Building a Large Curved Trestle on a Grade – Part 4
Odd-sized Stories and Bent Assemblies
After a break for the Holidays and the generally unpleasant freezing weather, I’m back at it. In this installment, I’ll be building the odd-sized stories, adding the mud sills and assembling complete bents.
A spreadsheet was used to calculate the height of the bottom story for each bent, as I’m building the terrain to fit the bridge. If you are fitting a bridge into existing terrain, you would measure the total height and use that to calculate the required stories.
Bent Calculation
To calculate the required heights, I’ll use a bent that is 26 13/32” tall as an example. The top story is 9 11/16” tall not including the bottom sill, the second story is 9 5/8” tall, so the third story height is:
Height = 26 13/32 – 9 11/16 – 9 58 = 7 3/32”
Allowing for the bottom sill, the third story post assembly should be cut to the bent height less the mud sill height:
Cut Length = 7 3/32 – 5/8 = 6 15/32”
(Note that the calculations are to the nearest 1/32”, but I doubt that level of precision can be achieved in practice. Shims and/or sanding will be used to adjust heights if needed.)
Odd-sized Stories
All of the third stories, and many of the top and second stories, are not full height. The posts will be glued in place before cutting. Then a rubberized push block will be used for sawing the odd-sized stories to even the posts. The friction will help prevent the posts from flexing when run through the saw. I made a brace using scraps to hold the posts in place, but that didn’t seem necessary. I had no trouble running a story assembly through the saw without any braces.
The only jig needed is for post gluing. Figure 1 shows the Third Story plan that will be attached to a plywood base. (The Top and Second story plans were in a previous post.) Note that on the Butterfly trestle, no additional inner angled posts (shown as dashed lines) are used. The extra angled posts between the normal pair of outer angled posts are used only on the beam support bents (7a, 9a, 11a and 13a). Gluing these odd-sized stories may require some creative use of blocks as shown in Photo 1.
Figure 1 – Third Story Drawing
Photo 1 Creative Clamping
To attach the full-size drawings to the wood base, I made a wheat paste from 1 Tbs of flour plus 5 Tbs water, heated in a small pan, stirring constantly until thickened. Spread a very thin layer on the surface of the wood, apply and smooth the paper plan and let it dry overnight.
Really Odd-sized Stories
The second story of bent 10 has the center posts resting on the heavy longitudinal beams, with a separate short sill as shown in Figure 2. The third stories of bents 11 and 13 are on split-level footings, so there are two separate cut lengths as shown in Figure 3. For these stories glue and trim the short ones first, then glue and trim the longer ones.
Figure 2 Bent 10
Figure 3 Bent 11-13
Cutting to Size
Photo 2 Cutting Supports
I built cutting supports for the stories to help position them and support the posts when they are run through the table saw to cut them to length. These differ from the gluing/cutting jigs because the cut length may vary quite a lot. Photo 2 shows the cutting supports for second and third stories.
Photo 3 Sawing Supports
One problem I had earlier was that the small post scraps cut off sometimes would travel back into the saw blade and get ejected in pieces, at high speed. The blade guard protected me, but the adjacent posts were sometimes not so lucky. Several were broken loose by ejected debris and had to be reglued. The solution that I found was to hot glue a scrap of rejected brace material to the ends past the cut line so as to make a single piece that travels through the saw intact. Photo 3 shows two stories with a support glued in place.
Mud Sills
Photo 4 Mud Sills
These odd-sized stories are all at the bottom of the bent (except #10), so a mud sill (or two spliced together) will be glued and nailed on to each one. Photo 4 shows a few bents with mud sills being attached. I’m adding a blue tape label showing the bent number as I complete them. Note that an acrylic sheet was used to prevent excess glue from attaching the bents to the workbench!
Braces
Since the braces for these odd-sized stories are all different, measure the distance between the corners of the top and bottom sills, then cut a brace about ½” shorter than that. These will be glued without using a jig, speeding the process considerably.
Bent Assembly
Photo 5 Top-Second Assembly
Take a top story and second story, put glue on the bottoms of the top posts and on each brace as shown in Photo 5, then slide the second story top sill into place, angling both slightly so that the brace glue doesn’t smear until it is in position. Add brace material scraps to support the pieces then clamp in place and let cure. Nail the glued braces to the sill for added strength.
Photo 6 Complete Bent Assembly
Repeat the glue and brace process for the third story. I’m improvising the braces as appropriate for the third story of each bent, roughly following the prototype. Photo 6 shows two complete bents being glued. Note that I numbered each with blue tape labels so I could keep them straight later.
Bents 7, 9, 11 and 13 have an adjacent support for the bent support beams. I added a very thin spacer between these before gluing the pairs to allow better drainage and drying. Note that bent 7 in the foreground of the photo has a doubled bottom story.
After repeating this process several times, Photo 7 shows many completed bents showing the height progression from end-to-end.
Photo 7 Complete Bents
Next time: Building the Center Beam Assemblies
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