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BUILDING RGS BRIDGE 45-B
By Bob Hyman
UPDATED: 02/07/02. This is an
updated version of the previous article of the same title.
Notes: To view the figures in
this article, just click on the thumbnail images. Click on the BACK button
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Abstract
This is a
"how-to" article for building a highly detailed model of one of the
most famous high line bridges on the Rio Grande Southern railroad. A
section on jigs and molds gives valuable information for making and using these
handy tools that simplify model bridge building. The article contains very
detailed step-by-step instructions for preparing plans, fabricating the various
sub-assemblies, and assembling the completed trestle. Some of the drawings
contained in the article are drawn to scale and can be used as templates for
making various components. The tips and techniques clearly explained in this
article will provide the modeler with valuable information for building this and
other RGS trestles.
Introduction
Bridge 45-B was located on
the Ophir Loop, just south of the depot. It crossed the Howard Fork of the San
Miguel River on a 24-degree curve and 2% grade. No other curve on the RGS was
tighter, although the curve at the upper end of Trout Lake at Bridge 51-A was
just as tight. Bridge 45-B was unique in that it had a pedestrian walkway beside
the track on the inside edge of the curve.
The original bridge was 112
feet long and 24 feet high. It consisted of eight bents placed sixteen feet on
centers. In 1919 the bridge was reported as108 feet in length. This corresponds
to the timeframe (1914-1918) when the other Ophir trestle, Bridge 45-A, and
several of the other high line bridges were extensively modified.
The modification to Bridge
45-B shortened the distance between the last two bents (#7 and #8) to twelve
feet. This is probably also when the pedestrian walkway was added, the concrete
foundation was placed under bent #4, and the guard rails moved from the inside
to the outside of the running rails. Judging from photographs of the bridge
after it was modified, it does not appear that the end panel between bents #7
and #8 was actually shortened. I think that another bent (call it Bent #8A for
reference) was simply added between bents #7 and #8. The pedestrian walkway
ended at the new bent #8A.
My completed model
represents Bridge 45-B as it would have appeared in the 1939 timeframe. This
model is applicable for most RGS layouts from the 1920’s through the end of
operations. The hand railing on the pedestrian walkway was repaired several
times over the years, and looks slightly different in various photographs.
Part 1. Prototype Trestle Construction
Stringers
The bridge engineers had to
make some adjustments when building a bridge on a curve this sharp. The three
8" x 18" stringers that made up each of the bridge beams were
staggered and spanned two panels each, just as they were for most trestles.
However, they were not parallel, as they would be in a tangent (straight)
trestle.
Figure
1 - Curved
Trestle Stringer Arrangement
On a 24-degree curved
trestle like bridge 45-B, the center stringer of each beam was angled. To
accommodate this arrangement, the beams were about 36 inches wide and were
spaced only a foot apart. This resulted in a total deck width of about 84 inches
when measured from the outside edge of one beam to the outside edge of the other
beam. With the beams resting on the tops of the 14-foot long bent caps, this
resulted in about 42 inches of exposed cap on each side of the beams at bents
#1, #3, #5, #7, #8, and #8A. On bents #2, #4 and #6, the exposed portion of the
bent cap on the outside curve side would be slightly greater than 42 inches and
the exposed portion on the inside curve side would be slightly less than 42
inches (approximately a six inch difference.) The 9-foot long bridge ties
overhung the beams by an average of twelve inches on each side.
Tie Placement
The 8" x 8" x
9’ bridge ties were laid 12 inches center to center, which is typical for RGS
trestles. This results in a 4-inch spacing between the ties. The ties were not
perpendicular to the beams; they were laid perpendicular to a chord drawn
between the midpoints of each bent cap. The track centerline also passes through
these same midpoints. Viewed from above with the ties in place, the trestle deck
appears to be constructed of seven straight segments, with angled joints above
each of the bent caps.
Curve Radius
Bridge engineers use basic
trigonometry to layout prototype curves. They reference curves using a term
called "Degree of Curvature". To convert degree of curvature
into an actual curve radius, simply take the sine of ½ the degree of curvature
and divide it into 50 feet. The result will be the prototype radius in feet. To
find the model radius, just multiply the prototype radius by the scale. The
24-degree curve of the prototype represents an actual curve radius of about 240
feet. My model trestle is built in S Scale (1:64) and scales out to a 45-inch
radius. In the case of Bridge 45-B in S Scale, the computation is as follows:
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Sine of 12° = 0.208
(rounded to three decimal positions)
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50 feet / 0.208 = 240
feet (rounded to nearest foot)
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240 feet x 1/64 = 3.75
feet or 45 inches |
Panel Configuration
The portion of a trestle
between two bents is called a panel. Bridge 45-B was composed of
eight bents (nine, if you count bent #8A) and seven panels. Since the
number of panels is uneven, the beam configuration does not appear perfectly
symmetrical when viewed from the sides. The odd panel was at panel #7 on
the south or uphill end of the trestle (toward Rico), between bents #7 and #8.
Figure
2 - Panel
Configuration
When viewed from the side,
the trestle appears to have three equal 32-foot beams between bents #1 and #3,
between bents #3 and #5, and between bents #5 and #7. The odd panel appears as a
16-foot beam between bents #7 and #8. Of course this is only how the beams
look from the outside; the middle stringer in each beam is staggered between
bents #2 and #4, between bents #4 and #6, and between bents #6 and #8, with the
short middle stringer between bents #1 and #2.
Pedestrian Walkway
The supports for the
walkway consisted of twenty-eight 4" x 8" x 12’ timbers placed
between the ties at every fourth tie, starting at bent #1 and ending at bent
#8A. They extended out about four feet past the tie ends on the inside of
the curve. Three 3" x 12" floorboards were nailed to the 4"
x 8" supports to form the walkway. The vertical handrail posts
were four foot lengths of 3" x 6". The horizontal railings were
a combination of 3" x 12" and 3" x 6" boards. A
3" x 6" cap ran along the tops of the posts.
Figures #3 and #4 show how
the walkway was attached to the trestle. The stringers that form the beams of
the deck are visible between the ties for reference. On the actual trestle, the
beams were covered with #22 galvanized sheet metal and the individual stringers
would not be visible. Also, note how the ends of the walkway support timbers
were butted up against, but did not extend under, the guard timbers on the
outside edge of the trestle.
Figure 3 - Pedestrian Walkway
Cross-Section View
Figure 4 - Pedestrian Walkway
Overhead View
Bents
On a straight or mildly
curved trestle, the two vertical posts within each bent were typically spaced
36" center-to-center. On Bridge 45-B, the two vertical posts of each bent
were spaced 48" center-to-center, probably due to the wider beams. This
causes the trestle to have a short, squat appearance when compared to other
bridges. Like most curved trestles, the angled posts of each bent were 3 in 12
batter. This means they spread 3 inches for every foot of height.
