City Bridge – Pt V

Oopsie …
BraceOopsieThis is a good example as to why I like modeling first in Sketchup. I used the .080″ Evergreeen angle for the Main Angles of the girder. I used the .060 angle for the cross-bracing .. and realized that .. oops! .. I would have a problem trying to use that same .060″ angle for the diagonal bracing. Since it is the same size as the angle under it .. it juts out the.014″ web thickness. The options were to (1) ignore it (2) taper the brace down (3) rework the girder. I went with option (3).
Cross-Frame Dimensions
CrossGirderExplodedI reworked everything. The Main Girders are 1.250″/60″ FS center to center. More about that in a bit. First ..
I brought a copy of the cross-Frame out so you can see it. On the right is a complete structure .. made up of two mirrored sub-structures. On the left I exploded one of these.

The cross bracing is made up of .080″ Evergreen angle. The top and bottom pieces are cut back to fit against the vertical pieces. The diagonal braces are .060″ Evergreen angle.

Notice there are eight assemblies which means the cross-girders are spaced at 6′-10″ (about). This is just shy of – the usual practice is to space cross-girders from 8 to 10 ft. apart.1 I may reduce this to six assemblies. That would have them spaced at nine feet. Then again .. I may not. It does say – “usual practice” after all!

Two sources I mentioned earlier .. have the center to center for the girders at 8ft2 and 6ft-6in.3 That’s for a standard gauge so figure they are then 1.7X and 1.38X gauge. The girders for my bridge measure 1-1/4″ .. OR 5ft/60in. With On18 *actually* measuring 17in gauge that means my girders will be 3.53X gauge. The reason that the bridges for my On18 are so sturdy (relative to what you possibly find for such a gauge) .. my story being that this nutcase built a 3ft line which went belly up. I say nutcase because he spent madly on the bridges. They pulled what rail had been put down and installed the On18 (17in) simply because they had already run from the mine to the Coke Ovens and said .. “Hey .. the roadbed is there anyway ..” …. and even for that 3ft gauge (which is the *why* for the Deck Girder) .. the girders would be spaced 1.6X gauge

Girder Dim – again
This is like the fourth or fifth time I’ve re-drawn the girder. I’ll pretty happy with this version I think .. it has good proportions and the angles all seem to work. The only change I may make is if I can pick up some 3/8″ strip styrene for the flange plates I will use that instead. (not going to happen – 1/4″ seems the largest strip styrene I can find.)

  • Main Angles are .100″
  • Cross Bracing are .080″ angle
  • Diagonal Bracing are .060″ angle
  • Split the difference and made the flange plates 1:35 of the span – 19.2″/0.400″

I WILL have to draw the cross-girders again. Looking closer at what I drew up .. it looks pretty – but doesn’t make sense mechanically. The various bits and pieces have to rivet to each other … and then to the main girders.

Intermediate Rendering
RenderThe only major thing I need to still add are the web stiffeners. Some gussets perhaps – but that is the sort of thing that can be best added during construction. This is a 54 ft. span so there would have been a splice .. so need splice plates. The rivets .. simply depends on what I have on my Micro-Mart rivet decal sheet.

  1. The Practical Design of Plate Girder Bridges, Harold Hughes Bird, 1920 []
  2. The Design of Typical Steel Railway Bridges – An Elementary Course for Engineering Students and Draftsmen , William Chase Thomson, 1908 []
  3. General Specifications for Steel Railroad Bridge and Structures: With a Section Making Them Applicable to Highway Bridges and Buildings, Virgil G. Bogue, 1906 []

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