I'm currently working on a model of the Chama River bridge for my HOn3 layout. I plan on building two spans just like the real thing although mine will be slightly shorter at 100' per span. I believe the prototype is 110' and 120'. My model is based upon measurements and drawing made from a recent trip to Chama. I planned on kit bashing a Central Valley bridge but I'm just not happy with the underframe. I decided to go ahead and build my own underframe. At first I was using styrene I beams that were purchased at the local hobby shop but these just didn't seem strong enough. After further thought I decided that I would build the underframe out of brass.

The first real challenge is those large I beams in the shot above. These are about 3' high with about a 7" head on them. Where would I find a scaled down brass version of this? I checked the structural shapes size and found nothing close. Time for some creativity here. It turns out that you can purchase a rectangular tube that is 3/8 x 3/16. If I could slit one of these in half and then solder the two pieces back to back I would have a reasonably sized I beam. So I proceeded to put one of these in the vise of my little Sherline mill. Before clamping down too hard I inserted a piece of brass rod in to the tube to provide more support for clamping. I then took a very small milling bit and proceeded to slit down the center of one of the 3/16 side. I then turned the tube over and slit down the other 3/16 side. I worked my way down the 12" long tube slitting about 2" at a time. When I was done I had two pieces that were exactly the same although they had curled a slight amount due to the cutting. This was ok though as I would then clamp the two pieces back to back and solder them together.

The next challenge would be the I beams that support the ties above. There are four of these that run beneath the track. The two outer beams are about 15" high and the the two center beams are about 20" high. This shot shows one of the center beams with the outer beam just visible in the upper left of this shot. Again these would be challenging to find in just the right size.  About the best I could come up with for these would be 3/16 and 1/4 C channel which is readily available. I figured that when viewed from the side you wouldn't really notice that it was a C channel rather than an I beam. The only problem with the C stock is that the lip is way too big. So once again I turned to the mill and took these lips down to about .070".

With lots of cutting and soldering I now have a reasonable frame to start working with. The C channels and I beams, 31 pieces, have all been solder together using a wooden jig to help maintain the proper spacing. Note that the two end pieces haven't been attached yet. The C channels for these are slightly longer and the I beams themselves are different than all the others.

Here is a side shot of the frame to provide a little different perspective. You can see how the I beams are two C channels soldered back to back. The seam doesn't really matter as it will be covered up by the vertical members later. The C channels have been soldered to the I beams on the back side  so that the solder will be less visible. If you look closely you can just barely make out the larger center C channel below the outer one.

The I beams on the end of the bridge are slightly longer than all the others. They are also tapered on the ends with a cap on each end. I cut the taper on the mill and then solder some thin strip stock to the end for the cap. I still need to do some cleanup on the solder.

This proto shot illustrates the difference in length of the end I beam compared to the others. Also note the angle and cap on the end of the I beam. The other I beams all butt up to the vertical members of the bridge.

Here is a sample of the girders that come with the Central Valley bridge kit. You can actually purchase these separate from the bridge kit and in retrospect I probably should have gone this route. But then again there are lots of other detail parts that you get with the kit. I'm going to try and use these girders for the bridge superstructure. The lower two will work well for the vertical members and the short diagonals. The middle ones might work for the upper horizontal members. The solid ones would be ok for the end diagonals but they are a little wider than I would like.

That is all for now.

Chama bridge - part 2

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