January 13, 2010

No ifs, ands, or abutments

Time for the home stretch run on the Potomac Creek bridge. I added the stringers and ties to the top deck. I just need to add the nuts and washers to the truss rods and the truss portion of the bridge will be complete.

With the stringers and ties added, I was able to begin work on the abutments. The abutments must be sized to hold the top of the bridge at the proper elevation. Thus you need to have the stringers and ties in place to match the ties of the truss with the ties on the ground.

Likewise, the roadbed at each end of the bridge should be installed to set the final elevation of the track. I had the south bank done, but I needed to add the sub roadbed at the north end. Here is a photo showing the 30 inch curve template and the laser cut (with kerfs) north bank sub roadbed. I used hot glue, yellow carpenter glue and Elmers wood putty to hold and smooth the roadbed section.

Note how the track curve begins on the bridge. The stringers underneath are angled to accommodate this curve. Wooden bridge stringers are almost always straight. Some contemporary bridges use curved steel and cast concrete stringers.









Here is the first substructure for the southern abutment. The near vertical walls will be covered with embossed styrene stone sheeting. The lower bearing surface will be covered with Durham's water putty to represent the bed rock of the out cropping. I will make final height adjustments with wooden shims under the sways backs of the truss.

I am using 1/8th inch soft plywood for the substructure. It is a nice material to use as it is easy to cut and very strong. Hot glue also works well to hold it together, but still allows easy dis-assembly for adjustments etc.







The holes will be filled in with scenery material to represent the natural slopes.





I decided to add a stepped front to the abutment to allow a place for the bridge stringer to rest. I did it by building up a stepped area with wood strips (actually left over stringers from the bridge). Then I covered it with stone plastic sheet. Like my earlier abutment, I filled in the gaps in the corners with CAA and Tamiya Putty. A bit of sculpting with a file and knife and it's almost impossible to find the seam. The rough stone plastic is very forgiving.
The plastic stone sheets are from a UK manufacturer. I got them from a US Supplier:

Railway Models
P.O. Box 871
Edgewood, MD. 21040
http://railwaymodels.tripod.com/

2 comments:

  1. Bernie,

    What did you use for abutments? The hewn stone looks great.....

    Mark Andersen

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  2. The stone material is a styrene sheet from a UK manufacturer available from a retail supplier in Edgewater, MD. He usually shows up at GMSTS in Timomium.

    The rough texture makes it easy to finish the edges. I used CAA and Tamiya putty to fill in the corners and then reshape to the desired final appearance. Once it is painted it looks very convincing. I also have some Plastruct stone sheet, but the texture is less pronounced.

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