Friday, August 23, 2013

A new SP bridge for Shumala, Part 2

In my first post about constructing this bridge (you can view it at: ), I described why I was replacing the original bridge on this site, my design considerations in dimensioning the new bridge, and the creation of new bridge girders with Archer rivets. In this post, I describe completing the bridge girders.
     Before continuing, I should mention a nice magazine article about a bridge of approximately the length of the one I am building. It was by Harold Russell, in Model Railroader for November 1979, pages 116 and 117. It is worth consulting if you want to simply follow a published drawing and photos.
     When I left off in the previous post, the outside flanges and stiffening-member uprights had been attached to the faces of the girders. Next I added cover plates to the top flange, of about 11 scale feet length, using the same scale 1 x 10-inch styrene strip. Cover plates resist buckling of the top flange, when the bridge is under load, and the upper flange of the girder is in compression.
     Now I was ready to place the outer surface rivets, again using Archer set 88025, in double rows. These of course represent the attachment of the outer flanges to angles which are also riveted to the plate underneath, so there are two rows of these rivets on the outer flanges. In fact, neither the 88025 set nor other Archer sets have rivet spacings which really match bridge photos, so it may be time to dig out the prototype dimensions and hand off the information to Archer, in hopes they may add more rivet sets for us to use.
     But along the right side of set 88025 (held so you can read the lettering at the bottom) are conveniently closely-spaced rows of rivets, and these make a nice double row that you can apply all at once. That’s what I did. The black rivets show up nicely on the white styrene.

The second girder is identical to this one, as both are the visible sides of the bridge (the girder backs are plain, since they can’t be seen on the layout).
     At this point, it is a good idea to paint any part with rivets newly attached, as the paint layer protects them from handling damage. I have sometimes used clear flat, but for these girders, decided to use the final color, a faded black. I have mixed this from mostly black, with some gray and brown, and keep a bottle of it at my spray booth. (You can click on the image to enlarge it.)

The girder looks dark here, and the rivets aren’t obvious, but under layout lighting they are quite visible.
     The girders were then weathered. I did this with my usual acrylic washes, using a mixture of Ivory Black, Neutral Gray, and Burnt Umber. (If you like, you can download the handout from the joint clinic on weathering techniques, written by Richard Hendrickson and me, at this link: .) As the wash of that mixture got close to dry, I added another wash of Raw Sienna (yellow rust), and allowed it to accumulate along the bottom of the girders. Here is a photo to illustrate what I was aiming for on the girders themselves. It is the west end of the trestle at Gaviota, California, and was taken in January 1987.

Although this is a deck girder bridge, not a through girder design like I am building, it does illustrate the yellowish tinge I wanted.
     My next step was to provide for the ballasting of the bridge deck, once it is installed. To do this, I wanted to glue a wide strip of styrene underneath the track. The problem here is that the ties are an engineering plastic (maybe Delrin or maybe something else), and most adhesives adhere poorly or not at all to such plastics. I had a bit of a “wrestle” with this problem, and will defer a description of it to a future post, along with final bridge installation.
Tony Thompson


  1. Hi Tony,

    I noticed that you really didn't mention how you built the floor. With regard to the articles and books mentioned, it seems that the model railroad press has ignored a very common type of bridge floor construction. Thankfully, the internet has changed everything! Here are two really good web pages, with dimensioned drawings:

    With your attention to prototype detail and accuracy, I think you'll find them interesting.


    John Totten

  2. Thanks for the references, John. This is well covered in Mallery's book, too. I said nothing about the floor structure in my new bridge because, unlike my previous bridge, it has no floor. The underside of the bridge can't be seen from below, and with a ballasted deck, can't be seen from above, so I didn't model it. The previous one DID have floor beams and diagonal bracing.
    Tony Thompson