Description: Through timber Howe truss bridge
Prototype Information
In 1939 the Milwaukee bought two covered Howe truss bridges over the Chehalis River along the abandoned portion of the line at Mays and Dryad from Weyerhaeuser, and moved them to Park Jct. on the Tacoma Eastern to bridge the Nisqually River.
The WWSL
In the WWSL history, this bridge complex is the second at this location.Engineering
This design is characterized by inward leaning members that are heavier than the outward leaning members. The inward leaning members are in compression. In a static situation (no movement or
shifting of forces), the outward leaning members are not even needed. See Figure 1 for a description of the different parts of a Howe truss bridge.
The top chord is in compression and the bottom cord is in tension. There aren’t many trees big enough to supply large timbers long enough to build these chords
Modeling Information
Fortenberry, Curt. 121' Through Howe Truss Bridge, An Alaska RR Standard. Mainline Modeler, February 1986. Includes engineering plan and bill of materials.
Barrett, Michael C. Modeling Timber Truss Bridges. Michael was President of the Black Bear Construction Co., Inc., who made model bridge jigs and component kits. He passed away in 2018 and the company was dissolved.
I have a bridge clinic
he published before his death, and the web site that it was located on had been lost. I hate it when knowledge is lost and I am in the process of reconstructing that clinic below for your modeling information.
Construction
Almost all timber truss bridges are of the Howe design.
* The author designed and wrote the instructions for this product.
DIAGONAL BRACE ASSEMBLY
TENSION PLATE
TENSION BOLT
PORTAL CAP TOP
U-CHANNEL
TOP CHORD
Bridge Assembly Drawing
LATERAL TENSION ROD
PORTAL BRACE
PORTAL CAP
BOTTOM
TRACK GUIDE
FLOOR BEAM
NBW
STRUT
END POST
ASSEMBLY
VERTICAL TENSION ROD
COUNTER BRACE
ASSEMBLY
BOTTOM CHORD
NBW
Figure 1 – Components of Midwest Products’ Fox River Bridge kit. Notice
that the bridge can sit directly on the floor, table, or plywood. (Used by
permission of Midwest Products, Inc.)
without joints. The joints are generally combinations of cleats, plates, and bolts.
Lots of drawings and photographs are covered in the clinic.
The stresses in the braces and rods are greatest at the ends of the bridge. It is
common for the diagonal braces to be larger in size and the iron rods to be more
numerous as they get nearer the ends of the bridge. It is rare to take the size changes
into account when modeling. Also, the bottom chord was sometimes reinforced by
“scabbing” additional members onto the sides out in the middle of the bridge.
MIDWEST PRODUCTS’ FOX RIVER BRIDGE
Fox River’s “Success Series” instructions are similar to Midwest Products’ offerings
in other product lines. The Success Series Kit instructions contain detailed, step-bystep,
illustrated instructions that virtually guarantee that a modeler with only
moderate skills can build a good-looking, craftsman-quality bridge.
A full size drawing is included in the kit. It is used to build an assembly jig on a
piece of plywood. (The plywood is not provided.) This jig ensures the side panels
are identical and straight. The artwork on the drawing lets you line up the tension
rods so they are all
vertical and parallel.
Using music wire makes
sure they stay straight.
A key feature in the
design of the kit is the
stacking of the elements
(chords, braces, spacers,
and iron rods) in the jig.
The braces are supplied
with a thin “guide tab”
spacer that centers the
brace where it needs to
go. Figure 2 shows how
this guide tab lines up
the diagonal braces and
counter braces with the
elements of the chords
when the parts are
stacked in the jig.
USING THE FOX RIVER IDEA ON OTHER BRIDGES
Building the jig and deciding on the sizes of the pieces is the first task. Figure 3
shows key design parameters for several Howe deck and through-truss bridges.
Figure 4 shows typical wood sizes for modeling the Fox River Bridge in other scales.
BRACE
GUIDE TAB
TENSION ROD
JIG
Figure 2 – Brace and guide tab being installed in Fox
River jig.
You do not need a lot of detail on the drawing that is used to build the jig. But you
do need an accurate 1:1 schematic of the bridge. First draw two horizontal lines
separated by the distance that represents the inside chord height. Draw a series of
vertical lines that define the panels. These lines will locate the tension rods and the
end support points. See Figure 5.
