In a book titled “Notes on track : construction and maintenance By Walter Mason Camp” pub 1903 I found a diagram showing that .. yepper .. the ties overhang. If I used 8′ bridge ties .. that means they would overhang the rail bearers by only about 33″ .. which is the same (pretty much) as the standard gage overhang.
On page 847 it says .. “The size of tie required for a bridge floor depends upon the manner in which the tie is supported. If the tie is supported by stringers directly under the rails (which mine does .. aka .. the rail bearers) or nearly so, it supports the rail without appreciable bending movement, and other then this its duty is merely to hold the rails in gage and in line. Under such requirements a tie of ordinary size is sufficient, and the common sizes are 6×8, 7×8 and 8×8 ins., the first and last mentioned being the most common.”
Well. That pretty much answers that. 8′ ties will fit with clearance between the triangular braces. 7′ might be better. Will have to wait to I get to that part! 🙂
The bridge was going to be right at the edge of the layout so I wanted as much detail as I could get. I drew up some On30 tie plates and had them 3d printed at Shapeways. In the photo I have painted them and piled them up for a photo. Notice that some of them have lines on the surface. This is due to the way that they were printed. Those lines are the layers of plastic formed during the 3d printing process. Depending on the orientation they show sometimes .. and sometimes they don’t. I had enough printed that I could pick and choose. I mean .. these things are small .. right at the edge of the maximum resolution of the printer.
Note: If you click the link to Shapeways you will see the tie plates arranged in a sprue of 20 plates with a $5.32 price tag. I had these printed before Shapeways tacked on a $5 handling fee so my cost was only the .32 cents bit. I need to go back and gang together multiple sprues so that $5 handling charge is minimal applied across all the tie plates.