One other difference was
the bent caps; on 45-B they were 12" x 12" x 14’ as opposed to the
normal 12" x 14" x 14’. Other than these unique differences,
Bridge 45-B followed typical RGS bridge building practices: 12" x 12"
posts and sills, 3" x 12" diagonal cross bracing, 8" x 8" x
9’ bridge ties, and notched 5" x 8" guard timbers.
The bents in Bridge 45-B
were all single-story. Bents #1, #8, and #8A did not contain any posts.
All bents except bent #4 rested on timber footings made from 3-foot lengths of
leftover 12" x 12" post material. Bent #4 rested on a concrete
footing. The following figures show Bents #1 through #8A.
Figure 5 - Bent #1
Figure 6 - Bent #2
Figure
7 - Bent #3
Figure
8 - Bent #4
Figure 9 - Bent #5
Figure
10 - Bent #6
Figure
11 - Bent #7
Figure
12 - Bent #8A
Figure
13 - Bent #8
Part 2. Model Trestle Construction
Note: Any
scale dimensions given in this section are for an S Scale model. Where
prototype dimensions are given, the equivalent model dimensions are sometimes
shown in parentheses for clarity.
Material Preparation
You will need the following
sizes of scale timbers:
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8" x 18" Stringers
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12" x 12" Caps,
Posts, Sills, Footings
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3" x 12"
Cross Bracing, Walkway Floor Boards, Walkway Railing
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3" x 6"
Walkway Railing, Walkway Posts
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4" x 8" Walkway
Supports
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5" x 8" Guard
Timbers
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8" x 8" Ties
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4” x 12” Dump
Boards
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Prepare the materials as
explained in steps 1 through 5.
Step 1. Obtain and Prepare the Wood
After you have purchased or
cut the required sizes of timbers, you need to prepare the wood. Distress,
age, paint, stain, weather, or otherwise "doctor" the wood according
to your preferred method. No one seems to agree on the best method but
here's how I did mine:
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First, I rubbed the
surfaces of each timber lengthwise with a file card to add grain texture.
The file card is a stiff, short-bristled, wire brush used to clean files.
It is sometimes called a file cleaner.
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Then I stained each
timber with Minwax brand water-based wood stain; Colonial Pine color.
I brushed on the full strength stain, allowed it to dry for a few minutes,
then wiped the timbers with a damp rag to remove the excess. At this
stage the timbers will be a nice, soft brown color.
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Note: For
items where multiple, identical-sized, short lengths are required (like the
ties or foundation timber blocks), I cut the pieces to length and stain them
all at once by dipping rather than brushing. This saves me the bother of
touching-up the cut ends later. You will need 113 ties (8" x
8" x 9') and 144 foundation timber blocks (12" x 12" x
3'.) When dipping, I thin the stain about 50 /50 with tap water.
Rather than wiping each individual piece with a rag, I simply take the stained
pieces and rinse them under running water to remove the excess stain.
Shake off the excess water, place the pieces in a large paper bag, then shake
the bag to remove the surface moisture and to keep the pieces from sticking
together. Empty the damp pieces from the bag onto a paper towel, and
microwave them at high heat for about 30 seconds to 1 minute, or until they
are slightly warm to the touch. Spread them out on a piece of newspaper
and they will dry quickly.
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Next I brushed each
timber with a coat of A-West "Weather-It" to tone down the brown
color and add blackish gray surface highlights.
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Note:
Again, for the small pieces, I used the dip, shake and "nuke"
method.
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During assembly, after
cutting the various sized timbers to the required lengths, I touched-up any
unstained cut edges before gluing them together.
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Finally, after the
major components such as the bents and deck were assembled, I brushed
on a solution of India ink in alcohol (about 25 drops per ounce.) The
ink ran into the cracks and crevices of the timbers scratched by the file
card and accentuated the simulated grain texture.
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Step 2. Prepare the Sheet Metal Pieces
You will need material for
the #22 galvanized sheet metal flashing that goes between the bridge beams and
the ties, and also on the exposed bent caps.
I used Avery #6873 white shipping labels for color laser printers and
copiers. These labels are 2” x 3
¾” each. Eight labels are on
each 8.5” x 11” page. You can
also use regular white 20-pound bond typing paper.
Color print or paint the paper pieces with rust and grays shades.
The advantage of using the shipping labels is that they are a
“peel-and-stick” item. If you
use plain paper, you will need to glue the paper to the wood with Elmer’s
White glue or equivalent. Use
whatever material you feel most comfortable with to duplicate the sheet metal.
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Make 14 strips 3' wide
x 17' long (9/16" x 3 3/16".) You will use these for the
flashing between the beams and the ties.
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Make 18 strips 18"
wide x 52" long (9/32" x 13/16"). You will use these to
cover the exposed bent caps.
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Step 3. Fabricate the Concrete Foundation for
Bent #4
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Bent #4 rests on a
concrete footing. I made mine from plaster, poured into a mold made
from strip wood. This allowed the wood grain from the mold to appear
on the concrete, just as on the prototype.
Here are the dimensions of the concrete footing:
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Figure
14 - Bent #4
Concrete Foundation
Note: The mold
and directions for its use are described in the JIGS AND MOLDS section of this
article.
Step 4. Obtain and Prepare the Rails
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You will need two
pieces each of code 40 and code 55 rail. These typically come in
3-foot sections. The code 40
rail represents the 30# guard rails. The
code 55 rail represents the 57# running rail.
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Note: If
you would rather use larger rail sizes, you can go up to code 70 for the
running rails and code 55 for the guard rails without looking too much
out-of-scale. Just remember to keep the relative size differences
between the running rails and guard rails.
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Paint the rail a
rusty-brown color. (I used weathered rail so I wouldn't have to bother
painting it.)
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Step 5. Obtain and Prepare the NBW Castings
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Obtain and paint 56 NBW
castings for the beams. I used Grandt Line, HO Scale #5099 (3"
Nut & 4 1/2" Washer). In S scale, this represents a 2
1/4" Nut & 3 3/8" Washer.
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Obtain and paint 74 NBW
casting for the guard timbers. I used Grandt Line, O Scale #62
(7/8" Nut & 2" Washer.) In S scale, this represent a 1
1/8" Nut & 2 5/8" Washer.
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I painted mine a dark
earth color, and then weathered them with rust while they were still
attached to the sprues.
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Deck Plans
The easiest method for
assembling a trestle is to mount scale plans on a flat surface and to build the
trestle directly on the plans. You will need two versions of the deck
plan; one in normal view (viewed from above) and the other in reversed view
(viewed from below.) You will need two copies of the reversed view and one
copy of the normal view. Draw the plan as explained in steps 6 through 11.
