Understanding bridges, Part 3: Details

The two preceding posts in this series attempted to provide a broad background for bridge design and appearance, including references, concentrating on the prototype for each of several types of bridges. You can read the second post (and find a link to the first one) at this link: https://modelingthesp.blogspot.com/2024/03/understanding-bridges-part-2.html . The present post presents some important bridge details that should be included if you are building a model. 

An example of such details is the bridge shoe. This is the support on which a bridge member actually rests. Modelers sometimes make the mistake of simply resting a girder bridge or truss bridge directly on its abutment. Here’s an example (I’d rather not identify the layout on which this was found.)

The prototype has shoes at each end, one of them fixed (though able to rotate about a horizontal axis perpendicular to the long axis of the bridge), and a moveable shoe that can slide, to accommodate expansion and contraction. 

Here is a drawing from the Mallery book (Paul Mallery, Bridge and Trestle Handbook, revised edition, Boynton and Associates, 1976), Chapter 6. Mallery correctly points out that bridge shoes on all but the biggest bridges are so small in HO scale that we needn’t differentiate between the fixed and moveable shoes, which at any distance look quite similar. This one is a moveable shoe.

Here is a different model example of a girder bridge, correctly including a bridge shoe, which appears to be a fixed shoe.

Another clear example, from the late Bill Neill’s excellent layout in the Detroit area, is part of his model of the Pennsylvania’s massive truss bridge over the Ohio River at Wheeling. This is a really large shoe for a large bridge.

The abutments at the ends of bridges vary considerably in design, and may be built with a wide variety of materials, often concrete in the modern age. Naturally an abutment rises to the height of the track, and has an inset at the level of the bottom of the bridge member, where the shoe rests (as in the model deck girder above). This may be the full width of the abutment, or may be quite a bit less, especially when the abutment has wings to support the underlying fill at that point. The stone abutment shown below is from the Southern Pacific steel trestle at Gaviota, California (the shoe isn’t visible from this angle).

Another important point is bridge “floors.” Many bridges have no actual floor, and the structural members of the bridge are visible between the ties. Here is another drawing from the Mallery book, Chapter 9, showing the names of members in a through-girder bridge, including the floor members: beams, stringers and diagonal (lateral) braces. 

My original bridge over Chamisal Road on my layout was designed from this drawing, with full floor members. But the girders were far too deep, as I showed in a post when I was beginning work on a replacement bridge (see that post at: https://modelingthesp.blogspot.com/2013/05/a-new-sp-bridge-for-shumala.html ). I used Mallery’s book to choose realistic girder proportions, and then scratch-built new girders of correct size. 

In the process, I also changed the bridge design, in that SP predominantly designed and built ballasted-deck bridges. My new bridge was designed that way too, so that the floor structure is hidden (shown in this post: https://modelingthesp.blogspot.com/2013/09/a-new-sp-bridge-for-shumala-part-3.html ). Of course, the floor members are still present underneath a ballasted deck, but the location of this particular bridge on the layout prevents any possibility that the underside of the bridge can be seen, so I omitted it.

Information about prototype bridges and bridge details is quite readily available, if modelers would only consult it. For a serious modeler, there is really no excuse for not including prototype bridge features and details.

Tony Thompson