New Bridge 44-B at Red Rock Creek

After a spring flood washed out the center bents of Bridge 44B and crippled the line at Red Rock Creek, the B&B gang came up with an ingenious solution.  They rebuilt the trestle by placing large timber beams over the creek bed on concrete foundations and supporting the vulnerable trestle bents on the beams above the high water mark.

This is an F scale (1:20.32) model of the Red Rock Creek trestle.  It is 81 scale feet long and is located on a 2% grade.   It is an example of how bridge engineers dealt with challenges on some of the narrow gauge railroads.  Although this particular trestle is NKP (no known prototype), it is representative of the methods used by the Rio Grande Southern at Bridge 44A (Butterfly Trestle) and at 45A (Ophir Loop Trestle.)  It follows common trestle construction practices that would have been in use during the 1914-1918 timeframe when many of the RGS trestles were upgraded or rebuilt.  You can see at the top of the photo where the bridge will be installed — just behind the block wall there is a section of temporary track supported by two vertical white posts.

All prototype hardware is represented on the model with appropriately sized detail castings.  Weathered aluminum foil was used to simulate the galvanized sheet metal pieces.  Running rails are Code 215 aluminum (65 lbs/yard rail in F scale) and the guard rails are Code 148 nickel silver (30 lbs/yard rail in F scale.)  Both the running rails and the guard rails are painted a rusty brown color.  Spikes are ⅜” blackened steel.  Running rails are gauged at 36 inches and are spiked through the tie plates at each tie.  Guard rails are located eight inches outside of the running rails and are spiked to every third tie.

The tops of the running rails are scored at 33 foot intervals to represent rail joints between individual new rail sections.  Similarly, the tops of the guard rails are scored at 29 foot intervals to represent joints between individual used rail sections.   Dummy fishplates were assembled from brass strips and NBW castings, painted a rusty brown color, and then attached to the rail sides at the simulated rail joint locations

Note regarding rail lengths: Most of the early, lighter steel rails were 30 foot long. During 1900s – 1920s, 33 foot lengths were common. The modern 39 foot length did not appear until heavier rail from the 1930s arrived. Therefore 25 to 60 lb/yard rails were usually 30′ long, 60 to 90 lb/yard rails were usually 33′ long, and 90 to 115 lbs/yard rails were usually 39′ long. Used rail that was re-laid elsewhere generally had a 6” section cut off from each end since this was where the primary wear took place. These sections of used rail were typically 29′, 32′ or 38′ long.

All wood for the model was cut and milled from Western Red Cedar.  All wood components were saturated with a water/UV resistant wood preservative prior to assembly.  After drying, the components were temporarily tacked in place with waterproof glue and then permanently connected with 3d (1 ¼”) finish nails, ½” or ⅝” brads, or ½” box nails.

I generally use the nearest fractional inch equivalent (in ¹⁄₃₂” increments) for timber dimensions, except for bridge ties which I cut to exact scale 8 inches.  For some timbers which are not scale specific, such as dump boards, I use a more relaxed fractional equivalent (in ¹⁄₁₆” increments.)  The following table lists the timber sizes I used to build this model (click to open table in a separate window.)  Timber Sizes

Four large beams span the creek bed to support the trestle bents.  Each beam is made from a pair of 12” x 30” x 35 foot timbers spaced two inches apart for ventilation.  Each beam has a galvanized sheet metal covering on the top of the timbers to protect from water and falling sparks.  The beams are supported on doubled timber bents at either end which rest on concrete foundations.  Standard single timber bents are spaced above the beams at 16 foot intervals to support the bridge deck.  The concrete foundations were made by pouring concrete into molds made from scale 12” wide boards to replicate the form impressions and wood grain patterns often seen on prototype piers.

The deck is composed of 8” x 8” x 9’ ties spaced twelve inches on centers.  The ties are supported by two stringers spaced one foot apart.  Each stringer is made from three 8” x 18” timbers bolted together above each bent cap.  The timbers in each stringer are separated by two inches for ventilation.  Each timber is nominally 32 feet long and supported on three bents.  The ends of the individual timbers are staggered so that all do not end at the same bent.  Galvanized sheet metal covers the tops of the stringers to protect from water and falling sparks.