This will result in a reversed view plan.
Step 6. Layout the Track Centerline and Bent
Positions #1 - #8
Figure
15 - Track Centerline Drawn and Bent Positions #1 Through #8 Marked
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Begin by laying out the
45-inch (240-foot) radius arc that will represent the track centerline.
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Mark the arc at eight
points in 3-inch (16-foot) increments.
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Label the marks as Bent
#1 through Bent #8 going from right to left.
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Step 7. Layout the Chords and Bent Position #8A
Figure
16 - Chords Drawn and Bent Position #8A Marked
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Draw reference lines
between adjoining marks on the arc to locate the seven panel chords.
These chords should be exactly 3 inches (16 feet) long.
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On the panel chord
between the marks for Bent #7 and #8, measure ¾" (4-feet) from the
mark for Bent #8 and make another mark. Label this mark as #8A.
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Draw reference lines
between the marks for Bents #1 and #3, Bents #3 and #5, and Bents #5 and #7
to locate the exterior stringer chords. These chords should be
slightly less than 6 inches (32 feet) long.
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Draw reference lines
between the marks for Bents #2 and #4, Bents #4 and #6, and Bents #6 and #8
to locate the interior stringer chords. These chords should be
slightly less than 6 inches (32 feet) long.
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Step 8. Layout the Bent Cap Positions and the
Tie End Lines
Figure
17 - Bent Cap Positions and Tie End Lines Drawn
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Lay a straightedge
between the arc center point and the mark for Bent #1 and draw a reference
line through the mark, extending 1 5/16" (seven feet) on either side of
the arc. Repeat for remaining eight marks. These reference lines
represent the centerlines of the 2 5/8" (14-foot) bent caps. They
will also be the cut line for all stringers that end at bents #2 through #7.
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Draw the outlines of
the bent caps sides by drawing lines parallel to each of the nine bent cap
centerlines and spaced 3/32" (6-inches) on either side.
Connect the ends of the side lines to outline the bent cap positions.
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Next draw reference
lines parallel to the seven panel chords and spaced 27/32" (4 1/2 feet)
on each side. These lines represent the ends of the 1 11/16"
(9-foot) ties.
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Step 9. Locate the Long Interior Stringers
Figure
18 - Long Exterior Stringers Drawn
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Each beam is composed
of three stringers: two exterior stringers and one interior stringer.
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The exterior stringer
on the outside of the beam (the side facing the tie ends) is called the
outer exterior stringer.
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The exterior stringer
on the inside edge of the beam (side facing the other beam) is called the
inner exterior stringer.
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The inner exterior
stringers are always 3/16" (1 foot) apart.
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To locate the long
inner exterior stringers, draw lines parallel to the exterior stringer chord
and spaced 3/32" (6-inches) on either side. Now draw another set
of lines parallel to the exterior stringer chord and spaced 7/32"
(14-inches) on either side. These sets of parallel lines locate the
edges of the 1/8" (8-inch) wide inner long exterior stringers.
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The outer exterior
stringers are always spaced 5/16" (20-inches) from the inner exterior
stringers. To locate the long outer exterior stringers, draw lines
parallel to the exterior stringer chord and spaced 17/32" (34-inches)
on either side. Now draw another set of lines parallel to the exterior
stringer chord and spaced 21/32" (42-inches) on either side.
These sets of parallel lines locate the edges of the 1/8" (8-inch) wide
long outer exterior stringers.
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These stringers end at
the centerlines of bent caps #7, #5, and #3, and at the right edge of bent
cap #1.
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Step 10. Locate the Long Interior Stringers
Figure
19 - Long Interior Stringers Drawn
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The long interior
stringers of each beam alternate back and forth; touching the outer exterior
stringer at one bent and the inner exterior stringer at the next.
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To locate the long
interior stringers, draw lines parallel to the interior stringer chord and
spaced 5/16" (20-inches) on either side. Now draw another set of
lines parallel to the interior stringer chord and spaced 7/16"
(28-inches) on either side. These sets of parallel lines locate the
edges of the 1/8" (8-inch) wide long interior stringers.
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These stringers end at
the centerlines of bent caps #6, #4, and #2, and at the left edge of bent
cap #8.
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Step 11. Locate the Short Stringers at Trestle
Ends
Figure
20 - Short Stringers Drawn
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The short stringers at
either end of the trestle are parallel to the panel chords.
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On the Bent #8 end
of the bridge, the short stringers are the four exterior stringers.
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On the Bent #1 end
of the bridge, the short stringers are the two interior stringers.
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To locate the short
inner exterior stringers at the left (Bent #8) end of the trestle:
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Draw lines parallel
to the panel chord between bents 7 and 8 spaced 3/32" (6-inches) on
either side.
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Now draw another
set of lines parallel to the panel chord between bents #7 and #8 spaced
7/32" (14-inches) on either side.
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These sets of
parallel lines locate the edges of the 1/8" (8-inch) wide short
inner exterior stringers.
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The short inner
exterior stringers end at the left edge of bent cap #8 and the
centerline of bent cap #7.
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To locate the short
outer exterior stringers at the left (Bent #8) end of the trestle:
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Draw lines parallel
to the panel chord between bents #7 and #8 spaced 17/32"
(34-inches) on either side.
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Now draw another
set of lines parallel to the panel chord between bents #7 and #8 spaced
21/32" (42-inches) on either side.
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These sets of
parallel lines locate the edges of the 1/8" (8-inch) wide short
outer exterior stringers.
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The short outer
exterior stringers end at the left edge of bent cap #8 and the
centerline of bent cap #7.
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To locate the short
interior stringers at the right (Bent #1) end of the trestle:
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Draw lines parallel
to the panel chord between bents 1 and 2 spaced 5/16" (20-inches)
on either side.
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Now draw another
set of lines parallel to the panel chord between bents #1 and #2 spaced
7/16" (28-inches) on either side.
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These sets of
parallel lines locate the edges of the 1/8" (8-inch) wide short
interior stringers.
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The short interior
stringers end at the centerline of bent cap #2 and the right edge of
bent cap #1.
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The deck plan is now
complete. Make a copy of the plan and label it as "REVERSED VIEW
PLAN."
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Make a mirror image
copy of the plan and label it as "NORMAL VIEW PLAN."
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Deck Assembly
Assemble the bridge deck as
explained in steps 12 through 15.
Step 12. Cut and Assemble the Stringers Into
Beams
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Attach the original
reversed view copy of the deck plan to a flat work surface. I used
masking tape to attach the corners of the plan onto piece of smooth
1/4" hardboard.
