Just down from where the pump house was a need for a bridge. The On30 track that serves the Hill Valley Coke Company Coking Ovens siding is owned by the Deep river Railroad. Since the purpose of a model railroad is to have trains that run on it .. a pump house or coke ovens are of little use without a rail connection. To that end I figured I needed to get my trains running .. and for that I need to be able to cross my river.

At one time I had intended to model HOn30 .. but .. at some point I was seduced by the Dark Side. During that pre-HOn30 time I had got my clamy hands on a Central Valley Model Works #1903 72’ Plate Girder Bridge (single track).

I was happy with the kit and when I went over to On30 I decided I wanted to take the bridge with me. It has great details and would be a shame to not make use of it.

The main side girders then were what was actually used. Using HO scale engineering structures in O scale effectively reduces everything by 1/2 (approximately). That’s not precisely true since you can reduce the gauge of a locomotive but people remain the same. True, you can reduce cab height and width but generally one of the neat things about narrow gauge locomotives is that the cab appears larger in comparison to the running gear then on a standard gauge locomotive. Let’s just say that a 3 foot gauge locomotive is about 2/3 the size of it’s standard gauge cousin. If you divide standard gauge into three foot gauge (36″/56-1/2″) you get 0.637 which is pretty darn close to 2/3. For On30 that would be 30″/56.5″ or a ratio of 0.53

Now .. don’t get me wrong .. I know that you can’t simply reduce a standard gauge locomotive by .53 .. the cab, walkways, ladders and all of that are still ‘people sized’ .. but the locomotives that I will be using are small anyway like the Davenport, they will be more industrial size engines. The point being after all of that long winded noise is that I will be happy using the main girders in O scale.

I measured the CV Cross Girders .. at 2.230″. Taking a quick look at the NMRA S-7 Clearances we find that for HO – centerline to extreme width is 1-1/32″ or 0.03125″. Doubling that we then get 2.0625″ for maximum width. Hold on .. just a little more math … (2.230″ – 2.0625″) / 2 = 0.08375″. Ok .. ok. Getting nutz with the ‘exact’ math .. but hey .. having fun here. That means that the Central Valley Plate Girder bridge measures 0.168″ wider then the NMRA clearance for HO. Cool. I simply need to find the NMRA clearance for On30 and add that 0.168″.

Referring back to the NMRA S-7 Clearances we find that this dimension for On3/On30 is 3″. Adding that 0.168″ we end up with 3.168″ for the width of the new Cross Girders.

Just for fun. When you double the size of something .. H x W x L it cubes the volume. Looking at that another way .. if you had a steel cube measuring 1″ x 1″ x 1″ it would equal 1 cu. in. .. if you double the dimensions so you get .. 2″ x 2″ x 2″ the volume now is 8 cu. in. A narrow gauge locomotive is about 2/3 that of a standard gauge locomotive or 0.666~ …. therefore that same 1″ sq. cube reduced to .666″ x .666″ x .666″ would be 0.295 cu.in.

Round that to .3 .. so something 2/3 the size will weigh about 1/3. From the ‘American Civil Engineer’s handbook’ we have a chart. An I-Beam 21″ high weighing 100 lbs per foot has a Coefficient of strength of 1,766,100 (calculated load in pounds that will produce a stress of 16,000 lb per sq in on the extreme fiber when the span is 1 ft. By dividing this coefficient by the span in feet, the safe load uniformly distributed in pounds is found). Figuring 1/3 of that we get 588,111 COS. That equals to around a 15″ high beam at 40lbs per foot. a 12″ beam (that’s what the CV beams work out to in O scale) would have a COS of about 500,000. Since in “my world” the locomotives that will be using this bridge will be on the light side .. even for narrow gauge, that 12″ beam would probably be ok.

With that in mind I looked at what I have in my parts box as far as I-Beams. I have .. 3/16″, 5/16″ and 3/8″ Evergreen I-Beams. Cool. The 3/16″ I-Beam is too small as it would only be a 9″ I-Beam. The 3/8″ I-Beam would be a 18″ beam. The 5/16″ I-Beam would be a 15″ beam. Shazam! That will work! Now, the Evergreen 5/16″ I-Beam #278 has a flange width of .134″ which equals 6-1/2″. That is close to a 15″ I-Beam running 85 lbs per foot and having a COS of 1,131,000 which means it will more then be sufficient for our bridge. Cool.

I had made both the Rail Bearers and the Cross Girders using the 5/16″ Evergreen I-Beam .. and that won’t work. Here’s why ..

This is the CV casting flipped over to look at the underside. To make the casting easier they have the Cross Girders and Rail Bearers the same height. The Lateral Bracing (X-Bracing) looks ok .. until you look closer. It actually passes through the Rail Bearers when it should clear that. I looked into it and the Rail Bearers are not as tall as the Cross Girders. The allows the Lateral Bracing to pass over the top and clear the Rail Bearers.