Page 26 - September October 2014

The Fildid & Wissat RR – a Garden Railway
PART 8: Building The Trestle; Assembly
In my last article I described the construction techniques for the trestle bents and the problems I had with placement of the bents due to the need to accommodate the intermediate and lower tracks running beneath. This time I will deal with the assembly of the trestle to its final form.
All the timbers in a trestle are in compression, not tension, and the space between adjacent bents is called a panel. In order to give stability to the trestle and form a roadbed for the track, adjacent bents are joined by timbers called stringers which rest at their ends on the caps of the bents, as shown in Figure 1, and form the so-called deck. The deck is completed by laying ties across the stringers, and guard timbers are often laid across the ends of the ties along the edges of the deck. On the ties, rails and usually guard rails are laid in the usual way.
Additional stability is given by wall bracing, which is made up of sway braces extending diagonally between adjacent bents. Where the bents are several stories, their cross-braces are joined by horizontal braces called girts and sway braces are arranged at each storey.
Wooden trestles were used on many railroads, from temporary logging lines to Class 1 roads. There was thus a wide variation in the strength and stability built into trestles depending on their intended use and loads expected. There was also a difference in the safety factors built in. Thus trestles used on logging lines might be quite rudimentary while those used on common carriers would be more sophisticated. Most trestles were built with open decks, but some were ballasted. Open-trestle decks may consist of several stringers side by side either in an open pattern as shown in Figure 2 or bolted together to form compound beams. In the latter case often three timbers were joined with spacers between them to permit water to flow through and inhibit rot. Again, the timbers used for stringers varied in size according to use, and their dimensions would depend on the spacing between the bents. The individual timbers used are typically 16–24 inches high and 10–12 inches wide, although the individual timbers used in compound stringers might be narrower. The number and location of timbers used on open trestles will also vary according to whether the track is tangent or curved. In the case of ballasted decks, the stringers are laid right across the width of the roadbed.
Figure 3 shows schematically that in the case of a trestle constructed on a tangent, the running rails may be laid directly over a stringer as in (a). However, where the trestle is curved, using straight stringers as on the prototype presents problems in view of the mismatch between the curved rails and the straight timbers. Thus, as can be seen in Figure 3(b), as the ties progress across the deck between one bent and the next, they become progressively more offset toward the mid-point, and this is more pronounced as the spacing between the bents (panel size) becomes greater. This is compensated for by positioning the stringers as shown in Figure 3(c) so as to be correctly aligned with the curved rail and the offset. As mentioned above, these views are schematic, and wider or more stringer beams will ensure complete support. Essentially, as can be seen, the width of the roadbed has to be greater in the case of a curved trestle than a tangent one.
With this in mind, when I constructed my trestle using the bents I had made, I first of all marked curves of the intended radius (4 feet) on 7 mm MDF and cut out curved strips 4 inches wide as guides for the positioning of the bents. With these guides in position along the intended track path in the manner illustrated in Photo 1, I tacked the guide strips to the caps of the bents. This enabled me to position the bents accurately and also fine-tune their heights. During this process, once all the bents were the correct height, I applied the sills along the bases of the bents.
With all the bents the correct height and disposition, I then marked across the guide strips, the positions and alignments of the bents, as seen in Photo 1. The strips are also marked with the numbers of the panels between adjacent bents. Aware as I
Nick Prior
was, for the reasons discussed above, that the deck had to be wider than the set track I was intending to lay, I had to decide how wide the road deck needed to be. I settled on a width of 7 inches and I used the curved guide strips to create a template of thin plywood as shown in position in Photo 2. Again I marked the panel numbers on the template, which for ease of use I made in several parts. This template, unlike the curved strips, has parallel straight edges of the correct 7-inch width between each pair of adjacent bent positions. Note that in this photograph and Photo 3 referred to below, the perspective is distorted but all panels are the same width. An illustrative section of the curved strips and template are shown side-by-side in Photo 3, in which it can be seen how the positions and alignments of the bents have been transposed from the curved strips to the template.
I decided that in order to provide the strength and stability I required for garden use, I would use stringers equivalent to 12” by 24” prototype size. While this was on the large side, it was within prototype bounds for heavy-duty use where the trestle bents are as much as 20 feet apart, as some are in scale on my trestle. As I mentioned, I was planning to use set track on the trestle. Commercial 45 mm track has ties of a predetermined width and this did not allow for the length of ties that would normally be used on a trestle, especially a curved one. I therefore decided the only practical solution was to provide prototypical cross-ties of about 9” by 9” along the length of the trestle as shown in Photo 4, these ties extending across the full width of the deck as in prototype practice. In view of the varying bent spacings and the awkward alignment of the bents, I settled on stringer panels constructed as shown in Photo 5.
The panels consist of three equally-spaced stringers connected at each end by cross-pieces. (In prototype practice, the adjacent ends of the stringers of adjoining panels are butted together or connected with overlap joints and sit on the bent caps; I decided to use unprototypical cross-pieces for strength and ease of construction and assembly. Also, the structure would not be visible except from the sides once the cross-ties were in place.) The stringers and cross-pieces were appropriately cut, using my bandsaw, to conform to each section of the template. The cross- pieces were screwed to the ends of the stringers to form a deck panel and adjacent panels were bolted together, making for easy and strong assembly as well as allowing for future disassembly. The stringer panel assembly was positioned on the bents (which had already been screwed down to the concrete foundation) and screws were inserted through the bent caps upwards into the stringer panel cross-pieces. By screwing the bents to the road deck to the bents and the bents to the foundation, a high degree of strength and stability was achieved. Dozens of ties were now cut and nailed (using the nail gun) to the stringer panels. The track was then laid over the completed deck. I was very pleased with the firmness of the trestle and it was apparent that wall bracing would not be necessary. I therefore decided not to use wall bracing at all, since this would block the views through the trestle to the layout features behind (and make the removal of leaves and other unwanted vegetation much more difficult). The stringer panels and ties were all cut from the same pressure- treated battens as the bents and were dipped in the same stain as had been used for the bents.
At each end of the trestle the deck is supported on cribbing, and at the left end there are two separate crib structures spanned by a simple gallows bridge that I made. I will describe these in another article.
22 ROUNDHOUSE