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Cut a 1/8" x
9/32" (8" x 18") stringer to fit over each stringer in the
plan, using the appropriate position of the angle cut jig.
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Note: The angle
cut jig and directions for its use are described in the JIGS AND MOLDS section
of this article.
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Drill #71 holes in the
outer exterior stringer of each beam to accept the NBW castings.
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Notes: The NBW
positioning jig and directions for its use are described in the JIGS AND MOLDS
section of this article. You can also drill holes for NBW castings on
the inner exterior stringers if desired. These inside NBW castings would
be almost impossible to see on the completed trestle. I left them off on
my model. If you decide to add them, you will need an additional 56
#5099 NBW castings.
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Touch-up any unstained
cut edges of the stringers.
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Glue the stringers to
each other and to the plan with small drops of Elmer's wood glue. The
deck is constructed upside down, so the surface of the stringers glued to
the plan will eventually be covered by the sheet metal flashing and the
ties.
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After the stringers are
assembled into completed beams, slide a razor blade between the plan and the
beams. Discard the plan.
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Sand the surface of the
beams smooth to remove any glue or paper residue.
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Add NBW castings to the
beams at each bent position in the previously drilled holes.
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Step 13. Add the Sheet Metal flashing to the
Beams
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Attach the normal view
copy of the deck plan to the work surface.
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Glue the beams onto the
plan with small drops of Elmer's wood glue at each end bent location.
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Starting at the left
(Bent #1) end of the beams, glue 9/16" x 3 3/16" strips of
simulated sheet metal flashing to the tops of the beams. Trim the
strip ends at the Bent #3, #5, and #7 centerlines. Trim the last pieces at
the right (Bent #8) end of the beams.
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Step 14. Add the Ties, guard Timbers, and
Walkway Supports
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First, glue ties to the
beams directly above bent caps #1 through #8 (not at Bent #8A.) These
eight ties are aligned parallel with, and centered over, the bent caps and
fit between the tie end reference lines on the plan.
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Next, fabricate
fourteen guard timbers using the notch jig.
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Note: The notch
jig and directions for making guard timbers are described in the JIGS AND
MOLDS section of this article.
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Touch-up any unstained
cut edges and notches of the guard timbers.
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Then, fabricate seven
tie / guard timber sub-assemblies using the tie positioning jig.
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Note: The tie
positioning jig and directions for making the tie / guard timber
sub-assemblies are described in the JIGS AND MOLDS section of this article.
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Starting with panel #7,
and working back towards the left end of the trestle, test fit the panels on
the beams. The lap joints should meet directly over the ties
previously glued to the beams above the bent caps.
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Carefully trim the
excess material from the lap joint areas where the guard timbers meet.
Touch-up any trimmed edges of the guard timbers with stain.
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Glue the tie / guard
timber sub-assembly onto the beams using the tie end reference lines on the
plans as a guide.
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As you glue on each
panel, add the pedestrian walkway support timbers that are next to tie #1/17
directly above bent caps #1 - #7. These support timbers are parallel
with, and on the right side of, ties #1/17.
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At the lap joints where
the panels connect (bents #2 - #7), at the tie#1 end of panel #1 (bent #1),
and at the tie #17 end of panel #7 (bent #8), drill sixteen #71 holes in the
guard timbers for the NBW castings. Drill the holes completely through
the guard timbers and ties.
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Add sixteen #62 NBW
castings to the holes in the guard timbers.
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Note: The
sixteen corresponding NBW castings beneath the ties will be added after the
deck is removed from the plan.
Step 15. Add the Rails
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Add code 55 running
rails and code 40 guard rails.
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The running rails are
located 9/32" (18") on either side of the track centerline.
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The guard rails are
located approximately halfway between the running rails and the guard
timbers.
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Use your preferred
method for attaching rail to the ties.
I used micro-sized spikes on each side of the rails above every bent
position.
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Cut the running rail
lengths to extend a minimum of six inches beyond the trestle deck so that
the rail joints will not be near the ends of the trestle. I used the full
36-inch length rails, overhanging the ends approximately equally on either
side.
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Cut the guard rails at
approximately 2 ½ to 3 inches beyond the ends of the trestle. The guard
rails extend onto approximately eight ties at both approaches and flare
slightly outward at both ends.
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Remove the deck from
the plan by sliding a razor blade between the plan and the beams.
Discard the plan.
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Remove any glue or
paper residue from the bottoms of the beams.
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Add sixteen #62 NBW
casting to the holes in the bottom ends of ties 1/17 at each bent position,
corresponding to the sixteen #62 NBW castings above.
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Trestle Assembly
Assemble the trestle as
explained in steps 16 through 19.
Step 16. Fabricate and Add the Bents
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Fabricate the nine
bents using the angle cut jig and bent jigs.
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Note: The angle
cut jig and bent jigs and directions for making the bents are described in the
JIGS AND MOLDS section of this article.
|
Lightly sand the tops
of the bent caps at an 88.854 degree angle using the bent attachment jig.
|
Note: The bent
attachment jig and directions for its use are described in the JIGS AND MOLDS
section of this article.
|
Attach the second
reversed view copy of the deck plan to the work surface.
|
|
Position the deck onto
the plan and secure with masking tape or weights.
|
|
Glue the bents to the
underside of the beams at the indicated bent positions with the bents
leaning toward the north or downhill end of the trestle (the Bent #1 end
toward Ridgway.) Use the bent attachment jig to insure the bents are
leaning at the proper 88.854 degree angle.
|
Step 17. Add the Bent Footings
|
Add timber bent
footings to bents #1 through #3 and #5 through #8A and the concrete footing
to bent #4.
|
Note: If
desired, the concrete footing can be left off until the trestle is ready to be
installed in final position.
|
Remove the trestle from
the plan and discard the plan.
|
Step 18. Add the Bent Cap Sheet Metal
Figure
21 - Bent
Cap Sheet Metal Arrangement
|
Temporarily support the
trestle in an upright position by placing supports under the various height
bents.
|
|
Glue a piece of sheet
metal to the exposed portion of each bent cap as shown.
|
|
The piece should
cover the top of the cap and extend down the vertical edges about 1/32"
(2").
|
|
The piece should be cut
to the length of the exposed bent cap plus about 1/16" (4") so
that about 1/32" (2") extends up the side of the beam and down
over the end of the cap.
|
Step 19. Add the Dump Boards Behind Bents #1 and
#8
Note:
Dump boards were located at most trestle abutments (end bents) to keep the
earth fill in place. These dump
board were usually made from leftover scraps of bridge timbers or heavy
planks. Since they were in constant contact with the ground, they
tended to rot quickly and were generally replaced at least once during the
life of a bridge. For the dump
boards on my model, I used scrap pieces of 4” x 12” timbers.
|
There
are four dump boards on Bent #1 and three dump boards on Bent #8.
