Technical Corner

From Sunrise Herald, May 2016

Gerry Glancy presented the May clinic on building timber trestles. We tried something different for the presentation this month. With Gary Myers as the cameraman, we took videos of the presentation that were projected onto a screen for better visibility by everyone. This approach was well received.

Gerry preparing for his clinic

Gerry began his talk by describing the three main structures of a trestle: deck, the bents and the abutments. He then sketched each of these structures on a whiteboard. In the diagram below, the deck consists of the stringers, bridge ties and guard timbers.

The deck is made from stringers arranged side- by-side with bridge ties secured on top. Stringers may be the length of the bent spacing on a curved trestle, or multiples of the spacing for a straight trestle. The diagram shows the stringers slightly spaced apart, but on a model the spacing may be omitted. Placing three stringers under each rail is typical. They are positioned so that the center stringer is beneath the rail. Bridge ties are usually ten feet long and centered over the stringers. However every few bents there will be a pair of longer ties, 15 to 20 feet long. These will project to the side of the trestle to provide a platform for water barrels, but also provide a refuge for maintenance crews if they find themselves out in the middle of the trestle when a train approached.

Railroads were adamant about full water barrels to provide water for extinguishing fires on the trestle. Fires could be caused by cinders and sparks from a locomotive (even diesels can emit these) or from badly overheated brakes or journals.

The bents are the support structures for a timber trestle. Bents are typically spaced about 15 feet apart. The center post of each bent is called the plumb post because it is always vertical. It is called a post if it is positioned on top of a sill, but for short bents it may be driven directly into the ground using a pile driver - in this case it is called a plumb pile. The posts or piles on either side of the plumb post are called batter posts / piles because they are angled (that is battered) outward toward the bottom. The innermost posts typically have a one foot outward horizontal displacement at the bottom for every eight feet of vertical rise. The outermost posts typically have a one foot outward horizontal displacement at the bottom for every four feet of vertical rise. Bents made from pile posts are usually limited to 30 feet or less that is about the limit of the usable part of a tree trunk. Construction crews often cut trees on-site when building a trestle if the timber was near-by. Taller bents had to be made from multiple short bents stacked one on top of another. A cap sill will be placed on the top of each bent and a mud sill at the bottom. Contrary to assumptions, the mud sill does not sit on the ground. It would quickly disintegrate when saturated with water. Instead it sits on a foundation of concrete blocks, dressed stone, gravel or concrete footers. After a few years this foundation will soon be covered so that it is no longer apparent.

The first step in construction is to build the foundation benchwork for the trestle that will eventually be installed on the layout. Construct it from quality lumber that is not warped. Each vertical block will be the foundation for a bent. They should be placed approximately 15 feet apart, or the spacing of the bents, and the top surface should be absolutely level. Allow extra spacing for items that will pass under the trestle. In this case Gerry allowed for a track to pass beneath and the creek will flow beneath at the center. Here there will be double bents with a longer deck structure between the bents. They also need to follow the centerline of the final trestle. The photo below shows Gerry’s foundation. Note that Gerry built the abutments right onto the end foundations that are at grade level.

Also, omit the stringers in the area where other deck structures will be placed. Now place the completed deck structure between the two end foundations. It should fit snugly against the abutments as in the photo below.

.Notice that Gerry has erected a vertical strut centered beneath the deck to support the curved deck from falling. It has been cut precisely to match the track elevation. Now it is time to begin construction of the bents. Gerry constructed a bent template from sheet styrene (not shown) with styrene strips to locate and support each post correctly. For each bent measure the elevation from the foundation to the underside of the stringers. Shorten this height slightly. Gerry emphasized that it easier to shim a bent that is a little too short than to cut a bent down that is a little too long. Locate the cap sill at the top of the template so that it is centered over the plumb post. Before cementing the plumb and batter posts to the sill, dab the end of the post with a little Dullcoat. This will seal the grain and allow the cement to hold better. When dry, apply a little ACC with a micro brush to the end of the post and set it against the sill. The cement should set quickly. Repeat this process with the four batter posts but angle the ends so that they will mate squarely with the cap and mud sills. Next cement the sashes and sway braces in place with a spot of ACC. Turn the bent over and cement the sash(es) and sway brace(s). At one point Gerry did not get a sash horizontally aligned with its opposite, so he applied some ACC release (un-cure), from the green bottle to release the sash so he could reposition it.

When all bents have been assembled it is time to attach them to the bridge deck. Gerry’s trestle and deck is on a 3.5% grade so the bents cannot be mounted square with the deck stringers. He used a T Bevel to set the correct angle so that each bent would be vertical with respect to the ground. He made two jigs to capture this angle, shown in the photo below. He used these jigs to hold each bent in position when he cemented them to the stringers.

Where his trestle crossed the track beneath, he built a short steel deck girder bridge to span the gap. A second bent positioned against the original bent supports the deck girder bridge. The stringers are not included in this gap. Refer to the photo below and notice the double bents at this location. Where his trestle crosses the creek at the center he built a queen post truss that spans the double bents in a similar fashion. A photo of a similar span taken from Model Railroader appears below. Note that that the truss rests on half of a double bent and that the deck stringers rest on the cross members of the truss.

A trestle with a steel deck girder in place. Note the double bents here. The shorter bents support the deck bridge and the longer bents the trestle stringers.

An example of a queen post truss from an article in Model Railroader.