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Clinic - Building Wooden Trestles

Building & Bridge Department

Report:  Bridge 1 (Cold Springs Trestle)

List of Materials:

Wood:  Basswood 
All posts and caps 8”x 12”  Northeastern Scale Lumber Company # HOSCAL81211
Cross braces  3”x 10”  Northeastern Scale Lumber Company # HOSCAL31011
Horizontal braces     6”x 12” Northeastern Scale Lumber Company # HOSCAL61211
Girts 6”x 8”   Northeastern Scale Lumber Company # HOSCAL6811
Stringers  6”x 20”  Open Stock from hobby shop
Bridge ties  8” x 8”   Northeastern Scale Lumber Company # HOSCAL8811
Platform railings  2” x 4” Northeastern Scale Lumber Company # HOSCAL2411

 

Paints & Stains:
Alcohol & Ink   5 Tsp of Black India ink in a pint of Rubbing Alcohol.  
Base coat    Antique Brown  Ceramcoat 
Mixed stain     ½ Tsp Drizzle Grey   Ceramcoat
  2 Tsp Antique Brown Ceramcoat
  3 Tsp Dark Chocolate Ceramcoat
  ½ Tsp Opaque Black   Createx
Rust (NBW) Cinnamon Ceramcoat
Rust (rails)  95% Rail Brown   Floquil
  5% Rust Floquil

 

Castings:    
Nut, Bolt, & Washers  3” Nut, 4 ½” Washer   Grandt Line #5099
Nut, Bolt, & Washers 2 ¼” Nut, 4 ½” Washer   Grandt Line #5098

Source Information:

Since the Purgatory & Devil River Railroad is supposed to be a sister line to the Rio Grande Southern, the Cold Springs Trestle is based on the RGS prototype.  The RGS was maintaining 111 bridges at the time of abandonment in 1951.  While no two of these bridges were alike, many do share a basic design.  It is this design that I used to build the Cold Springs Trestle.  For information on the prototype bridges of the RGS I used the book “Silver San Juan” by Mallory Hope Ferrell and the “RGS Story” series from Sundance Publications.  I also used scale drawings from the Black Bear Construction Company.

Basics of Trestle Building:

Wooden trestles are a marvel of engineering and craftsmanship.  Each vertical section is called a bent.  Each bent is constructed of posts with various types of bracing.  All the timber on a prototypical trestle is bolted together.  This was done with a large rod with a nut and washer on each end.  This not only holds the bridge together but also allows it to be tightened during maintenance.  Since most trestles are taller than one single length of lumber will allow the bents are constructed in panels.  The panels are stacked on top of each other until the necessary height is reached.  On the RGS these panels were usually 10 to 12 feet tall. 

The bents in a trestle are connected to one another with a horizontal brace called a girt.  Many trestles also have cross bracing on the sides between bents.  This does add a lot of stability to the structure and keeps it from racking (swaying).  On the RGS only a small handful of bridges used this cross bracing (I have only found photos of 2 that did.) 

Once the bents are held together by the girts, and cross bracing if you use it, you are ready to add the stringers.  The stringers are several large pieces of lumber that are bolted together and run across each of the bents.  This provides a surface for the ties and rails to sit on.  The stringers on the RGS were usually 8 x 18 timbers.  The RGS’s stringers were built with 3 of these large pieces of wood running parallel to each other.  Between each piece of lumber is a spacer.  The spacer is a piece of lumber cut from the same 8 x 18 lumber.  At the location of the spacers a truss rod is run through all the boards of the stringers and is bolted on both sides.  On the RGS the spacers were placed at the joints of the stringers.  They would run two truss rods on each side of the joint.  These rods would be located on a 45-degree angle from one another.  (If the upper rod were in the upper left corner of the spacer the second rod would then be in the lower right corner.)

On top of the stringers the ties and rails are then laid.  Most wooden bridges have a platform on each side that will have a water barrel.  During the days of steam this was a necessity to put out any hot spots on the bridge.  Around this platform will be some kind of railing.  Along the outer edge of the ties a board was bolted along the length of the bridge.  On the RGS they typically bolted the boards down every 3rd or 4th tie.  This would help to keep the ties from twisting out of place. 

The RGS also included 2 guardrails on most of their bridges.  The extra rails were there to hopefully keep any piece of rolling stock or locomotive that derailed from traveling off the bridge.  Depending on which bridge you look at, the RGS placed the guardrails both the inside and outside of the rails.  On most bridges a lighter rail was used for the guardrails.

Building the Cold Springs Trestle:

Before construction could begin all the scale lumber would have to be sanded and stained.  I don’t sand the lumber too much because it takes away from the character of the wood.  Railroad timbers are rough and shouldn’t look too smooth.  I sanded each piece by pulling it through a 220 grit sanding sponge.  Any dust left behind was removed by pulling the strip wood through a tack cloth.

All of the wood was then stained.  The first step was the base coat of Antique Brown.  Pouring a small amount of paint on a rag and pulling the strip wood though added the color.  The idea here was to get an uneven coat on each piece so the amount of paint on the rag varied from piece to piece.  Also changing the pressure of the rag on the wood will affect the amount of paint applied. 