Refer to Figure 5 and Figure 13 for details. |
|
Cut
the dump boards to length so that the lowest board extends approximately one
foot beyond the bent footing on either side.
|
|
The
next higher board is two feet shorter than the one below it, so that the
outer edges of the dump boards have a stair-step appearance.
|
Step
20. Add the Pedestrian Walkway
Note: Over
the years, there were various handrail configurations, and possibly some
floorboard changes as well. Several photographs show only 27 handrail
posts. No photographs clearly show the actual south end (uphill or Rico
end) of the pedestrian walkway.
|
Add the floor boards to
the pedestrian walkway support timbers as follows:
|
Carefully
cut-to-fit three 3" x 12" floor board to fit at each segment
of the deck corresponding to panels #1 through #7 of the tie / guard
timber sub-assemblies.
|
|
The angle for the
cut ends is 88.0898 degrees or position #2 on your angle cut jig, unless
otherwise noted.
|
|
Touch-up the cut
edges with stain.
|
|
On panels #2
through #6, the ends of the floorboards meet at the support timber
closest to bent caps #2 – #7.
|
|
On panel #1, the
left ends of the floorboards extend to the left edge of bent cap #1 and
have perpendicular end cuts.
|
|
On panel #7 the
right ends of the floor boards extend to the right edge of the
right-most walkway support timber and have perpendicular end cuts.
|
|
After you have
glued the floorboards to the walkway support timbers, use a needle to
impress nail holes at each support timber.
|
|
|
Add the handrail posts
as follows:
|
Cut twenty-eight
pieces of handrail posts, each 3" x 6" x 4’.
|
|
Touch-up the cut
edges with stain.
|
|
Glue a post to each
walkway support timber flush against the outer floorboard. The posts go
on the right (Bent #8) side of the support timbers.
|
|
After you have
glued the posts to the walkway support timbers, use a needle to impress
nail holes at the bottom of the post at each support timber.
|
|
|
Add the bottom railings
as follows:
|
Carefully
cut-to-fit seven 3" x 12" bottom railings, one per segment.
|
|
Touch-up the cut
edges with stain.
|
|
Glue the bottom
railings to the inside of the handrail posts, just touching or about an
inch (1/64") above the floorboards.
|
|
After you have
glued the bottom railings to the handrail posts, use a needle to impress
nail holes in the rails at each post.
|
|
|
Add the center and top
railings as follows:
|
Carefully
cut-to-fit fourteen 3" x 6" center and top railings, one of
each per segment.
|
|
Touch-up the cut
edges with stain.
|
|
Glue the top
railings to the inside of the handrail posts, with the tops of the
railing level with the tops of the posts.
|
|
Glue the center
railings to the inside of the handrail posts approximately halfway
between the top railings and the bottom railings.
|
|
After you have
glued the top and center railings to the handrail posts, use a needle to
impress nail holes in the center and top rails at each post.
|
|
|
Add the railing caps as
follows:
|
Carefully
cut-to-fit seven 3" x 6" railing caps, one per segment.
|
|
Touch-up the cut
edges with stain.
|
|
Glue the railing
caps to the handrail posts, on the post tops and flush with the inside
edge of the top railing.
|
|
After you have
glued the railing caps to the handrail posts, use a needle to impress
nail holes in the railing caps at each post.
|
|
|
Brush the pedestrian
walkway with an India ink / alcohol solution (about 25 drops per ounce) to
highlight the grain detail and nail head indentations.
|
Step 21. Add the Step-down at the Upper End of
the Pedestrian Walkway
Note: No
photographs clearly show the actual south end (uphill or Rico end) of the
pedestrian walkway. In most photographs taken from this end of the
bridge, either a large shrub or a locomotive blocks the view. Some
photographs indicate a possible step-down at this end with the walkway railing
angling downward similar to a staircase handrail. In any case, use your
own judgment and feel free to deviate from my directions if you desire.
|
If desired, add a
step-down at the upper (Rico) end of the pedestrian walkway. I fabricated mine using leftover pieces of 3” x 12”
boards from the pedestrian walkway floorboard material.
|
|
The step-down consists
of two four-foot wide steps, each with a 12-inch step and rise.
|
|
The steps are supported
on pieces of 4” x 12” dump boards mounted behind bents #8 and #8A.
|
|
The step arrangement is
shown in the following figure. The
handrail for the steps is omitted from the figure for clarity, but is
similar to the walkway handrail.
|
Figure
22 - Step-down
at Upper End of Pedestrian Walkway
Part 3. Jigs and Molds
General Information
Several types of jigs and
one mold were used to simplify construction of the model. These items can
be fabricated from various types of material such as wood, acrylic (Plexiglas),
or metal. Most of the jigs were used with my Dremel 4" table saw to
cut the various timber pieces. The jigs and molds that I used to build the
model of Bridge 45-B are described in this section of the article.
I use .093-inch thick
Plexiglas as the backing material for most of my jigs. It is available at
most home improvement stores (Lowe's, Home Depot, etc.) and comes in various
size pieces (8" x 10", 11" x 14", and larger.) For
most S-scale jigs, the 8" x 10" size is about right.
For jigs that slide in the
slots on my table saw, such as the angle cut jig and bent jig, I start by gluing
a strip of Plexiglas to the bottom of the Plexiglas sheet. This strip is
the width of the slot in the table saw, and will align the jig so that it slides
across the table of the saw in the same position every time. I glue the
strip a little further from the edge of the sheet than the actual distance from
the blade to the slot. After the glue is dry, I run the jig through the
saw blade. This will trim the edge of the jig precisely. This edge
of the jig will always be the "cut line". At his stage, I refer
to the jig as a "jig blank".
Note: You can
make jig blanks for either side of your table saw, depending on if you are
left-handed or right-handed. On my Dremel table saw, the blanks are not
interchangeable -- the slots are not quite the same distance from the blade on
the left and right. My personal preference is to make my jigs to slide
in the right side slot to the right of the blade. The plans shown for
the various jigs are drawn accordingly.
To make a specific jig, I
glue my paper plans onto the Plexiglas jig blank with spray adhesive or a glue
stick. I align the "cut line" of the plan with edge of the jig
blank that touches the saw blade. The cut line of the plan should just
touch the blade as you slide the jig through the saw. I glue small blocks
of wood to the paper plan with Elmer's Wood Glue as required to hold the pieces
being fabricated. When I no longer need the jig, I can peel the paper plan
from the Plexiglas, clean the adhesive residue from the surface, and re-use the
jig blank with another plan.
