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

Figure 1 – Third Story Drawing

Photo 1 Creative Clamping

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 2 Bent 10

Figure 3 Bent 11-13

Figure 3 Bent 11-13

 

Cutting to Size

 

Photo 2 Cutting Supports

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

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

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

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

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

Photo 7 Complete Bents

Next time: Building the Center Beam Assemblies

Building a Large Curved Trestle on a Grade – Part 3

Mass Production of Bent Stories

To efficiently build a large trestle such as this, I will take advantage of the many identical assemblies of bent stories. Of the 20 bents, four are less than full height. I’ll handle exceptions like those individually. In this article I will describe how I build the bent stories in batches. These will be 16 top stories (with 5/8” x 11/16” cap) and 6 second stories.

CAD Drawings

LibreCAD is used to draw the construction plans. It is a free open-source CAD program that is relatively full-featured for 2D drawing. The learning curve is a bit steep, but I find it invaluable to get a good, accurately measured drawing.

Link: http://librecad.org/cms/home.html

User Manual: http://wiki.librecad.org/index.php/LibreCAD_users_Manual

Figure 1 Bent Top

Figure 1 Bent Top

Figure 2 Bent Second

Figure 2 Bent Second

Figures 1 and 2 are (not to scale) plans for the top and second bent stories. There are many of these that are identical, so producing them in a batch makes sense. I had a few drawings printed full-size (18”x24”) through Staples to use during construction. (At the bottom of this post are links to the DXF and PDF files.)

Link: http://www.staples.com/sbd/content/copyandprint/engineering-prints.html

Assemble materials

Photo 1 Material

Photo 1 Material

I first ripped the materials needed so I could do a whole run of many bents without interruption. Photo 1 shows my supply during production, and the spreadsheet I used to track how much of each size and length I would need. I need a LOT more than what is shown, but I have enough to start producing bents.

I used a spreadsheet to keep track of how many pieces of each size wood I needed. I’m going to do the 15 Top and 6 Second stories in a batch. The others are all unique sizes that I will leave for later. For the top story there are (15) 5/8” x 11/16” x 8 ½” Caps, (30) 5/8” x 5/8” x 9+” vertical posts (leave them just a bit long) and (30) 5/8” x 5/8” x 9 ½” angled posts (3-in-12 batter, or 14.0 degree angle). The second story will need (6) 5/8” x 5/8” x 11” sills (at the top), (24) 5/8” x 5/8” x 9” vertical posts and (48) 5/8” x 5/8” x 9 ½” angled posts.

Construction jigs

Photo 2 Gluing

Photo 2 Gluing

Post jig – This is used to glue the cap or top sill and attached posts. It is positioned with the bottom of the posts at the edge of the jig so a table saw can be used to even the bottom up for a perfect fit onto the next story assembly or the mud sill. Photo 2 is a saw jig being used for gluing posts.

Photo 3 Saw Jig

Photo 3 Saw Jig

 

 

 

Photo 3 shows a Saw Jig in place on the table saw. Note that the blade guard was raised for the photo, but was in place during cutting!

 

 

 

 

 

Photo 4 Brace Jig

Photo 4 Brace Jig

Brace jig –Braces as seen from behind are top-left to lower-right, so when they are viewed from the front of a bent are top-right to bottom-left. Photo 4 shows the Brace Jig in use. The brace being glued is placed on the jig, dots of glue applied and the bent is set on top of that and weighted.

Glue the paper plans to plywood, for example, one plan with the bottoms of the posts at the very bottom of the jig to use on the table saw to evenly trim the posts, and another to be used for brace gluing and bent assembly. I attached a strip of plastic using a glue stick where the posts meet the cap, so the glue won’t stick to the paper. (It is not visible in the photo because it is clear.) For the braces, I add brace scraps under the posts so the bent sits level during the gluing process.

Production

Photo 5 Stories

Photo 5 Stories

So working from saw jig to saw to brace jig, glue the posts to the cap or sill, then the brace on one side, then a brace on the second side. I let the glue set for about 1 hour before moving on to the next step, so the mass production process is a few small steps spaced apart by a bit of time. (20-20 hindsight made me realize that braces could have been glued twice as fast if I had positioned the saw jig guides so that it could also be used for braces.) Photo 5 shows the result of a few days of work.

