Concrete Abutment

A Basic Abutment
In “Railroad Structures and Estimates” Chapter VI “Bridges, Trestles, and Culverts” there is a drawing of an Abutment and a table showing dimensions for the bridge seat depending on the type of bridge and length.

This book was published in 1918 so the depth of the bridge seat in the table was based upon the engineering at that time and reflects the depth of the bridge needed for different spans. There were so many different ways to engineer the same type of bridge (such as a Plate Girder) that I imagine this was just a general starting point. For a Model Railroader and Scratchbuilder it works fine .. for me since I am modeling the 1930-1940 time period.

The depth of the bridge seat (Dim A in the drawing) though .. impacts directly on the design of the bridge shoe. Think about it. The height of the bridge seat (Dim B in the drawing) depends on the length of the span .. and what engineering was used. In other words, how deep are the girders and how did the engineer arrange them. The depth though .. that is the part of the abutment that takes the weight of the bridge through bridge shoes. That to some extent also relates to the engineering/bridge design since different engineering and materials (iron vrs. steel) would dictate the weight of the bridge and therefore the width.

Still. For me .. scratchbuilding a Model Railroad bridge .. this gives me the dimensions I need to make the bridge shoe!

Some Dimensions
Just below the drawing of the abutment is a table labeled “Quantities in Abutments”. It shows the approximate cubic yards of cement used for various types of bridges – Deck Plate Girder, Half-Deck Girder and Through Bridges. What caught my eye though was that these quantities are give for bridges of various spans for each of these types of bridges along with dimensions for the bridge seat. Cool. Since my abutment was for my On30 plate girder bridge I was only concerned with dimensions for the Half-Deck Girder bridge. I’ve pulled out the section dealing with Half-Deck Girder bridges – which is on the left here.

My bridge ended up spanning 10-3/16″. That converts to 40′ 9″ full size. If you look at the chart that tells us that for a Half Deck Girder bridge with a 40ft span, dim B is 1ft 8in – which translates to .416 in.

Oops. This is where I screwed up. With all the attempts I made to ‘get it right’ .. at the very last .. looked at the wrong column. I should have pulled that 1ft 8in from column B .. but somehow I pulled the 2ft 6in from column A .. which is the seat depth .. not height. I managed to recover from this which I will talk about in a bit but I must say it was irritating at the time.

Making a master
With the information in hand on the depth from base of rail to base of bridge (which was wrong), an approximate size for the bridge shoe, the measurement from the base of rail to the water, I could modify the abutment from the book. They have the bridge wings folded back at 30° and sloped at 1-1/2 to 1. I sliced some from the center to fit 8 ft On30 bridge ties and modified the tie seat to 0.112 in (On30 switch ties on hand).

The part concrete colored will be what’s above ground. The footing is all of the yellow concrete. I would only be building the part that shows.

I used .040″ Styrene to build the master. I again used Sketchup to print the plans out full-sized in O scale. You use kid’s stick school glue to paste the prints to the Styrene and you simply score and snap on the lines. The glue-stick glue dissolves in warm water so you can remove the paper afterwards. You end up with a stack of plastic pieces .. your own personal ‘kit’ .. as it were.
I found the multitude of angles on the abutment interesting. If I hadn’t already been slightly mad before starting .. this would have caused me to trip over that thin line I imagine. There are angles everywhere! It does though make for a very ‘pretty’ structure.
I was pretty much happy with the finished abutment. This is after a couple coats of primer, some putty (automotive), sanding and finally a coat of Rust-Oleum “Fine Texture” paint. (I hadn’t at this point realized that I had the bridge seat height too deep)

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