Once the Antique Brown was dry I then stained the lumber with a mixed stain (for the formula see the list of materials).  I started with a large airbrush bottle and the paints.  I poured the airbrush bottle about ¾ of the way full with distilled water and then added the paints.  I use distilled water since the contents of some tap water can affect the color of acrylic paints.  Once I achieved the color I was looking for I filled the rest of the bottle with more distilled water. 

I stained each piece with the new stain using the same technique as before.  I didn’t shake up the bottle after the initial mix.  The settling of the paint in the bottle created various shades of color as I worked.  This effect gives some of the bridge timbers a more weathered look than ones next to it.  Once all the lumber was stained I left it to dry overnight.

Construction:

All of the necessary pieces and parts were then cut on my Northwest Shortline Chopper.  I then used the mixed stain and a small paintbrush to touch up the ends of each cut.  All of the lumber was left to. 

Since I can make the posts on the bents from one solid piece of wood I constructed a “False Panel” bent to take advantage of the added strength.  Using scale drawings, the bents were assembled with Super Z - RC56 glue.  The first step was to glue the top cap and the posts together.  After each post was laid in place and the cap glued on the horizontal braces were added.  These are not only the supports for the girts they also represent the top cap of each panel in false panel construction.  After the horizontal pieces are in place the cross bracing was glued on.  On RGS trestles they typically had two cross pieces on 45 degree angles on the lower panel and only one on the upper panels.  Once all these pieces had a chance to dry the bent was turned over so the horizontal and cross braces could be added to the other side.  The bents were left to dry before adding the NBWs (Nut, Bolt and Washer).

The next step was to add the NBW castings.  Each NBW was first painted with Ceramcoat Cinnamon.  Next I drilled a hole at the location for each NBW with a #69 bit.  The NBW was cut off leaving a small amount of shaft underneath.  This tail was then dipped into Elmer’s white glue and then placed in the hole.  Each NBW was pushed in until flat on the surface of the wood.  On the horizontal braces I used the larger of the two NBW castings.  All other locations received the smaller castings.  Each full bent on the Cold Springs trestle required 44 of the larger castings and 88 of the smaller ones.  This totals 132 castings on each of these bents.  There are a total of 1248 NBWs on the bents of this trestle.  The stringers required another 57, as did the outer rail on the deck of the bridge.  This brings the total number of NBWs to 1361.  This does not include the dozen or so that slipped out of the negative tweezers and shot across the room.  (The $20 spent on NBWs was more than the cost of the scale lumber.)

After all the bents had their NBWs attached I then set the bents up in my Midwest Products Trestle Buddy.  This allows you to evenly space the bents and support the entire bridge during construction.  I then added the girts to the trestle.  These were glued in place based on the scale drawings as well as photographs of various RGS bridges.  This assembly was left to dry.  Since the RGS did not use cross braces between bents on most of their bridges I also chose to forgo the cross bracing.

The stringers were constructed next.  During my research on the RGS bridges I found that the stringers were usually built to span between 3 bents.  Since I could purchase scale lumber that was long enough to make the stringers in one long length I chose to do so.  This I felt would provide better strength to the structure.  I found in several old photos that most stringer sections would span between 3 bents on the outer edges of the trestle and sometimes span just between two bents on the inner pieces.  I chose to use this type of design on the Cold Springs trestle.  To simulate the seams in the bents I used a razor saw to scribe a seam in the outer edge of the stringers.   I then placed the spacer blocks behind this seam and placed NBWs on either side of the seam in the same manor as the prototype.

Since the entire visible track on the P&DR’s El Lobo Division is hand laid Code 70 rail I did not use flex track for the bridge.  The ties were cut from the 8 x 8 lumber to a length of 10 scale feet.  These were stained in three batches.  I used the same method as above where I first stained several of the ties with the Antique Brown and then the mixed stain.  A second batch of ties was done with just the mixed stain without the Antique Brown base coat.  The third batch of ties was stained with a coat of the ‘Opaque Black’ from Createx and then the mixed stain.  This was only done to 7 or 8 ties to simulate newer ties on the bridge. 

On both sides of my trestle is a platform for the fire barrel (one on either end of the bridge).  The ties here were cut to a length of 16’ to act as the platform base for the fire barrel.  The railings around the platform were made from scale 2” x 4” lumber.  Again the design was based on photos of various RGS trestles.   The timber bolted along the edge of the ties was done with the same lumber that I used for the girts.  Every 4th tie I used the smaller of the NBWs to ‘bolt’ it down.

Once all the wooden parts of the structure were in place I applied a coat of Alcohol and Ink stain over the entire trestle.  I next painted the rails to give it the rusty look.  The paint is mostly Floquil Rail Brown with a dash of Floquil Rust added.  This was applied with an airbrush.  After cleaning off the bottom of the rails they were coated with a thin layer of Pliobond glue.  Once the rail was in place I ran a hot soldering iron along the rail to melt the glue and bond the rail to the ties.  I placed my guardrails on the inside of the rails.  I chose this because I use several D&GRW Mikados, which are outside frame locomotives.  With the wheels being inside the frame and the counterweights on the outside I was concerned that the counterweights would strike the guardrails if they were on the outside.

 

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