City Bridge – Pt III

Bridge Span
BridgeSpanWith the abutments glued down – it fixes the bridge span – and therefore fixes the dimensions:

Note: It was only when looking at the graphic below that I realized that unless I am VERY lucky – I will have to tear the abutments out and re-work them. I happily stumbled along making the abutments match those for my trestle – and forgetting entirely that I hadn’t decided at the time what kind of bridge/trestle I was going to create. A wood trestle .. fine .. I could use the same abutments – BUT – a steel girder bridge will have different requirements. Oh well. It’s white glue! 🙂

  • The distance between the faces of the bridge seats comes to 13-5/8″ .. which translates to 54′-6″
  • The bridge seats are 5/8″ or 30″ long/tall FS
  • With that bridge seat length I ended up with a clear distance of 12.375″ or 49′-6″ FS
  • The seat width is 2.43″ or 9′-8″ FS

I am going to leave this up since these dimensions work fine for a timber trestle – the 5/8″ seat depth works fine for a stringer + sill system. I’ll change this as I re-work everything for a deck girder bridge.

Engine Weights
From On18 Trestle – Pt IV

On18 Engine Weight

  • An On18 Mikado would weigh .. 112,000 x .42 .. or .. 47,000 lbs .. or .. about 24 tons. No .. I have no intention of an On18 Mikado .. but that puts the upper limits on the weight that a structure would need to support. Looking at (for me) logical N-scale mechanisms that might be used .. I think 0-4-0, 4-4-0, 0-6-0. I like the 4-4-0 American loco simply because they had the large driving wheels which I think would scale better to an On18 loco.
  • The 4-4-0 was relatively light .. let’s say 50,000 lbs. That means that my On18 4-4-0 would be about 21,000 lbs or about 11 tons. I’ll keep those numbers in the back of my head .. but look at the structure needed at say .. 11 tons operating weight but with a maximum at about twice that. I think that is probably good engineering anyway .. figure the load at twice the operational weight for safety purposes makes sense.
  • So. 21,000 lbs/11 tons for a 4-4-0. Double that for safety margins to 42,000 lbs. /22 tons. The theoretical Mikado 47,000 lbs./24 tons – doubling for safety margins 94,000 lbs./48 tons.

With those numbers .. we have a place to at least start.

Basic Dimensions
It has been found, as a result of calculation and observations in practice, that the greatest economy of metal is obtained where d is made from one-eighth to one-fifteenth of the span, one-tenth or one-twelfth being a good average value.

The depth of a girder is often limited by considerations of headroom, clearance, etc., but whenever possible the limits given above should not be exceeded. The standard depth should be one-tenth to one-twelfth of the span, and this should not be departed from in a single web plate girder of varying section without due consideration.

If a depth greater than one-eighth span be adopted it will be found that the amount of metal in the flanges will be comparatively small, but that in the web will be very much increased. The reason for this latter is, that although a thinner web, thinner than that required for one-tenth span, would suffice to carry the shearing forces, a thicker one would be required in practice to allow for the greater buckling tendency due to the increased depth. If a depth less than one-fifteenth is taken, difficulty will be experienced in providing for the heavy end shear on the web.1

New Dimensions
BridgeSpan2Since the On18 locomotive load is so relatively low compared to even light locomotives, I was quite comfortable with going with the one-fifteenth of the span for the flange depth. Given 13.625″/54′-6″ as the total span, then one-fifteenth of that is 0.908″/43.6″/3.6′. The Bridge Shoes I had previously had printed are 0.220″ tall. This gives a total bridge seat height of 1.128″.

  • The distance between the faces of the bridge seats comes to 13-5/8″ .. which translates to 54′-6″
  • The bridge seats are 1.128″ .. or 54″ tall
  • The bridge seats are 0.714″ deep .. or 34″
  • with that bridge seat deepth of 0.714″/34″ I ended up with a clear distance of 12.198″ .. or 48′-9-1/2″
  • The seat width is 2.669″ .. or 10’9-1/2″

I changed my mind about going with the one-fifteenth ratio after I made it up in Sketchup. It just looked .. wrong .. to me. I changed to the one-twelfth ratio. Hey .. I model by tinkering until it comes together!


  1. The Practical design of plate girder bridges – Harold Hughes Bird, J.B. Lippincott Co., Philadelphia, 1920 []

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