Install blocks of wood along the horizontal lines. Place them so they do not interfere
with any pieces of the bridge. Stack up your chord stock and install blocks of wood
on the outer sides of the chords and at the ends of the chords. See Figure 6.
DECK BRIDGES Span Panel Inside Chord Inside Cl Top Chord Bot. Chord
RGS *
Bridge 45A (original) 108' 10'-3" 23'-3.75" 13'-3" 2-7x12, 2-8x12 2-7x15, 2-8x15
RGS
DRG Design, 1890 202' 12'-3/8" 23' 12.5' 4-8x14 4-8x16
THROUGH TRUSS BRIDGES
Fox River - HO Scale
Midwest Products 97' 12'-6" 26' 15'-6" 4-8x14 4-8x16
DRG/RGS * 128' 10'-2" 21.872 14 2-6x12, 2-7x12 2-6x14, 2-7x14
64A-Burns Cany on
CMStp&P 147' 10'-11" 25' 14'-6" 4-8x12 4-8x15
1891Natl Trans Mus.
Northern Pacif ic 150' 11'-1" 28'-4" 14'-6" 4-7.5 x 14 4-7.5 x 18
Kalmbach, p.40
NMRA Standards (Allow .5212" f or f loor beams, stringers, and track.)
HOn3 20.77' 12.23'
HOn3-Large 20.77' 14.95'
Std Ga, HO 25.53' 14.95'
* Narrow Gauge Ref erences: Carsten's Bridge and Trestle Handbook, Paul Mallory
Kalmbach's Model Railroad Bridges and Trestles
SPAN
PANEL
SIZE
INSIDE
CHORD
HT
INSIDE CL
WIDTH
TOP CHORD
BOTTOM CHORD
Figure 3 – Typical timber Howe truss bridges.
It is difficult to cut slits in the sides, or ends, of the braces in a non-production
environment. Therefore, a different method must be used to line up the braces. The
solution lies in using short pieces of the same music wire that is used in the tension
rods.
It is easy to estimate the lengths of the diagonal braces and counter braces. Cut your
material for these pieces. Make them about ¼” too long. Countersink the ends of
each brace with a pointed motor tool bit. Drill holes in the ends of the braces for the
iron rods. Twirl the brace in one hand while holding the electric drill with the other.
FOX RIVER THRU-TRUSS BRIDGE - BUILDING IT IN OTHER SCALES
IN THE KIT O Scale Sizes
HO Scale Sizes Narrow Ga. Standard Ga. S Scale N Scale
Top Chord 3/32 X 5/32 5/32 x 1/4 3/16 x 5/16 1/8 x 7/32 1/16 x 3/32
Bot Chord 3/32 X 3/16 5/32 x 5/16 3/16 x 3/8 1/8 x 1/4 1/16 x 1/8
End Post 5/32 X 3/16 1/4 x 5/16 5/16 x 3/8 7/32 x 1/4 3/32 x 1/8
Diagonal Brace 1/8 x 1/8 .2083 x .2083 * 1/4 x 1/4 5/32 x 5/32 1/16 x 1/16
Counter Brace 3/32 X 3/32 5/32 x 5/32 3/16 x 3/16 1/8 x 1/8 1/16 x 1/16
Portal Cap Top 1/8 x 3/16 3/16 x 5/16 3/16 x 5/16 5/32 x 1/4 1/16 x 3/32
Portal Brace 1/16 x 3/32 3/32 x 5/32 3/32 x 5/32 1/16 x 1/8 1/32 x 1/16
Styrene U-Channel 1/8 3/16" 1/4" 5/32" .080"
Struts 1/16 x 1/16 1/8 x 1/8 1/8 x 1/8 3/32 X 3/32 1/32 x 1/32
Vertical Tension Rods .025 dia music wire .047 dia .047 dia .032 dia .015" dia
Tension Plate .030 x .125 Styrene N/A N/A N/A N/A
Tension Bolt 1/4" Heavy Duty Staple N/A N/A N/A N/A
Lg NBW (Grandt Line) 5093 16 16 5123 5066
Sm NBW (Grandt Line) 5066 5093 5093 5066 N/A
* O Scale 10 x 10 or 7/32" x 7/32"
FOX RIVER PROTOTYPE FLOOR MATERIALS - HO
MUSIC WIRE .025" DIA .047 dia .047 dia .032 dia .015" dia
SPACER .0208" x .1041" .0416" x .125" .0416" x .125" 1/32 x 1/8 .0208" x .1041
STRINGERS 3/32" x 1/8" 5/32 x 7/32 5/32 x 7/32 1/8 x 5/32 1/16 x 1/16
Lg NBW (Grandt Line) 5093 16 16 5123 5066
Sm NBW (Grandt Line) 5066 5093 5093 5066 N/A
FLOOR BEAMS 1/8" x 1/4" 7/32 x 7/16 7/32 x 7/16 5/32 x 5/16 1/16 x 1/8
