by ¾” wide. The two outer strips are 48” long and extend about five inches past the ends of the stringers so that they can be attached to the PVC ladder roadbed. The two inner strips are 960mm long (37.8”) to match the length of the stringers.
The top edges of the strips are painted flat black. The aluminum strips are bonded between the cedar stringers with Gorilla™ construction adhesive. The bottoms of the aluminum strips are flush with the bottoms of the cedar stringers. The tops of the strips are recessed 1/8” below the tops of the stringers. The two innermost stringers are spaced ½” apart.
I spiked 6-foot lengths of painted Code 215 aluminum rail to the bridge ties. Here is a disclaimer before someone says “Better get some guard rails on that bridge!” I follow Rio Grande Southern standards (boy, that’s an oxymoron) where the guard rails were generally placed 8 to 9 inches outside of the running rails to avoid interference with the blade flangers.
Here is a close-up of the deck showing the running rails (with shiny tops) spiked with 8 spikes per tie, and the guard rails (unpolished tops) outside of the running rails and spiked with 8 spikes every fourth tie. The N-B-W castings om the guard timbers are from Ozark Miniatures help make the trestle look more realistic.
Here is the deck from the underside:
I threaded Llagas Creek plastic tie strips onto the excess rail at each end. Here is how the transition looks from the top.
And here is how the transition looks from the underside:
The actual weight of the bridge and any live loads are transferred from the stringers via the aluminum strips back to the PVC roadbed.
I removed the flex track on the temporary bridge and cut out a section for the new trestle. I drove a PVC stake into the ground between the splines of the ladder roadbed on each side and touched-up the cut edges with spray paint. Here is what it looked like with the section removed:
I inserted the trestle deck and attached the aluminum strips to the ladder roadbed.
The bents under the trestle are basically for appearance only and do not actually support the deck. I used a 2½ in 12 batter for the angled bent posts. Here is one of the bents:
And here is one of the abutments. The backing boards behind the end bents are scraps of ¼” x 3/8” cedar.
After the bents were installed, I re-contoured the slopes at each end of the trestle. I filled-in behind the abutments and beneath the bents with decomposed granite.
The completed trestle is 960mm long (37.8”) which equates to 64 scale feet in F scale. It is tangent (straight), and on a 2½% grade. Since the trestle is on a grade the bents, which are vertical, are not perpendicular to the deck.
On my last project, Ken commented that there were no trains in the photos. Okay Ken, I get the message. The following photos are for you and any other RGS narrow gauge enthusiasts out there.
C-19 #41 thunders upgrade through Red Rock Canyon. This train has the honor of being the first revenue run over the new bridge.
And waiting patiently on the mine spur while the uphill freight passes by, Climax #6 awaits her turn to be the first freight downgrade across the new bridge.
Here are the coal loads behind the Climax on the span.
Later, Motor #7 has the honor of being the first mail and passenger run across the bridge.
Until the next project strikes,
Bob
