Page 20 - July August 2014
P. 20

 with and more prototypical for a wooden trestle that had lasted into the 1930s, the period I model.
With all this in mind I chose a bent structure as shown in Figure 3. I was helped in this decision by having purchased a sample Fn bent from Caboose Hobbies in Denver of the same design. It was not large enough for my purposes but served to illustrate the structural techniques. The number of posts or uprights in a bent varies according to load, but I chose to use five posts for each bent for stability and strength. This is also prototypical and looks good. The centre post is vertical and the outer posts lean inwards. As noted above, the posts are held together by the horizontal cross­braces and diagonal sway braces. The cross­ braces are arranged at intervals with an upper cap sill and a lower support sill. As you can see in the side view, there is a cross­brace on both sides of the posts at every level. The angled braces, so­called sway or transverse braces, are secured diago­ nally across the bent between each pair of cross­braces. This arrangement ensures that the bent remains rigid. (The sway braces are omitted in the side view but, as with the cross­braces, are fitted on both sides.)
Longitudinal beams called stringers extend between the bents, resting on the bent caps, to form the roadbed. On a curved trestle these are wider than the track to accommodate the curve. More of that next time.
Next, it was necessary to choose the cross­sectional dimension of the timbers to be used. In Fn scale (1:20) a 12­inch timber scales down to 5/8" so I aimed for square timbers of that dimen­ sion. I found suitable rough sawn and pre­treated roofing bat­ tens at a wood yard in High Wycombe. These battens were 5/8" × 1 3/8" and I trimmed them to size on my band saw to produce two 5/8" square lengths plus a small waste strip from each batten.
Since I would be making 11 bents in all, I felt it was worthwhile to make a jig, which is shown in Photo 3. I used a sheet of 3⁄4" MDF as a base. I had built a curved concrete base for the trestle at the lower level and smaller concrete sills on the intermediate levels at each end to ensure that the bents rested on firm foun­ dations and would be protected from rotting. I knew that the trestle had to be aligned in height with the upper­level bridge over the river. Using a long 2" × 1" piece of wood resting at one end on the bridge deck and with a level on it, I was able to determine that the six main trestle bents would need to be 31" high. I therefore made the MDF jig base 15" × 34" to allow some leeway.
On the Denver & Rio Grande the top sills or caps were usually about 14 feet long, which in Fn (1:20) equates to just under 81⁄2". After looking at a lot of pictures and a bit of experimentation I settled on a cap length of 7". This enabled a prototypical look while providing bents that did not look oversized. Thus the wider the cap, the wider the base of the bent. With a 7" cap, the
base of a 31" bent is 151⁄2". An 81⁄2" cap would give a base of 19" using the same angles. This seemed too much and not pleasing in appearance.
Having cut the jig base, I marked the position of the bent cap across one end of the base and then posi­ tioned a length of post section vertically along the centre of the base to form the centre post. I then arranged other posts, not at all scientifically but just until they looked to be at the right angle and were uniformly spaced. I then marked the positions of the posts in pencil on the base and removed the post sections. Following this I marked the positions and
lengths of the cross­braces at 6" intervals down the base. This interval was again based on observation and look rather than prototype. Thus D&RG recommend sills about 20 ft apart, which would make the spacing 12" in Fn and just did not look right. The 6" spacing certainly looks pleasing and more like the prototypes I have seen than the 12" option.
The cross­braces/sills project beyond the outer posts by about a foot on the prototype so I used an overhang of about 5/8". Lengths of the post sections were now nailed to the base as shown in Photo 3 to create the framework of the jig using the previously drawn pencil lines as guides. This produced an easy­ to­use jig allowing the rapid construction of the 11 trestle bents required.
The tall bents would need six cross­braces/sills from top to bot­ tom, as shown in column 1, while the lengths of the required five sway braces are shown in the same order in column 2:
7" 7" 81⁄2" 81⁄4" 101⁄4" 93⁄4" 113⁄4" 111⁄4" 131⁄2" 131⁄2" 151⁄2"
Photo 4 shows a partial kit of parts for one tall bent including five posts, six (of 12) cross­braces/sills and five (of ten) sway braces. Thus the kit only shows the braces for one side, there being an equal number applied to the other side. Photo 5 shows the post lengths inserted in the jig. You will note they are not cut to length at this stage. Photo 6 shows the cross­braces also in position.
While indoor trestles in smaller scales are generally glued to­ gether, other considerations apply for large­scale outdoor trestles. People have certainly used glue (Gorilla or NoMoreNails) to assemble the bents, but reading the literature it seems the success of this is limited, with a number of adverse reports. I therefore decided to use nails, and since the best results includ­ ing long­term security are achieved with nail guns, I purchased one. I settled on an electric gun, Tacwise Master Nailer 191EL Pro, which would fire either nails or staples and was relatively inexpensive. The gun is shown in use in Photo 6. I am pleased to say that after six years there have been no pulled nails or any sign of deterioration of the joints.
So, the cross­braces are positioned on one side and nailed in place using nails just shorter than the combined thickness of brace and post. The sway braces can then be nailed in place between the cross­braces, as shown in Photo 7. I have used sway braces 5/8" × 3/8" and so shorter nails are required. These sway braces were thicker than the typical prototype thickness of 3" since at this scale, using true scale thickness of just over 1/8"
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