4.1.9 Satsop River Bridge N5 (Timber Pile Trestle)

Prototype Information

Trestle. Trestles are a single bridge which require one or more intermediate supports. Timber trestles were common in the early days of railroading. The intermediate supports were either piles - tree trunks driven into the soil to provide a better bearing and then fastened together with various size timbers, or timber frame, sawmill cut lumber supported on a timber or concrete foundation. More modern trestles trestles are built of steel from a variety of "I", "WF" and "T" shaped steel. 

Trestle bridges first rose to prominence during the 19th-century railroad boom. Timber was plentiful and crews needed a quick solution to bridge ravines, wetlands, and flood-prone valleys. In many cases, wooden trestles served as temporary structures, allowing materials to be transported across the intervening terrain to continue railroad construction. The trestle could then be abandoned, upgraded by filling in the trestle footprint with stone and dirt (creating a permanent embankment beneath the tracks), or rebuilt at some later time with more permanent material - stone, or steel. 

A trestle is a common general design. essentially a series of short spans supported by closely spaced frames—or bents—that transfer loads from the deck to the foundations. A trestle deck can be built as either an open deck or ballasted deck.

The bridge's primary components include (from top to bottom):

Deck: The flat surface span that carries traffic—rail, ties, ballast if the bridge is a ballasted deck design, and stringers. 

Bents: Vertical frames, often composed of timber posts or steel beams, that carry the deck’s weight. The cap beams collect deck reactions, transferring vertical shear and bending moments into the bent posts or piles.   

Bracing: Diagonal and horizontal members that stabilize the bents against wind and dynamic loads. They resist lateral drift, ensuring that the bent acts as a rigid frame under side loads.      

Piers/Foundations: The substructure elements (piles or footings) that transfer loads from bents into the ground. They convey all forces—vertical and lateral—into the soil.

Trestle Nomenclature Bent Nomenclature

Engineering - Timber Pile Trestles

The Timber Pile Trestle is generally used where the ground is quite soft, and may either occasionally or constantly be covered with water; or where the distance from the rails to the surface of the ground is not very great. Prior to the mid-1950's pile trestles were primarily 5 pole bents.  Post 1950's 6 pole bents were used on heavy traffic lines. The pile is a trimmed butt of a log, 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.

The main support (pile) should be of straight, sound, live heart timber, perfectly free from wind shakes, wanes, large, loose, black, or decayed knots, cracks, worm-holes, and all descriptions of decay. Piles should be stripped of bark. The pile is driven deep into the ground (narrowist diameter down) until it reaches a rock surface (foundation). The piles are 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 bracing.

Individual bents are connected together by stringers (also called "chords") laid longitudinally atop the bent caps, usually spanning at least two bents, and staggered. These stringers support wood cross-ties to which the steel rails on which the trains run are attached. The individual piles of each bent are interconnected with horizontal braces called girts, also spanning at least two bents, and staggered.

The WWSL

The Satsop River Bridge N5 is a Timber Pile Trestle. The prototype structure was on the original Tacoma eastern main line, purchased by the Milwaukee Road, and after abandonment used by the Mt. Rainier Scenic Railroad (MRSR). 

A search of the internet found a couple of pictures of the trestle when it was used by the both the Milwaukee Road and the MRSR. pictures show nine 6-timber pile bents, and one 6-steel pile bent with a concrete cap at the point where the timber trestle meets the plate girder bridge. You can just barely see that there is a small wood frame bent resting on the concrete cap holding the wood stringer.

North Bank 1

North Bank 2

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.

Modeling Information

The WWSL uses Union Pacific Common Standards whenever possible. In this case I could not find any Milwaukee Road timber pile trestle plans, but I did find a Union Pacific 5-bent trestle standard drawing. I also found a Southern Pacific Standard Drawing for a 6-pile ballasted deck trestle. 

Union Pacific 5-pile deck trestle

Southern Pacific 6-pile ballasted deck trestle

 

 I have made several modeling decisions based on the common standard diagrams above for my model:

1. The bridge is front and center in the Satsop River Section and it will be built of dimension wood, with appropriate bridge details (bolts, spikes, guard rails.
2. I will use the SP standard drawing for the generic 6-pile bent construction, using UP standard materials. 
3. I will use the UP common standard drawing for all other trestle construction (guard rails, stringers, fire safety, bracing, etc.
4. The piles will be field cut timber close to 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. 
5. 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.
6. 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.

The timber bulkhead construction information is located at Satsop River Bridge N5 (Wood Bulkhead).
 

The steel bent construction information is located at Satsop River Bridge N5 (Steel Pier) .

 

Designing the bridge

Selective compression of the pile trestle will require construction of a wood bulkhead, five 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 (Timber Pile Trestle)

Tool List Pile Trestle Jig SuperJet Glue

Materials Top caps - 12x14 Horizontal Braces - 6x8 Diagonal Braces - 4x8 Pile Legs - Stringers - 8x17 Girts - 6x8 Nut Bolt Washers

 

Preliminary Preparation

1. Stain the wood.
2. Cut the wood for the bents and stringers.
3. Distress the wood as necessary. 
4. Nut Bolt Washer (NBW) parts will be painted prior to installation.
5. Bridge rails and ties 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. I used the Common Standard drawings above, formatted the size to HO scale, and glued the drawing on a styrene sheet. Scrap styrene was used to xxx accurately place the wood parts for quick assembly. See 11.xxxx Jig Construction for further information

BUILDING THE BENTS

1. 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.  
2. Cut the side bracing on the legs while the wood is still in the jig.  Remove the bent from the jig and .
3.  
4. drill the and  Install NBW
   

BUILDING THE STRINGERS

1. Construct the stringer jig.
2. Cut the stringers to length and placed in the jig. Use the jig as a guide to place the stringer separator washers.  
3. 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

1. 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.  
2. Glue the girts on the outside of one side of the assembly.
3. Glue the girts on the inside of one side of the assembly.

WEATHERING THE TRESTLE

1. Pan Pastels
2. Timbers can be creosoted, the creosote end driven into the ground.
   
   
   

 Reference:

Southern Pacific Standard Drawings Pile Trestles

 Milwaukee Road's S-Curve Trestle (Tacoma)

Southern Pressure Treaters' Association, Timber Pile Design and Construction Manual

TRESTLE BUILDING CLINIC Michael C. Barrett