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Building the Wood Arch Truss Bridge The mid-1800's saw a lot of bridge building activity, and one type of bridge was the Arch Truss. In the East, and in places where iron was easily acquired, iron components were coming into fashion to a greter degree. Out west, we had lots of wood but iron was more scarce and thus more expensive. Wood was the more popular construction component, with more precious metal reserved for nuts, bolts, rods and so forth.  Black Bear Construction's Arch Truss Bridge kit
An excellent wood arch truss bridge kit is available from Black Bear Construction. This kit came with materials, a special plastic jig, and instructions including a full-size blueprint template. All parts, tools, stains, and weathering solutions to build this bridge (and other wood truss bridges) came from Black Bear Construction. Being able to get everything from one place, including advice as needed, was a big help and made doing this project much more straight-forward. I highly recommend getting the right tools for the job, too: specialty devices like the NWSL chopper and true sander made it all go together the way it was supposed to, with a minimum of fuss and virtually no wasted time or material.  Designing The Installation: Angled Piers?
I ran into an issue in designing the bridge installation: the arch truss is a good bridge design, and can span 46' with ease in the Black Bear's arch of 44 feet long and 13 feet tall. But the bridge I replaced was a modern steel arch spanning 92 feet! With a track running diagonally below the bridge, a straight bridge with right-angle piers would be too close to the track, and would prevent ever re-doing the lower track to ease its curve radius. Research to the Rescue: OGR Forum input!
With a quick post to the O Gauge Railroading Magazine's online forum, I was able to find out if angled bridge piers would have any basis in reality. Starting a thread called Angled bridge piers, I asked if there were any prototype precedents for this. The answer was yes, and several forumites posted photos of bridges using angled piers. These included an older steel Through Truss railroad bridge: definitely close enough, considering the wood bridges are almost all replaced or gone with the ages. I settled on a design with a modest 6' offset. This would minimize the disruption of the chord loading, thus increasing its feasibility, and give it an offbeat look that might not be too unusual-looking. And most important, it would give me the lower-level track clearance I needed.  Setting the Bridge Footings
The new bridge is going into place well in advance of the rest of the scenery updating for this area. For now, I just want to anchor the new bridge in the same spots as the old bridge. But the old bridge, a Lionel plastic structure, had a flat sheet-metal base that easily slid into place between the cork roadbed and the track. Since the new bridge will have the arch truss for the main span and then a typical wooden trestle in the area that doesn't need such a big span, I want to build the arch truss into a long deck that will include the trestle portion. This will make a stronger bridge, even if it isn't prototypical of how these bridges were built: in real life, the under-deck stringers would not run the full 95 feet of the truss span plus the trestle.  The Work Space
An overview shot of the work space. Construction really never expanded beyond this area. The large sheet of white paper was useful for laying out the wod peices as I stained them. Stain (in the larger brown bottle) went on easy with a sponge and a quick wipe with a paper towel. You can see the white plastic truss jig sitting on the table, behind the Chopper III and the True Sander jigs. The trusses are resting on the blueprint, which is about 2' x 3'. It's very useful for lining up pieces to cut and for lining up the pieces as they're assembled. Since I did some customizing to make the 6' offset for the angled piers, the blueprint really helped me keep things lined up and true.  Trusses on blueprint
Here are the two trusses resting on the blueprint. One in the back is standing upright on the bird's eye drawing of the decking. You can see it's lined up for the shoe (the small 2' long wood piece under the stringer) to sit in its place, the arch to sit in its place, but the base stringer sticks out to the left another 6 feet beyond the drawing. One in the front is laying down, directly behind the elevation view of the truss.  Trusses lined up with under-deck braces
The arches are lined up and glued down to the cross braces that run under the decking. The inside base stringers are also added. You can see in this shot how the 4 shoes are lined up to keep a good, straigh support for the eventual addition of the angled bridge piers. A small square, similar to a carpenter's square, is critical for this part. Everything must be nudge into place just rigt to keep it square, plumb and true. You can see the square I used (also from Black Bear Construction) in the next couple photos: it was never far from the work, especially in the assembly process. You can see traces of the brown stain on the paper and box.  Underside of the bridge
Once the first couple cross braces were in place, it was actually easier to add most of them with the structure propped upside down. This made keeping it square and true much easier. I also kept checking for plumb during this operation. This view makes it easy to see how the 6-foot offset is coming together, especially how the cross braces at each end are flush on one end and stick out on the other. Note the semicircle protractor-shaped angle guide: the kit comes with this to aid in cutting. The arch bridge has many different cutting angles - a different angle for each arch piece and for each arch diagonal. This guide makes it much easier to get the angles correct.  Adding the Decking
Here we've got the decking just built across the arch area, and we're ready to start the "home run" down the trestle to the other end. The instructions describe how to notch these decking ties to work them in around the truss pieces. This is accurate to the way these bridges were built, and makes for a much stronger assembly since there are so many more connection points. Making all those notches can be tedious and is the most risky part of the project, since you end up working with knives and exacto's in very close proximity to your fingers. I had to keep reminding myself to cut away from my fingers - the temptation to do it the easy way is pretty strong.  The finished deck with top stringers added
The bottom inside stringers and the top stringers line up directly below where the track's rails will be. This is also true to prototype: the actual railroad track ties would lay on the top stringers, and the rails would be spiked to those ties. The whole assembly is sitting on the old Lionel sheet-metal bridge deck; I put weights on the stringers and gave the glue a full day to set and dry.  The "new" old bridge takes its place on the pike!
I built just one pier assembly roughly following the jig and instruction set. Since I've got angled piers, the pier post spacing is slightly different from the jig's. For most builders and for most other arch truss bridges I may make, the jig's spacing will be right on. I slipped the bridge ends into the pre-drilled holes in the homasote, first deep into the trestle end, then pulling back a half inch into the arch end. The pier went in next, and snugs the whole thing up just right: the pier is not even glued to the bridge or the base.  Looks like it's holding the weight!
One worker on the bridge chscks the truss bracing while NP 1178 lumbers past. Below, a crew cleans up the pile of wood left over from construction. Eventually I'll re-do the scenery under the bridge, and the pier posts will be shortened when some sloping terrain is placed under the trestle end. At that time I'll also build the trestle bents: one every 16 feet should do it.  Looks pretty solid from down here, too!
The cross bracing and the pier's cross bracing is highlighted in this shot from ground level. Time to complete the bridge, once construction started, was roughly 20 hours. This includes the time for every step described above, from removing the old bridge to placing the little railroad people into the finished scene. Well I hope you enjoyed looking at this project, as much as we enjoy the bridge - and the building of it! Cheers, Bob Davis |
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