November 6, 2009

Arched Truss at Potomac Creek

I have reaffirmed my decision to build the arched truss. I got some interesting feedback from several folks about how a spindly trestle might look cool. It's true that most of the USMRR building projects were by necessity expedient. But in the case of the Aquia and Fredericksburg line, the Union Army was encamped for several months. That whole time the highly industrious USMRR construction crew under the command of W.W. Wright and Herman Haupt were hard at work.

If you look at the pictures on this blog you'll see a RR that for the most part is built to a high standard. The first trestle, the “bean pole” one, was built quickly and it captured Lincoln's imagination. But it was gone by the time I am modeling. Over that winter the new trestle was substantially better built using sawn lumber as opposed to local logs. In early spring Haupt replaced that trestle with the truss to avoid washouts that he worried could take out the trestle. It utilized the existing stone piers and abutments.

Also consider that the USMRR Aquia and Fredericksburg line used the existing roadbed of the RF&P. It was built before the war with good drainage, stone abutments and a level right of way. The roadbed did suffer damage when the confederate cavalry churned it up during their retreat, but the USMRR construction crews quickly re-graded it, albeit using compacted soil and not gravel ballast.

So having a sturdy bridge, as opposed to a flimsy bridge, better suits the image of a railroad serving a powerful army with nearly unlimited logistic might behind it.

I did a quick spreadsheet calculation to see what the truss will involve. Each section of arched truss will require about 438 individually cut wood parts and over 600 pieces of hardware including about 256 NBWs. Multiply by three for three sections. That's a lot of pieces. Thank goodness for the laser.

Wood Part


No. per panel

Number of panels

Total per section

Diagonal Braces and Uprights

48

2

96

Top & Bottom Chord

4

2

8

Arch Laminations


3

2

6

Vertical Hanger


10

2

20

Horz sway braces


32

1

32

Lateral Hanger


16

1

16

Sway Braces


32

1

32

Main Stringers


2

1

2

Bridge Deck


64

1

64

Rail Stringer


2

1

2

Bridge Ties


64

1

64

Arch Spacers


16

2

32

Chord spacers


16

4

64





Total wood parts

438

Metal parts & Hardware




Spikes



256

1

256

Vertical Rods


10

2

20

NBW



128

2

256

0.015 Pins


48

2

96



I built the first panel of truss last night. My approach was to scale down the bridge by 50 percent, both in

overall layout and individual members. This keeps the overall look correct, but I am concerned that the truss members are too small for an O scale bridge. For example, the prototype chord is a lamination of 3x12 beams, 4 on each side of the truss members for a thickness of 24 inches of wood across the chord. That works out to 8 pieces at 0.062" thick and 0.25" inches tall. I used 0.062" by 0.125". The overall proportions are good, but the individual members look too flimsy.

I consulted Haupt’s book (I was able to borrow a copy of the original print run from 1864 – reading 145 year old source material is cool) and determined that even for shorter spans the chords should be 12-by lumber. That means I need to rework the drawings with thicker chords. I also now better understand the means of fabricating the chords, so I should be better able to replicate it.


As to proportions, Haupt says that trusses, be they deck or through, should be 16’ - tall enough to clear a locomotive. This allows the use of standardized parts in either type of design. That works out to a truss depth of 4 inches in O Scale. I think that won’t look right in my application. I need to do some more mock-ups to check it out.

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