Building a Large Curved Trestle on a Grade – Part 1
Materials
This is the beginning of a semi-regular series describing how I go about building a complex trestle. I am writing this series so that, hopefully, others may benefit from my experience and (probably more than a few) mistakes. I model in 1:20.3, but will provide many measurements and plans in 1:24 as well.
Photo 1: Prototype Photo of remains ca. 1970?
I am modeling the Rio Grande Southern and one of the bridges I am building for my railroad is the Butterfly Trestle. This will be a great example as it is curved, on a grade and very tall. This trestle is built using framed bents, which are divided into manageable sections called stories. The bridge has 20 bents of varying heights, up to 3 stories tall, with two bents carried on very large beams.
Figure 1: Butterfly trestle plan
The top member (that the stringers rest on) is the cap which is 12×14 x 14 feet, which scales to 5/8”x11/16” x 8.5” (1/2”x9/16” x 7”). Each story is about 16 feet tall, which scales to about 9.5” (8”), with 12×12 posts with the two outer posts on a 3-in-12 batter. Between stories are 12×12 sills, and a 12×12 mud sill at the bottom. The 12x12s scale to 5/8”x5/8” (1/2”x1/2”).
Sway bracing is 4×10, which scales to 7/32”x1/2” (3/16”x7/16”), and longitudinal braces (girts) are 8×8, which scales to 3/8”x3/8” (5/16”x5/16”).
The two larger openings provide clearance over the waterways where a bent in the center is carried by eight 9×30 beams, as shown in the small plan at the bottom. These scale to 7/16”x1-1/2” x about 17.5” (3/8”x1-1/4” x about 16”).
The track is supported on two sets of 3 stringers of 8×18 wood, which scales to 3/8”x 7/8” (5/16”x3/4”).
I used a spreadsheet to calculate about how much wood of each size I will need as I am going to rip my own material from cedar reclaimed from very old siding.
| Piece and Size | Quantity (feet) |
| Cap: 5/8×11/16 (1/2×9/16) | 13 |
| Post: 5/8×5/8 (1/2×1/2) | 245 |
| Brace: 7/32×1/2 (3/16×7/16) | 168 |
| Beams: 7/16×1-1/2 (3/8×1-1/4) | 27 |
| Girts: 3/8×3/8 (5/16×5/16) | 95 |
| Stringers: 3/8×7/8 (5/16×3/4) | 29 |
Figure 2: Rip list
Well, this will keep me busy for a while! Next time, I’ll describe what I learned about processing reclaimed wood.
Bridge Terminology
Batter – the angle of posts to provide lateral stability, typically measured in inches of offset per 12 inches.
Beam – the members that are supported by the truss that in turn support the stringers and track.
Bent – the term for the vertical assembly of one or more stories that support the stringers and track-work.
Brace – diagonal members used to enhance the structure of a bent.
Cap – the top timber that the stringers rest on.
Chord – the horizontal pieces that run the length of the truss on the top and bottom.
Deck – the assembly of beams and stringers that supports track-work (ties and rails).
Girt – longitudinal braces between trestle bents.
Mud Sill – the bottom-most sill of the bent that rests on the footing, pier or the ground.
Plate – the piece at the ends of the chords that rest on the piers.
Rods – typically the tension members of a truss.
Sill – the bottom timber of the story.
Story – The vertical section of a bent between sills.
Stringer – Beams that run parallel to the track which support the ties.
Truss – the whole assembly of chords, compression members and tension rods that form the support structure on the sides of the bridge.
Wall Brace – Braces between trestle bents on the outside of the structure.
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