Timber trestles come in two varieties -- frame and pile. In frame trestles, the individual bents (the vertical structures that support the roadbed) are made of lumber and sit on a prepared pad, which itself rests on a footing made of piles or masonry on a rock surface usually. Pile trestles are made of timber, trimmed at the butt of the log
Pile bents are generally used where the ground is quite soft, and may either occasionally
or constantly be covered with water; also where the distance from the rails to the surface of
the ground is not very great. Their main supports are driven deep
into the ground until it reaches a rock surface.
Pile trestles are of general design, the major difference is the size of
the timbers and braces, and the number of piles in the bent. Prior to
the mid-1950's timber trestles were primarily 5 pole or frame bents and
post 1950's 6 pole bents were used on heavy traffic lines.
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| Timber Pile Trestle |
Engineering - Timber Pile Trestles
Timber should be of straight, sound, live heart timber, perfectly free from windshakes, wanes, large, loose, black, or decayed knots, cracks, worm-holes, and all descriptions of decay. Timbers should be stripped of bark.
Timbers can be creosoted, the creosote end driven into the ground.
Round piles are, as a rule, from 12 inches to 15 inches across the butt after being cut off, and when they are wider than the cap, the portion which projects on either side -should be adzed off to an angle.
Pile trestles are built with a variable number of bents. Bents are close together, usually about 15 feet. insuring that the settling or failure of one, or possibly more, does not render the entire structure unstable. The bents are constructed on site.
The timber piles driven
into the ground at different angles -- the center pile or piles straight
down, and those on either side leaning inward, resulting in a
triangular-shaped bent with a wide base. When all the piles in a bent
have been driven as deep as practicable, their tops are cut level and
capped with a large, transverse timber beam (top cap). The individual piles of each
bent are interconnected with horizontal, diagonal and lateral bracing.
Bents are connected by stringers (also called "chords") laid longitudinally atop the bent caps, usually spanning at least two bents, and staggered. These support wood cross-ties to which the steel rails on which the trains run are attached.
The WWSL
Modeling Information
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Another interesting point is that the steel bent pier had a steel bulkhead protecting the bent and there was a sizable rock rip rap along the base to protect the bent from erosion.
Scratchbuilding
I wanted to build a 6 pile bent. I could not find any Milwaukee Road timber pile trestle plans. I did find a Southern Pacific Standard Drawing for a 6 pile ballasted deck trestle.
I have made several assumptions based on the photographs above for my model.
- The bridge is front and center in the Satsop River Section and will be built of dimension wood. I will be using the SP standard drawing for engineering and materials.
- The piles will be field cut timber close to (but not necessarily) the common standard dimension. The piles will be field creosoted at the base, with additional creosote along its length depending on the age of the structure.
- The bents will be of the same height but appear to be different heights to accommodate the unevenness of the terrain on the Satsop River Bridge Section.
- There is one short timber bent located on top of the concrete cap. That bent will be a timber frame with a horizontal footer, and custom cut to height.
Selective
compression of the pile trestle will require construction of a wood
bulkhead, 5 - 6 pile bents. The maximum height of the pile bents will
be distance from the top of the section base (the 3/4 inch plywood) to
the top of the subroadbed (bottom of the mainline ties). That distance
has been measured as 3.64 inches (actual) but for modeling purposes I
will size the jigs for 3.5 inches and shim the bridge as required at
installation.
Project Process
Satsop River Bridge N5 (Pile Trestle)
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Tool List |
Materials Top cap |
Preliminary Preparation
- Cut the wood for the bents and stringers.
- Distress the wood as necessary.
- Stain the wood.
- Nut Bolt Washer (NBW) parts will be painted prior to installation
- Bridge ties will be painted prior to installation.
- Bridge rails will be painted prior to installation
Construction
BUILDING THE BRIDGE JIGS
There will be two jigs necessary. Jig 1 is for building the pile bents. Jig 2 is for building the stringers. See 11.xxxx Jig Construction for further information
- Construct the pile bent jig.
- Cut the legs and top cap to length and placed in the jig. Use the jig as a guide to cut the angles on the tops of the legs.
- Cut the side bracing on the legs while the wood is still in the jig. Remove the bent from the jig and .
BUILDING THE STRINGERS
- Construct the stringer jig.
- Cut the stringers to length and placed in the jig. Use the jig as a guide to place the stringer separator washers.
- With a razor blade, slightly slice the outside stingers to mark the board joins. Add the connector plates while the stringer is still in the jig. Remove the stringer from the jig and touch up the stain as necessary.
ATTACHING THE BENTS TO THE STRINGERS
- Place the stringer construction jig on a horizontal surface. Attach the bent to a machinist square with small rubber bands. Adjust the bent and glue the top cap of the pile bent to the stringer at the proper location. Use the machinist square to make sure the bent is perpendicular to the stringer.
- Glue the girts on the outside of one side of the assembly.
- Glue the girts on the inside of one side of the assembly.
WEATHERING THE TRESTLE
- Pan Pastels
Reference:
Southern Pacific Standard Drawings Pile Trestles
Milwaukee Road's S-Curve Trestle (Tacoma)
https://preservedwood.org/wp-content/uploads/TimberPileManual.pdf
TRESTLE BUILDING CLINIC Michael C. Barrett
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