Angle Cut Jig
Description
The angle cut jig has four
positions. Starting from the top the positions are used to cut:
|
90 degrees - All
perpendicular end cuts
|
|
88.0898 degrees -
Walkway floorboard angled end cuts
|
|
86.1796 degrees - Long
stringer end cuts (both ends) and short stringer end cuts (at Bent #2 end
and Bent #7 end)
|
|
75.9638 degrees - 3 in
12 batter bent post end cuts
|
Figure
23 - Angle Cut Jig
How to Make the Jig
To make the jig, attach the
plan to a right side jig blank. Align the cut line at the left of the plan
with the left edge of the jig blank. Glue strips of wood along the four
position lines, directly below the lines so that the lettering is still visible.
I used leftover strips of 8" x 18" stringer material. Let the
left ends of the strips overhang the left edge of the jig. Run the jig
through the saw to trim the edges of the wood strips along the cut line.
How to Use the Jig
To use the jig, place a
piece of timber to be cut in the desired angle position, on the lettering and
with the left end slightly overhanging the left edge of the jig. Hold the
timber against the wood strip glued to the jig and push the jig through the
blade. Pull the jig back and remove the cut timber.
Notch Jig
Description
The notch jig is used when
cutting the notches in guard timbers that fit over the ties. The notch jig
is actually a type of "stop block" that is clamped to the table of
your table saw. The steps on the jig are spaced 12 scale inches apart.
This allows you to make a series of cuts in a piece of timber stock that will be
spaced 12 inches apart by making repeated passes through the blade, while
stepping the stock to the next notch position on the jig. Depending upon
the thickness of your saw blade, you will need to make a series of passes in
order to cut the 8 scale inch wide notches in the stock.
Figure
24 - Notch Jig
How to Make the Jig
The notch jig can be made
from any stiff material (plywood, acrylic, or metal). I made mine by
attaching the plan onto a piece of six-inch square, .063 thick, 6061-T6 aluminum
with a glue stick and cutting out the metal along the outside edges of the
"stair-step" lines.
Note: All
dimensions shown are actual dimensions.
How to Use the Jig
Here are examples of the
steps involved in making the notched guard timbers.
Figure
25 - Steps in Using the Notch Jig
To make the notched guard
timbers:
|
Place the angle cut jig
on the right side of the saw.
|
|
Adjust the blade height
for a 1/32" (2-inch) cut above the height of your angle cut jig.
|
|
Place a left side jig
blank on the left side of your table saw. This will raise the height
of the notch jig by the thickness of the jig blank. It will also
support the cut portion of the guard timber as it is notched.
|
|
Clamp the notch jig on
the left side of your table saw, on top of the jig blank, with the bottom
edge of the jig aligned with the front edge of the table. The
stair-step notches face the blade. The first (lower right) notch should be
about 3/8" (2-feet) to the left of the blade.
|
|
Make the first pass
through the saw:
|
Place a six-inch
long piece of 5/64" x 1/8 (5" x 8" x 32') guard timber
stock on the perpendicular cut position of the angle cut jig with the
1/8" side facing up.
|
|
Butt the left edge
of the stock against notch position #1 of the notch jig and run the
angle cut jig and stock through the blade. This will cut a 2"
deep slice into the 5" thick timber.
|
|
Pull the jig back
to the front of the table and advance the left edge of the stock to the
next (#2) notch position.
|
|
Continue making
cuts, advancing the stock to the next notch position after each pass,
until you reach notch position #17 for a total of seventeen cuts.
|
|
Your piece of guard
timber will now look like Example #1, assuming your blade is .050"
thick like mine.
|
|
Continue until you
have notched all fourteen pieces.
|
|
Note: A
1/8" thick blade would be perfect for this task since the eight scale
inch notches would be cut during this first pass through the jig notch
positions. Since most of you will have blades thinner than 1/8",
you will need to make at least one additional pass thorough the jig notch
positions. If your blade is 1/8" thick, skip the second
pass.
|
Make the second pass
through the saw:
|
To make the second
pass, move the notch jig a small distance to the left. The
distance is equal to 8 scale inches minus the thickness of your saw
blade. For me, this amount was 0.075" (0.125" -
0.050".)
|
|
Starting back at
notch position #1 on the notch jig, make seventeen more cuts in each
timber, just as before. These cuts will be slightly to the right
of the cuts made on the first pass.
|
|
Your piece of guard
timber should now look like Example #2, again assuming that your blade
is .050" thick like mine.
|
|
Continue until you
have made the second pass on all fourteen pieces.
|
|
|
Now clean out the
notches for each tie position.
|
Remove the notch
jig from the saw.
|
|
Carefully remove
the material between the cuts made on the first and second passes for
each tie position.
|
|
You can either do
this with a sharp hobby knife, or by running the timber through the saw
blade repeatedly while moving it slightly along the edge of the
perpendicular cut position of your angle cut jig.
|
|
Your piece of guard
timber should now look like Example #3.
|
|
Continue these
steps until you have made fourteen identical pieces.
|
|
|
Trim the fourteen
pieces of guard timber to length.
|
Trim the ends at
4" left of notch #1 and 4" right of notch #17.
|
|
Your fourteen
pieces of guard timber should now look like Example #4.
|
|
|
Extend the first and
last notches.
|
Extend notch #1 to
the left end of the timber and notch #17 to the right edge of the
timber.
|
|
Your fourteen
pieces of guard timber should now look like Example #5.
|
|
|
At this point, the
directions change, depending on which particular guard timber you are
making. Two guard timbers for panel #1 will not have lap joints on the
left ends. Ten of the guard timbers, for panels #2 through #6, have
lap joints at both ends. The remaining two guard timbers for panel #7
will not have lap joints on the right ends.
|
For the two guard
timbers that go on panel #1 (lap joints at right end only):
 |
Trim the 3-inch
thick portion of the guard timber above notch #17 into a 1 1/2"
thick lap joint.
|
|
To make this
lap joint, remove a 1 1/2" section from the bottom of the
timber at notch #17.
|
|
Your piece of
guard timber should now look like Example #6.
|
|
Repeat for a
second guard timber.
|
|
|
For the ten guard
timbers that go on panels #2 through #6 (lap joints on both ends):
|
Trim the 3-inch
thick portion of the guard timber above notches #1 and #17 into two
1 1/2" thick lap joints.
|
|
To make these
lap joints, remove a 1 1/2" section from the top of the timber
at notch #1 and a 1 1/2" section from the bottom of the timber
at notch #17.
|
|
Your piece of
guard timber should now look like Example #7.
|
|
Repeat for nine
more guard timbers.
|
|
|
For the two guard
timbers that go on panel #7 (lap joints at left end only):
|
Trim the 3-inch
thick portion of the guard timber above notch #1 into a 1 1/2"
thick lap joint.
|
|
To make this
lap joint, remove a 1 1/2" section from the top of the timber
at notch #1.
|
|
Your piece of
guard timber should now look like Example #8.
|
|
Repeat for a
second guard timber.
|
|
|
|
Now modify the seven
guard timbers that go on the inside edge of the bridge to fit over the
walkway support timbers by removing the tabs between the tie notches as
follows:
|
(1 piece) On one of
the two guard timbers for panel #1, remove the tab between the notches
for ties #1 - #2, #5 - #6, #9 - #10, and #13 - #14. This piece of
guard timber should now look like Example #9.
|
|
(5 pieces) On five
of the ten guard timbers for panels #2 through #6, remove the tab
between the notches for ties #1 - #2, #5 - #6, #9 - #10, and #13 - #14.