Guard timbers are notched pieces of 5” x 8” timbers that fit over the ties.  They help to hold the ties in alignment on the trestle deck.  They also provide some additional protection in the event of a derailment.  Each guard timber is 16’ 8” long and spans one panel between two bents.  The notches are 2” deep.  The ends of the guard timber utilize lap joints where they join together.  Guard timbers are bolted to the ties at each lap joint and at every fourth tie.

Each single bent is composed of a 12” x 12” sill, two 12” x 12” vertical posts, two 12” x 12” angled posts, a 12” x 14” cap, and two 2½” x 10” diagonal sway braces.  The doubled bents supporting the beams have an extra pair of 12” x 12” angled posts.  All angled posts have a 2 in 12 batter. The tops of all bent caps are protected with galvanized sheet metal.  All bents except bent #4 are tied together at the sills with 6” x 10” girts.

I designed the trestle using a computer drawing program (Microsoft Visio) and printed the drawings to use during construction.  Here are a few examples.  The drawings have been reduced to fit the screen and are not to scale.

 

Side View

 

Beam Layout

Guard Timbers

Bent 4

Stringer Layout
Here are a few photos taken during construction:

Bent 2 Jig

Here is the bent jig for Bent #2.  It is made by attaching a full-scale drawing of the bent to a piece of Plexiglas with a glue stick.  That way the paper can be peeled off when no longer needed and the Plexiglas used for another jig.  Small blocks of scrap wood are glued to the paper to align the bent cap and posts.  Small pieces of the paper have been cut away where the posts will be glued to the cap and sill.  This is just in case any glue leaks out of the joints.

Back of Bent 2 Jig

This is the back side of the jig.  The horizontal strip of Plexiglas attached to the back of the Plexiglas sheet will ride in the miter slot of my table saw, as you will see a bit later. 

Posts and Cap are Test-fit in Jig

I have test fit the cap and posts into the jig.  The angled tops of the two battered end posts were cut on a separate angle cut jig (not shown.)  The angle cut jig allows me to cut precise 90 degree perpendicular cuts as well as angled cuts for 1:12, 2:12, 2.5:12, and 3:12 batters.

Posts are Glued and Clamped to Cap

I glue the posts to the cap with waterproof glue and clamp them in place.

Jig Moved to Table Saw

After the glue sets, I remove the clamps and take the jig to my Dremel 4″ table saw.  The strip on the back of the jig fits into the miter slot on the table saw and positions the jig to make the cuts on the post bottoms.

Bottom of Posts Cut to Length

One pass through the blade trims all four posts evenly at the bottom.

Bottom Sill Added

I glue the sill on to the post bottoms and reclamp the bent. 

Drilling Pilot Holes

After the clamps are removed, I take the bent off of the jig and drill 1/16″ pilot holes through the cap and sill into each post.  I drive 1 1/4″ (3d) finish nails into each hole to secure the posts in place.  I lay the bent flat and nail on the diagonal braces with 1/2″ 19 gauge brads. 

Assembled Bents

Here are a few of the finished bents. 

Double Bent

Here is one of the double bents. 

Underside of Deck

This is the underside of the deck.  The three 8″ x 18″ timbers that make up each stringer are gapped 2″ like on a prototype trestle.  I used 1/4″ zinc plated steel washers for the spacers. 

Top of Deck Before Adding Rails

This is the top side of the deck before the rails were added.  The 8″ x 8″ ties are nailed to the stringers with 3/4″ 18 gauge brads.  On a prototype trestle the tops of the stringers are protected with 22 gauge galvanized sheet metal.  I simulated this with aluminum foil tape painted gray.

Tie Plate and Running Rails added

Here is the deck with the tie plates and running rails added.  The guard rails and splice plates for the rail joints have not yet been added.

Lap Joint on Guard Timbers

The guard timbers are notched to fit over the ties and utilize lap joints where they meet.