The On-Line Magazine of Rideable Model Railroading
  NUMBER EIGHTY ONE

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©APRIL 4, 2007   
 

©Scale Diversions.com where it originally appeared. Article used here with permission.
This material may not be republished, rewritten or redistributed without written permission of the author.
It may however be reproduced for use as a construction guide at the local level.
If you attempt a project like this, you are responsible for calculating the load handling ability.
 

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Bridge and Trestle Construction
"Building the North Pacific Coast Railroad"


This bridge is a focal point on the North Pacific Coast Railroad.


Written by Ken Stanfield

The bridge and trestle span 90 feet on a 2 percent grade. The 30 foot radius curve covers 90 degrees of turn. The bridge is very rigid, there is no detectable deflection in the bridge even when a person tries to bounce on it. The structure is sturdy enough to handle heavier equipment. There have been no derailment on the trestle or bridge. I am very pleased with the results.

While the bridge design is based on actual truss bridges, it is not intended to be to scale. The bridge has a 15 foot span and is supported by double bents at each end. This area of the NPCRR is very dry in the summer and very wet in the winter. Water flows across the meadow and under the bridge during rain storms. The gravel embankments could have been extended to do the same job but I preferred the look of the bridge and trestle.

Planning and construction took several months. First the right of way was marked on the grass using upside down marking paint. Control points were set by locating center points for the approach curves at each end, then using a string line to mark the curve centerline. The next step was to mark the tangent between the curves. The first bent was marked at the point where the curved trestle ends and the straight portion begins. Working both directions, bents were positioned every five feet. I established the end points and decided on the length and position of the bridge. Finally, I received design and right of way approval from the governing authority, my wife.

The first step in construction was to repaint the centerline and mark the position of each of the two foundation holes that would support the base. With a rented power auger, it took about three hours to drill all of the holes. The post holes are 2 to 3 feet deep because of the soft ground. Click thumbnail images to enlarge.

The concrete footings were cast in place. First, the forms were positioned over the holes. Rebar was added and foundation bolts were positioned to hold the bents in place. and left to cure. I poured two footings at a time (the forms are reused) with a total of 13 single footings for the trestle, two double footings for the bridge and two end wall footings. The tops of the single footings are 49 inches long and 4˝ inches wide. The sides slant outward at about a 15% angle. Slanting the sides gives a better “look” and allows the form to be easily removed. They support a bent with a 4 foot long base. On the curve, the forms were elevated ˝ inch on the outside for superelevation. I did this so that I could build bents with the top beam parallel to the bottom.

This is the mockup bent (left) that I used to verify the lengths of the bolts that would be needed. It also allowed me to better visualize proportions and check for interference between fasteners. One change that I made was to use standard treated 2x4s for the outside stringers and rotate the 3x4s stringers to match. Using standard dimensional lumber saves lots of money.

While the bridge and trestle are not “scale”, they have the right “look”, are functional and safe. Because the tallest bent is about 24 inches, I was able to use a standard 4 foot bottom length on all bents. Taller bents should have a longer base to provide more side to side stability. If you are not sure how long to make the base, build a mockup of your tallest bent from 2x4s. All of the others can be measured from it. Taller bents also require diagonal bracing. Lag bolts were used to attach the bent posts to the top and base. The top bolt heads are exposed but the bottom heads and washers are counter sunk into the underside of the base. This is not how a “real” railroad bent is built but it simplifies construction . Each bent was custom built for the required height. Thus, the height of the concrete footing was not important, only that it be properly located on the centerline and above ground level to keep the wood dry.

The end walls were built last in order to match their height to the trestle. The 2% grade of the trestle was extended out onto each approach then transitioned to a new grade. In order to maintain good track work and prevent derailments, I avoided sudden grade changes on the trestle and at both ends. The sides of the bridge were cut and assembled in the shop. While the parts are the same, the right side is a mirror image of the left. They were assembled from 2x4s with 2x6s used for the end diagonals only. The idea is for the bridge to hang from the top rails while the diagonals provide end to end stiffing. I used 3/8 inch steel rods 4 feet long to hang four 3x4 lateral beams below the stringers. The center 3x4 stringers are bolted to these lateral beams for strength.

Oversize washers and square nuts were used for aesthetic effect. I used cut lengths of 3/8 all-thread rod for bolts so that I could use square nuts on each end. Lockwashers hold the nuts in place. All hardware on the trestle and bridge was painted black before installation for looks. The inside clearance is just under 4 feet. The sides were spaced so that the four foot decking would fit within the structure on top of the 2x4 sides. Consequently, the top beams of the support bents are wider – 5 feet – to support the bridge sides. Notice that the ends of the stringers are notched to provide a lapped joint with the next stringer. Also, a layer of blue foundation foam keeps the wood bents from direct contact with the concrete. I believe that this design could easily be extended to 20 feet if desired.

The bents are located about five feet apart at the centerline. Since I had a clear area to work in, I used a string from the center to swing an arc to each bent to mark the centerline. I then worked out from the center line of each bent to position the stringers. A 16 foot length of lumber made three stringers for a curve or covered 15 feet of straight trestle. I used a lapped joint between stringers that allowed me to fasten both stringers to the top beam of the bent with one bolt. This also provided a pivot point for the curve. These lapped joints were heavily caulked to retard water infiltration. All treated wood cut ends of the bridge and trestle were brushed with preservative.
Flat washers are used with all bolts and nuts. I used standard nylon lock hex nuts on the trestle. The decking was made from used cedar 2x4s and fastened with deck screws . I set them with a 1 ˝ inch spacing on the straight while on the curve I pinched the inside spacing and expanded the outside spacing when necessary but only at the pivot points. I adjusted the position slightly to keep the bolt heads exposed below them. This will provide access to the bolt heads for tightening. The only ones that I can not tighten are the hidden ones in the bottom bent beams. One reason for using the cedar is the warning on the treated wood that says that it should not come in direct contact with aluminum.


click image to enlarge

The final step was to install rail, guard rail (optional) and a wooden edge rail (must have). Because of slight variations in height, some 2x4s had to be belt sanded to provide a flat plane for the rail. With careful 2x4 selection and placement, this is a minor problem. Both the guard rail and the edge rail are safety items. The guard rail captures a wheel if it derails and prevents the engine or car from traveling sideways over the edge. The guard rail needs to be spiked about every fourth tie. The edge rail is a backup system. If the wheel in not trapped as designed, the wooden edge rail will usually stop a derailed car from going off the trestle. I used a Trex 2x2 for the wooden edge rail.

Continuing maintenance includes tightening of nuts and bolts and treating exposed cut ends to prevent deterioration of the wood. Special tools that are helpful in construction include: quick clamps, a 10 inch miter saw, long drill bits, a 3/8 inch by 16 thread die and handle.

Be Safe. If you attempt a project like this, you are responsible for calculating the load handling ability of your structure. Failure of a bridge or trestle can result in injury or death. The author and this website are not responsible for checking your calculations and workmanship. 

 

Written by Ken Stanfield Ken operates "SCALE DIVERSIONS"

 


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