|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!