These five pieces of guard timber should now look like Example #10.
|
|
(1 piece) On one of
the two guard timbers for panel #7, remove the tabs between the notches
for ties #1 - #2, #5 - #6, #9 - #10, and #13 - #14. This piece of guard
timber should now look like Example #11.
|
|
|
Place the two guard
timbers for a panel into a labeled zip-lock plastic bag, along with fifteen
ties and four pedestrian walkway support timbers.
|
Each panel takes
two guard timbers, one on the inside edge and one on the outside edge of
the ties.
|
|
The inside edge
guard timber is the one with the tabs removed to clear the walkway
support timbers.
|
|
Note: The final
trimming of the lapped ends of the guard timbers is performed after the
tie / guard timber sub-assemblies are built and added to the bridge beams.
Bent Jigs
Description
I made a bent jig for every
different bent size in the trestle except bents #1, #8, and #8A. Since
bents #3 and #4 are the same size, they can be made on the same jig. This
example is for bent #5.
Figure
26 - Typical Bent Jig
How to Make the Jig
Glue a scale sized bent
plan onto a right side jig blank with the cut line of the plan aligned with the
left edge of the jig. The small rectangles on the plan indicate small
blocks of scrap wood glued to the plan to hold the bent components in proper
alignment. The shaded areas show where the cap and posts fit into the jig.
The dashed line shows where the diagonal cross brace attaches to the cap and
posts.
Notes:
Before I start to assemble the bents, I cut all of the bent caps and sills to
finished length and touch up the cut ends with stain. I cut the posts to
approximate length (about an inch or so longer than final length.) I cut
one end of these posts at a perpendicular angle and the other end at a 3 in 12
batter angle using my angle cut jig (so that it can be used either as a
vertical post or as an angled post.) I touch up both ends of the posts
with stain. I also cut the diagonal cross braces to final length and
touch up the ends with stain. I take all of the pieces for a bent and
put them into a labeled zip-lock plastic bag to keep them together until I am
ready to assemble the bent.
How to Use the Jig
|
Place the cap into the
bent cap position of the jig.
|
|
Put a small drop of
wood glue on the 3 in 12 batter angle cut ends of two posts, place them into
the angled post positions of the jig, and butt them up against the cap.
The posts should overhang the cut end of the jig by about an inch.
|
|
Put a small drop of
wood glue on the perpendicular cut ends of the other two posts, place them
into the vertical post positions of the jig, and butt them up against the
cap. The posts should overhang the cut end of the jig by about an
inch.
|
|
Run the jig through the
table saw. This will produce a perfectly flat cut along the cut line.
|
|
Touch up the cut ends
of the posts with stain. Allow stain to dry before continuing.
|
|
Glue on the bottom
sill, aligning the ends with the sill end lines.
|
|
Glue on one of the
diagonal cross braces at the reference position.
|
|
Carefully remove the
bent from the jig.
|
|
Glue on the other
diagonal cross brace for the opposite side of the bent.
|
|
Using a pin or awl,
insert nail head indentations into the diagonal cross braces at each
position where they would be nailed to the cap, posts, and sill.
|
|
Brush on an India ink /
alcohol solution to bring out the grain detail and nail head indentations.
|
Bent Attachment Jig
Description
The bents on a trestle are
always vertical. On a trestle where the deck is level, the bents would be
perpendicular (90-degrees) to the deck. Since Bridge 45-B is on a 2%
grade, the bents are not perpendicular to the deck. To calculate the
correct angle, just take the inverse cosine of the grade. The inverse
cosine of 0.02 (2%) is 88.854 degrees. Therefore we know that the bents
are attached to the deck at a 88.854 degree angle.
Figure
27 - Bent Attachment Jig
How to Make the Jig
|
The jig shown is
made from a stiff piece of material, with the top edge angled at 88.854
degrees from the sides.
|
Notes: All
dimensions shown are actual dimensions.
|
Think of this angle as
equivalent to a 2 in 100 batter. To lay it out on a piece of material,
start with a square rectangle. Starting at the upper right corner, measure
100 units along the top edge from right to left. From this point, go
in 2 units. Make a mark. Draw a line from the corner to the mark
and extend it to the left side of the rectangle. Cut or sand the
rectangle along the line. This is a very slight angle; it works out to
about 1/4" rise in a foot of length. My jig is made from a
two-inch square of 3/4" thick wood. The jig can be any width as
long as it can fit between bents. |
How to Use the Jig
|
First, I use the jig as
a template to set the tilting table of my Dremel 5" Table Sander.
|
I slightly sand the
tops of my bent caps to match the angle. This is not
prototypically correct, but the angle is so slight that the material
removed is not noticeable.
|
|
This angle provides
maximum contact between the bent cap and the deck when the bent is glued
in place.
|
|
Adjust the table
until the UPHILL BENT side of the jig is parallel to the table and the
DECK side of the jig is parallel with the sanding disk.
|
|
Remove the jig and
lay the bent flat on the table with the bent cap against the sanding
disk.
|
|
Turn on the sander
and remove only a slight amount of the bent cap.
|
|
Note: This is
easier to do before the diagonal braces are added to the bent. If the
diagonal braces are already on the bent, place shims between the bent and the
table as required.
|
Secondly, I use the jig
as a tool when gluing the bents in place.
|
Lay the upside down
deck on a flat work surface, and place the jig on the deck with the side
labeled DECK against the underside of the deck.
|
|
Glue the bents into
place, aligning the bent posts with either side labeled BENT.
|
|
Notes: To
use this method, the bents need to be glued to the deck before the pedestrian
walkway posts / railings and the bent footings are added. If a
cross brace on the bent is in the way, I remove a small piece of the jig BENT
edge so that the edge can fit snugly against the bent post. Make sure
you angle the bents in the correct direction toward the downhill or bent #1 (Ridgway)
end of the trestle.
