City Bridge – Pt VIII

Dimensions – again
GirderDim6This is where I am currently at with the development of the girders. I should point out that the precision of the numbers is due to Sketchup. I put in some basic numbers to start such as the 1.250 center-to-center for the girders .. then the 0.040″ web plate, the 0.100″ angles .. adjusted the back-to-back spacing of the main angles and put in the .400″ flange plates .. and the numbers just .. ‘happened’.
Notes on Bracing …
The top lateral bracing of deck girders shall be so designed that the unsupported length of the flange shall not be greater than 12 times its width.1

In deck bridges there shall be transverse sway bracing at each panel point (…) In deck-plate girders there shall be cross frames (or diaphragms) not more than 16 feet apart, and at the end of each span.2

The girders of a deck bridge are generally connected by stiff diagonals – which can take both tension and compression. The shorter the span the heavier should be the transverse bracing, since short spans are light weight and liable to oscillate under live load. The term sway bracing is used for all the transverse systems except the end one, this is often being called portal bracing (text consolidated).3

Railroad deck plate girder spans up to 50ft. length usually have a top lateral system when on a tangent. All spans on a curve and spans above 50 ft. length should have a top and bottom laterals. There should be a frame at each end and intermediate cross frames about 15 ft. apart each frame to consist of a top and bottom strut and two diagonals.4

A plate girder bridge consists of two or more, usually two, plate girders fastened together by lateral bracing, and in the case of deck bridges by transverse bracing consisting of two or more cross-frames.5

Railway Deck Plate Girder Bridge
Fig1RDPGBThe drawing to the left is from The Design of Highway Bridges ..6 and really shows a lot. It would be even better if I could actually read the darn thing but .. better than not having it. At the top is a view from the top, in the center a side view with an end-view on the right center, and on the bottom the bottom view. I can see ..

  • Top View
    • Cross framing (yellow) – visible in the top view. If you look down to the center view there is a dimension of 9’9″ for the stiffeners that pretty much match the spacing on the center cross frames.
    • Gussets (green) – General shapes obvious.
    • Flange plates (blue) – Note that they extend the full length. The bottom plate is not needed but is there for keeping out trash and water. That could be a thin plate for that purpose.
  • Side/End View
    • Flange plates (blue) – you can see where the bottom flange plate stops just shy of a cross frame
    • Gussets (green) – shape and how they rivet to the transverse sway bracing. Note that there is another on the outside that ties into the Base Plate.
  • Bottom View
    • Flange plates (blue) – it’s really clear how the flange plates stop short 10′ or so from the end of the girder.
    • Gussets (green) – a little more detail on how they differ from the ones up top .. and also the one that braces against the base plate (appears about 28″ sq.)
Cross Frame
CopperRenderLowUsing the bridge drawing as a guide I created this cross frame. The outside angle framing is .080″ Evergreen angle while the transverse bracing is .060 Evergreen angle.

The spacer plate where the two transverse angles cross is the same thickness as the gussets – 0.015″. Not concerned about rivets .. this is a plan to make the sucker. Rivets are easy .. as I will be using Micro-Mark water slide resin rivet decals for that.

Spacing of the Cross-Frames
CrossBraces2This may *look* simple but too a bit to get right. The cross-frame nearest is turned 180° from the other four. That’s because if you look at the 2nd back, the diagonal angle extends beyond the Cross-Bracing. That is fine except for the near end where that means it would extend past the end of the girders.

Normally in Sketchup I would copy one cross-frame and then array copies to the end. This as I said would have put that diagonal outside the end of the girder. Spinning the last one around and moving the cross-brace to the edge of the girder works .. but that messed up the spacing between the cross-bracing .. which caused a problem.

I want the lateral bracing between the cross-frames the same .. so I had to get the spacing the same. What I finally did was place a cross-frame at one end .. copy to the other end .. spin that 180° .. align to end of girder. Then went back to the first cross-frame ..arrayed 5 copies so the last one sat on top of the one nearest the camera (so two occupied the same space). That spaced everything correctly .. so finally I deleted the last cross-frame that had been arrayed.

This is a continuing process in that the more of this I do the more I learn. I had put the gusset plates along the lower angle .. and they are completely wrong. I’ll leave them so I can refer back to them in the next bit.

Lateral Gusset Plates
LateralGussetsNext up is figuring out the placement of the lateral bracing. The good thing being that I should be able to dupe the lower laterals for the top. In the pic here, I placed a gusset plate in place (in red) centered between the cross-frames so I could start to work out the placement of the diagonal bracing – Insert minor ‘Oops’

In the photo showing where I was setting the spacing for the cross-frames you can just see the gusset plates that I had just ‘stuck on’ between the cross-frames. That makes no sense once you take a good look. – It is just hanging out there .. literally! It has nothing to attach to. It could be welded on today I suppose .. but not riveted. Instead .. it slides under and centered on the cross-frame where it can be riveted nicely (shown in the remaining gusset plates in white).

lateral Angles
WhereLaterialsCrossI tried the .060″ angle and didn’t like it .. too weak looking. I changed to the .080″ and am happy with how it looks. Have it were I want it .. correctly on the gussets. The only question I have is where they cross. I’m pretty sure that one angle is full length – with the one that crosses in two pieces with a plate connecting everything.
crossing_lateralsWell .. shut my mouth! “The method of crossing the laterals at the center of a panel is shown in Fig. 43: one diagonal a continues unbroken, and the orther bb leads up to it on each side, and is spliced by a plate cc, which is also riveted to the angle a.” 7

I looked at that diagram and was puzzled for a bit until I realized what I was looking at. The piece highlighted in blue is a couple pieces of angle rotated 180° and snugged up to the area where the short angles lead up to the longer one. This is one of those little items that is easy to add and really adds to the effect (if noticed at all).


  1. General Specifications for Steel Railroad Bridges and Structures: …, Albert Wells Buel, Virgil Gay Bogue, 1906 – pg.20 []
  2. Common Standard Specifications; Structural Steel for Railway Bridges, Steel … – pg.17 []
  3. A text-book on roofs and bridges: …, Volume 1, Mansfield Merriman, Henry Sylvester Jacoby, 1920 – Pg.239 []
  4. Design of Steel Bridges: Theory and Practice for the Use of Civil …, Volume 1, F.C. Kunz, 1915 – Pg.166 []
  5. The Design of Highway Bridges of Steel, Timber and Concrete, Milo Smith Ketchum, 1920 – Pg.157 []
  6. The Design of Highway Bridges of Steel, Timber and Concrete, Milo Smith Ketchum, 1920 – Pg.157 []
  7. Bridge specifications: Design of plate girders, International Correspondence Schools, 1908 – Pg.61 []

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