Tip: You will need clamps for posts and gluing weights for sway braces. For weights, I use a combination of small snack-size bags filled with sand and a couple of old gallon pails about half full of sand.

To finish construction, drill a pilot hole through the cap or sill into each post and drive a 1 ¼” finishing nail into that for extra strength. Each brace should also be nailed to each post it crosses.

Next time: Odd-size Stories and Bent Assemblies.

What is Batter?

Batter is the amount of offset in the angled posts expressed as feet offset per 12 feet in height, so 3-in-12 would mean 3 feet horizontal offset for each 12 feet in height. The Rio Grande Southern typically used 2-in-12 batter for tangent (straight) trestles, 3-in-12 for curved and 2½-in-12 for combination trestles. Since this trestle is curved, it uses a 3-in-12 batter. The angle to use on a miter saw is 14.0 degrees.

Note: If you want a different angle, the conversion from batter to degrees is angle = tan-1 batter, where batter is expressed as a decimal, so 3-in-12 = 3/12 = 0.25. For example, a 2-in-12 batter would give an angle of tan-1 (2 / 12) = tan-1 0.16666 = 9.46 degrees. Or, here you go:

Batter to Angle
Batter (in 12) Angle (deg)
1 4.8
1.5 7.1
2 9.5
2.5 11.8
3 14.0
3.5 16.3
4 18.4

 

Drawings of bents, 3-in-12 batter, 1:20.3 scale. The PDF files are designed for printing on 18″x24″ paper.

Drawing DXF PDF
Top Story DXF PDF
Second Story DXF PDF
Third Story DXF PDF

Building a Large Curved Trestle on a Grade – Part 2

Processing Reclaimed Wood

I have a house that is over 100 years old, and in the past got a load of cedar channel siding that was reclaimed from a similar house. Many of the pieces were only 1-2 feet long, so not very useful for repairs, but very useful for model bridge building (and other scale lumber use) because it is all old-growth cedar as shown in Photo 1. It is very likely that you can find a recycled building material company in your area where you can obtain old cedar or redwood.

Raw Material

Photo 1 – Raw Material

I did quite a lot of research into reclaiming wood on the internet to learn from pros. There are a few things that need to be done to prepare reclaimed wood for processing. First, reclaimed wood is likely dirty. Hose and brush it off thoroughly to remove dirt and small stones. Next, and most important, remove all nails and screws before cutting or ripping with power tools. A nail will ruin a saw blade and perhaps cause dangerous kick-back. Suitable tools are medium hammer, pry bar or pliers. If you can’t pull a nail because of the tight bond with the wood, you can drill right next to the nail with a small bit to relieve some of the pressure holding it in place. Use an old or cheap bit, as it will get dulled by contact with the nails.

Use a sander to remove irregularities and smooth the surfaces for easier handling on the saw. Remaining nails may show up after sanding, as they will perhaps be shiny, but don’t count on that. Old, rusty square (iron) nails are not shiny.

Magnet Nails and Wood

Photo 2 – Magnet, Nails and Ripped Wood

Completely hidden nails can be located using a rare-earth magnet  suspended with fine string or fishing line as shown in Photo 2. This can be passed closely over the wood surface, and will be strongly attracted to any remaining iron or steel. Mark the location with chalk so you can easily find it later. The magnet allowed me to find a ½” piece of square nail that was completely inside the board – no head and nothing visible on either side.

The severe weathering of the siding only extends a little ways in from the ends in most cases, and a fraction of an inch in depth. Note in the photo how tight the grain is, and how nice the cedar coloring still is even after 100 years except where rust has discolored the wood.

To prepare the wood for ripping, I took a small amount off the edges of each piece to provide a straight edge. If both edges are too uneven to get a good rip cut, temporarily nail a straight board slightly overhanging the ragged edge to run against the rip fence.

I finish the ripped wood using 60-grit sandpaper in a palm sander to smooth the sides and get to correct finished size.

Next time: Mass-production of Bents.

Tool List

  • Stiff brush
  • Medium-sized hammer
  • Wood chisel
  • Regular and needle-nosed pliers
  • Small pry bar
  • A drill with a small bit
  • Rare-earth magnet
  • Palm sander with sandpaper

 

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.

Building-Pt-1-Photo-1

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.

Building-Pt-1-Figure-1

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.