Wednesday, April 10, 2013

Designing a Trestle Bridge for Geyser Gulch

Testing the waters (so to speak) with a temporary trestle bent. This was built to see if a bridge using lighter 8" x 8" posts would be convincing in the scene. 

It's always exciting when trains cross over water and as Geyser Gulch is such a focal point on the layout it will require a very special bridge. Coming up with a pleasing design was a bit of a challenge since I want a wooden bridge but at the same time don't want it to obscure or distract from the scenery in the Gulch. Then there are the purely logistical problems of engineering a trestle for a tight 15" radius curve while trying to make something that might be believable in the real world. I want something light and airy looking, but also sturdy.

I'm no bridge expert, but with the valuable help and advice of fellow modelers at the On30 Railroad Line Forum I was able to narrow many options down to a final design. My first design decision was to use scale 8" x 8" timbers for the posts and sills instead of the more prototypical 12" x 12"s. This reduces the "visual mass" of the bridge right away and gives a nice spindly look to the bents. The next decision was to reduce the angle of the outside posts to just 7º, adding to the tall, narrow look. Next I opted for a rather unusual king post truss arrangement between the bents instead of the usual straight stringers which will allow for a wider spacing of the bents than would otherwise be possible on such a tight curve. This arrangement is modeled after a bridge in Franklin New Hampshire and should make for a beautiful and graceful looking trestle. The final design decision was the toughest to make. The question of whether to use round piles driven into the stream bed or square timbers on footings.

I started out with a pile trestle very firmly in mind; creosoted timbers sitting right in the water, encrusted with travertine and slowly rotting away. Of course, placing wooden timbers directly into water is never a good option and most railroads avoided it whenever possible. It might be cheeper in the short run to use a pile driver and smash the posts into the stream bed, but it would be a temporary solution at best. It really comes down to which construction method was more likely to have been used "back in the day" when the railroad was first constructed. Finally I decided that since the TMMC was once a rather well off little railroad, the brass hats would have spent the money to build the bridge right the first time with stone or concrete footings keeping the bents above water level. But the clincher was a purely aesthetic decision.

I have always been a great admirer of National Park architecture, particularly the native wood and stone buildings of Mary Elizabeth Colter at Grand Canyon and other southwestern parks. Since Nature's Wonderland shares so much in common with our National Parks, it finally occurred to me that what I really wanted was a bridge that looked like it might have been designed by Mary Colter.

Phantom Ranch Canteen, Grand Canyon National Park. Designed by Mary Elizabeth Jane Colter.

So, my solution is to build a bridge that will compliment and reflect its surroundings, rather than detract from them, using local Ponderosa Pine timbers from the top of the Mesa and local sandstone for the footings and abutments.

Mixing light and airy construction with sturdy stone piers, this design will require only three bents to cross the water at Geyser Gulch. It will be supported with a stone abutment on the right and a short stone approach bridge on the left side.

I'm pretty pleased with this design. Hopefully it will make for a beautiful, graceful bridge that really compliments the scene. I'm currently waiting on orders of materials from several suppliers but will detail the construction of the bridge here when they all arrive. Adios for now!

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