When finished, the Rio Grande Southern Railroad had 142 bridges scattered along it's 162 miles of track . The most spectacular bridge was 9A. This wooden trestle was 836 feet long and had a 202 foot Howe deck truss in the center. It was 134 feet high, prompting the early crews to call it the high bridge. The original 9A strained the capabilities of wooden bridge building to their limits and, as a result, it was never really successful. The crews didn't like it or trust it, and neither did management. Unsurprisingly it was among the first of the bridges to be replaced. 9A was removed and replaced with an earthen fill and box culvert in 1903. When this culvert washed away in 1908, it was replaced with the second 9A, which was a more typical curved open deck trestle.
Like 9A, almost all of the original bridges were rebuilt or replaced at one time or another. Most of the Howe trusses were replaced with simple open deck trestles, and many of the smaller bridges were replaced with earthen fills and culverts. By the end of operations, 111 bridges were left on the railroad; virtually none were original. The reasons for replacing a bridge were many, but for the most part, the arrival of heavier motive power or damage caused by weather and high water resulted in replacement.
At a glance, most of the bridges appear to be alike, a closer look however, shows that all differ in detail. The RGS built their bridges following accepted and proven engineering practices. Thus, all of the bridges look similar in overall design. Detail differences in the bridges occurred due to unusual circumstances in placement or geography. Changes also occured when the bridges were repaired. Thus, over time, all of the bridges slowly changed.
What is presented here is the basic design standards and building techniques that all of the bridges adhered to, no matter their differences in detail.
The drawing above represents a typical bent from a strait or tangent RGS trestle. Straight trestles were built with a batter, or slope, of 2/12 on the outer posts, while curved trestles had a batter of 3/12. The drawing shows the maximum height of each story. In normal practice, the heights varied as necessary for the bridge to fit into the terrain. For heights under 16 to 20 feet, one story was used. For heights over this, anywhere from two to six stories would be used; the upper stories were normally the full height and the bottom one was adjusted to what ever height was necessary. The bents were placed 16 feet apart, with stringers of 8 x 18 inches. If the bents needed to be farther apart, stringer size was increased to 8 x 24 inches. Many bents sat on footers made of large wooden blocks, while others used formed concrete. Many of the bridges used a combination of both.
Two major styles of trestle bents were used on the RGS. One used two vertical posts in the center and the other three. Although an engineering reason probably exists for this difference I am unaware of what it might be. The following is a list of the standard sizes used for the various components of a typical RGS bridge:
These photos are of the underside of the Trout Lake trestle deck. Trout Lake is the only one of the large Rio Grande Southern trestles still standing, making it a gold mine of information. It was maintained for a while after abandonment of the railroad by the state of Colorado, but for the most part, it exists today just as it did when the RGS ran trains over it.
The dimensions of the wooden members of this bridge are consistent with normal railroad practice, and the metal hardware used to tie the bridge together is the same as observed in the remains of other bridges. These photos show details not normally visible in historic photos. Additionally, most drawings available to the modeler do not show this area well.
Perhaps the most unusual aspect of trestle deck construction is in attachment of ties to the stringers. In the Trout Lake bridge, as in all other sites I have examined, the ties are not connected to the stringers at all. The only things holding them in place are gravity and the rails spiked to their top. The guard rails are notched to accept the ties and are bolted to every fifth or sixth tie, and so they help in maintaining spacing and alignment. It is believed this was done to allow the rail to expand and contract slightly with out affecting the trestle structure beneath.
This sketch below shows the method used by the RGS to tie the bridge bents together. This style of construction has been observed at all of the major bridge sites on the railroad that contain enough material for examination. It is also evident in the water towers still standing.
A cap was attached to the posts underneath it by driving a round steel pin down through the cap and into the post. This was done for each post and for each story of every bent. Anywhere from one to three small bridge nails (9" long with 3/8" shaft) were used to attach the sway braces to the posts. These sway braces ran from the lower left to upper right. This unit or story was often assembled off to the side and then moved into position on the bridge.
Each story was attached to the one below by toe nailing several full sized bridge nails through the post and into the cap of the story underneath. In the bottom story, the mud sill would probably be attached before the story was moved into place. As each story was positioned and attached to the story below, girts were added between the bents to stabilize them.
When finished these bridges were strong and stable. The bridge at Ames, (43A), which was built in a very precarious location, stood for almost thirty years with no maintenance whatsoever.
These photos show some of the remains of bridges 44A, Butterfly, and 45A, the high bridge at Ophir. Although neither of these two bridges are still standing, most of their timbers remain where they fell. Today, they form a kind of three dimensional exploded view showing bridge design and construction. The drawings above, that detail this, were developed from the close examination of the remains of these two bridges.