STRUTS 1/16" x 1/16" 1/8 x 1/8 1/8 x 1/8 3/32 X 3/32 1/32 x 1/32
Styrene U-Channel Midwest P/N M-724 3/16" 1/4" 5/32" .080"
FOX RIVER LAYOUT PARAMETERS
Panel size (inches) 1.724 2.547 3.125 2.344 0.938
Inside chord ht. (inches) 3.586 5.155 6.500 4.875 1.950
Figure 4 – Building Midwest Products’ Fox River Bridge in other scales.
If you have been trying to come up with a reason to buy a battery powered Dremel
tool, this is it. See Figure 7.
Follow these steps to construct the bridge:
1. Place one piece of the top and bottom chord stock in the jig.
2. Cut pieces of music wire to length and glue them in place with CA glue.
3. Cut end posts to rough length. Drill holes in the ends. Cut angles to fit. Insert
pieces of wire. Glue end posts between chords.
4. Cut pieces of diagonal brace stock to rough length.
5. Drill holes in the ends of the braces. Place short pieces of music wire in the
ends of the braces and glue them to the chords using CA glue. See Figure 8.
6. Repeat these steps for all the braces on this layer.
7. Place additional chord material in the jig and continue installing the diagonal
braces or counter braces until the side panel is completed.
8. Build two side panels and install the flooring and bracing to join them together.
SPAN
PANEL
SIZE
INSIDE
CHORD
HT
Figure 5 – Drawing the lines for the jig.
2: GLUE BLOCKS ALONG OUTSIDE AND ENDS OF CHORDS
3: CUT BRACES TO LENGTH
1: GLUE BLOCKS ALONG INSIDE OF CHORDS
Figure 6 – Build the jig by fitting blocks to the chord stock.
CONCLUSION
Many very difficult challenges can be overcome by using the techniques presented
here to build a Howe truss bridge of just about any size. The easy-to-build jig and
registration wires allow the stacking of the components. This gets around the very
difficult task of drilling straight and properly aligned holes. Thus, a great looking
timber bridge is within the reach of any moderately skilled modeler.
COUNTERSINK HOLE
POWERED
DRILL BIT
TWIRL BRACE BACK AND FORTH
WHILE DRILLING HOLE
ANGLE CUT ENDS
WITH WIRE
INSERTED IN HOLES
Figure 7 – Procedure for drilling centered hole in the end of the diagonal and
counter braces.
BRACES WITH
WIRE IN ENDS
Figure 8 – Building the truss bridge with wire “guide tabs.”
Key points to remember:
1. Build two identical side panels.
2. Drill holes in sides of top and bottom chords. Holes on inside are for lateral
tension rods. Holes on outside are for NBW’s.
3. Insert tension rods on one side panel.
4. Glue two floor beams to one side panel. Make sure they are square.
5. Glue the first two floor beams to other side panel.
6. Install rest of floor beams.
7. Slide lateral tension rods into other side panel.
8. Install struts, channel iron, and other details.
References.
Harold Russel that I referred to in my last post, in the Dec. 1988 MRR.
It was actually part 6 of a series that Harold did, starting in July of 1988.
The book Model Railroad Bridges & Trestles contains drawings of a Northern Pacific 150-foot Howe truss bridge.
Fox River Through-Truss Bridge kit
Fox River Through-Truss Bridge -- 11-7/8 x 2-5/32 x 3-1/2" 29.7 x 5.4 x 8.8cm
Midwest Products Co., Inc. - no longer in their catalog
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