Building a Large Curved Trestle on a Grade – Part 6

Assembling a Trestle Section

Construction Jig

To accurately position the bents I created a construction jig. I created a full-size CAD drawing that I had printed at Staples as shown in Figure 1. The bottom view is the one I will use to position the bents properly, as I am building this “upside-down,” much like you’d do in a smaller scale. The drawing is “glued” to a piece of plywood with wheat paste (flour and water) and stops are added at each bent location as shown in Photo 1. I added two protruding nails to each to allow a rubber-band to wrap around and hold the bent tight against the stop.

Figure 1 - Bent Stringer Plan

Figure 1 – Bent Stringer Plan



Photo 1 - Bent Assembly Jig

Photo 1 – Bent Assembly Jig

If you want to draw the diagram – and you really only need a center line and lines for each bent – you can lay this out with the tried-and-true string compass method. (The red lines are what you would draw.) To calculate the span between bents in inches, which will give you the desired angle in degrees between each pair of bents, use this formula (diameter in feet): span = angle * diameter / 9.55. For example: 6 foot radius (12 foot diameter), 7.5 degree angle, span = 7.5 * 12 / 9.55 = 9.424 (just under 9 7/16”).

Bent Angle Supports

Trestle bents are always vertical, so there will be an angle other than 90 degrees between the stringers and the bents. On a 2.5% grade, the angle is a 1.43 degree lean toward the downhill end if building upside-down. (This is the same angle as the 2.5% grade is from horizontal.) To create an angle guide to use during construction use a piece of rectangular material such as thin plywood and measure along an edge 20”, where the angle is offset ½” from square. To find the distance to any rise for any grade, use distance = rise / grade. So for a ½” rise on a 2.5% grade, distance = 0.5 / 0.025 = 20”.

I made several identical plywood angle guides by building an angle jig to hold a rectangular piece at the correct angle when run through the table saw as shown in Photo 2. I added scrap cedar nailed and glued in place to provide a clamping surface. The jigs ready for use are shown in Photo 3.

Photo 2 - Bent Angle Jig

Photo 2 – Bent Angle Jig


Photo 3 - Angle Jigs

Photo 3 – Angle Jigs

Section Assembly

I positioned bents 15 through 18 on the assembly jig as shown in Photo 4. The girts must be added “upside down.” So unlike the conventional assembly method, I’ll need clamps to hold them in place while the glue sets. (I will need more clamps!) After the girts are in place, wall bracing is added between these four bents. (I use a small piece of brace material where the bent is on the outside surface of the pair to prevent excessive bending as shown in Photo 5.

Photo 4 - Bent Assembly Fixture

Photo 4 – Bent Assembly Fixture



Photo 5 - Small Block Under Top Brace

Photo 5 – Small Block Under Top Brace

The complete assembly of bents with girts and wall bracing is shown in Photo 6.

Photo 6 - Complete Assembly

Photo 6 – Complete Assembly

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