Thursday, April 24, 2014


As the mainline begins the climb up the mountain RR-West of Oakridge, it immediately crosses Salmon Creek.  The mainline bridge is a four-span ballasted-deck girder bridge.  Very nearby is a lower bridge and trestle that serves the Pope and Talbot sawmill.  The immediate concern, though is for the mainline bridge.

The mainline bridge was built around a core composed of the three splines in the middle of the spline roadbed used for the mountain grade.  I use nominal ¼ inch hardboard cut into one inch strips for splines.  The core bridge splines were depressed below grade to account for the thickness of the deck.   The ballasted deck was located on top of the spline core.  Girders were attached to the underside of the deck, concealing the spline core.  Piers and abutments will be fitted to this assembly (deck and girders), completing the bridge.  The spline core maintains roadbed continuity and a modest curve through the bridge.

Construction of the bridge began with a template tracing of the spline core.  Using this template, I planned the deck base sections, using two segments for each girder.   The deck width is 16 scale feet, per SP Common Standard plans published by Steam Age Equipment Company.  In contrast to the deck on a pile trestle, this deck uses 4 inch thick longitudinal boards supported by transverse (across the girders) 12x12 beams.  I cut out the scale 16 feet wide deck base segments from 0.040 inch thick styrene.  I used my NWSL Chopper to cut the 12x12 (1/8 inch square) styrene beams, making every eighth beam 18 feet long to support the railing posts. 

Ballast deck under construction.

Once the basic deck and its support beams were assembled, I attached the handrail posts and handrails.  Note there was a lot of styrene cement used on the deck.  While the primary bond occured fairly quickly, it took quite a while for the remaining solvent to evaporate.  I did fairly well with getting weight on the assembly to hold it flat, but I still got a little bit of warpage.  That complicated the handrail construction and subsequent handling, but was well within bond strength of the girder attachment and eventual use of adhesive caulk to hold the deck flat as roadbed.

Ballasted deck with handrails installed test fit into location.

The girders were cut from Central Valley plate girder bridge kits (210-1903). Only the rounded girder ends of two center girder sets were cut off.  The outer girder sets were cut shorter by a panel on each end, just as the prototype bridge.  The top and bottom plates were attached and trimmed to length, followed by the bridge shoes.  The rest of the Central Valley kit is surplus to this bridge.  I painted the girder sections Grimy Black. Rust weathering was done with Bragdon powders, followed by a light overspray of Tarnished Black. 

The bridge deck was placed on the spline core and the spline outline scribed on the bottom (cross-beam) surface.   The front girder segments were attached to the deck assembly using gap-filling acc cement.  Once this set, the bridge assembly was removed from location, inverted and the back girders attached, staying outside the scribed lines for the spline core.  Once all of this set up, the deck and girders were placed in position again, awaiting piers and abutments.

Ballasted deck in place with front girder segments attached.  Rear girder segments are laying on the “creek” surface below.  Cans of beans are very handy weights!

Completed ballast deck and girder assembly in place, spanning Salmon Creek.

No comments:

Post a Comment