NBW Positioning Jig
Description
The jig shown is used when
drilling the holes for the NBW castings in the exterior stringers. The jig
is used in three different areas: on the left edge of a stringer, on the right
edge of a stringer, and in the center of a long stringer.
Figure
28 - NBW Positioning Jig
How to Make the Jig
Draw the plan to scale on a
piece of paper.
Note: All
dimensions shown are scale dimensions.
Glue the paper onto a piece
of stiff backing material. I used thin brass shim stock. Drill #71 holes
through the plan and backing material at the six indicated points. Glue
two pieces of scrap 8" x 18" stringer material to the plan at the top
and bottom positions indicated by the darker shaded areas. Mark the center
reference line on the exposed surfaces of the two pieces of wood.
Carefully trim the backing material and wood along the outer edges of the jig
plan.
How to Use the Jig
|
Place the stringer to
be drilled into the jig, with the outside surface of the stringer (where the
NBW casting goes) against the paper.
|
|
Make sure the top of
the stringer (the edge where the ties go) is at the top of the jig.
|
|
To drill the two holes
at the left end of a stringer:
|
Slide the right end
of the stringer flush with the right edge of the jig.
|
|
Turn the jig over
and drill through holes #5 and #6.
|
|
|
To drill the two holes
at the right end of a stringer:
|
Slide the left end
of the stringer flush with the left edge side of the jig.
|
|
Turn the jig over
and drill through holes #1 and #2.
|
|
|
To drill the four holes
at the center of a long stringer:
|
First lay the
stringer on the plan and mark the centerline of the bent cap position
onto the stringer.
|
|
Then, position the
stringer in the jig and align the mark on the stringer with the center
reference line on the jig.
|
|
Turn the jig over
and drill through holes #2, #3, #4, and #5.
|
|
Notes:
Drill all holes from the backing material side of the jig into the stringers
with a #71 drill bit. The holes only need to go into the stringers about
1/16" or deep enough to accept the sprues of the NBW castings.
Tie Positioning Jig
Description
The jig shown is used to
assemble the tie / guard timber panel sub-assemblies. It will insure that
the ties are perpendicular to the guard timbers, and that the guard timbers are
uniformly spaced at the ends of the ties so that adjacent panels will mate
properly at the lap joints.
Figure
29 - Tie Positioning Jig
How to Make the Jig
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Draw the plan on a
piece of paper.
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Note: All
dimensions shown are actual dimensions.
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Attach the plan to a
piece of stiff backing material. Glue six pieces of 1/8" x
1/8" and 1/8" x 1/32" strip wood to the plan where indicated
by the shaded portions of the plan.
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How to Use the Jig
Note: All of
the pieces for each tie / guard timber sub-assembly should be in a labeled
plastic zip lock bag before you start.
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Open the zip-lock bag
for a sub-assembly.
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Place the two notched
guard timbers into the jig, notches facing up.
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Note: Insure
that the timbers are placed into the jig in the proper orientation: tie #1 end
to the right, tie #17 end to the left; inside edge guard timber on bottom,
outside edge guard timber on top (the inside edge guard timbers are the ones
with the tabs removed where the walkway support timbers will go.)
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Glue tie #2 into the
tie #2 notches of both guard timbers. Insure that the tie is flush
against the right side of the jig.
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Glue tie #16 into the
tie #16 notches of both guard timbers. Insure that the tie is flush
against the left side of the jig.
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Glue ties #3 through
#15 into the respective notches in the guard timbers. When completed,
there will be fifteen ties in each panel, corresponding to tie positions 2
through 16 on each guard timber.
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After the glue is dry,
remove the panel from the jig.
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Turn the sub-assembly
over and add three of the four 1/16" x 1/8" x 2 1/4" (4"
x 8" x 12') walkway support timbers between the ties.
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The walkway support
timbers butt up against the outside edge guard timber and go under the
inside edge guard timber.
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The walkway support
timbers go between ties #5 - #6, #9 - #10, and #13 - #14.
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Note: The
walkway support timber between ties #1 - #2 will be added later when the
sub-assembly is added to the deck beams.
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Place the tie / guard
timber sub-assembly on the work surface with the guard timbers on top.
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Drill a #71 hole into
the guard timber above the tie ends at tie positions #5, #9, and #13.
Add three #62 NBW castings to the holes in each guard timber.
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Place the
completed sub-assembly back into the labeled zip-lock bag with the single
unused walkway support timber.
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Note: On the
prototype, the guard timbers were attached to every fourth tie with 5/8"
x 8" Lewis lag screws (except at lap joints). At the lap joints,
the lapped ends of two adjacent timbers were bolted to the tie with a
3/4" x 12" bolt, washer and nut.
Refer to Figure 30 for details. This results in a NBW casting on
the timber above the lap joint (tie #1/17) and a NBW casting below the tie.
These NBW castings at the lap joints will be added after the panels are glued
to the beams.
Figure
30 - Guard Timber Attachment to Ties
Concrete Foundation Mold
Description
The concrete foundation for
Bent #4 was made by pouring plaster into a mold made from 3/16" square
strip wood. The surfaces of the strip wood on the interior of the mold were
distressed with a file card to impress grain texture into the wood.
How to Make the Mold
Figure
31 - Mold
Side Pieces
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Glue together pieces of
3/16" strip wood as shown to form the mold side pieces. Make two
identical sides. Don’t worry about the alignment holes; they will be
drilled later.
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Figure
32 - Mold
End Pieces
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Glue together pieces of
3/16" strip wood as shown to form the mold end pieces. Make two
identical ends. Again, don’t worry about the alignment holes; they
will be drilled later. |
Figure
33 - Mold
Assembly
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Assemble the four
pieces of the mold as shown.
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Now, drill the
alignment holes about the diameter of a straight pin through the ends and
into the sides after the four mold pieces are assembled. Exact placement of
the holes is not critical.
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Disassemble the mold
and spray the surfaces with a thin coat of paint to keep the plaster from
sticking to the wood. I used clear polyurethane spray paint. Let the paint
dry thoroughly before using the mold.
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How to Use the Mold
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Re-assemble the mold
pieces and insert straight pins into the alignment holes to hold the pieces
together.
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Set the mold on a flat
non-stick surface (I used a piece of Plexiglas) and place it on top of the
clothes dryer (or use anything that vibrates.)
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Turn on the dryer and
slowly pour plaster into the mold until it is slightly above the surface of
the mold sides. The vibration from the dryer will help release any trapped
air bubbles.
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Smooth the top surface
of the plaster level with the top of the mold with a straight edge. This
surface becomes the bottom of the foundation so absolute smoothness is not
critical.
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After the plaster sets,
remove the pins and carefully remove the mold ends first. Then remove the
mold sides.
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Note: The
surface of the casting that was against the Plexiglas is the top of the
foundation.
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