Building a concrete arched bridge
matching angle on the underside of the arch sides. After this, I applied liquid cement and added scrap styrene rein- forcing blocks on the inside of the joint. Once everything dried, I trimmed the arch sheet to the outside face of the arch sides and filed and sanded the joint smooth.
Once this initial work was done, I found I didn’t like the appearance of the bridge in track-level photos. It was too high above the equipment to look right for its era. Before permanently in- stalling it, I was able to trim about three scale feet off the bottom of the arch and piers. This is shown in the fi- nal photos, and I like the effect much better. My railroad doesn’t need clear- ance for intermodal or auto racks. Dou- ble-stacks and car carriers haven’t yet run to Maine in 2014, and they probably never will on the old Eastern Railroad.
The beam bridge For the beam portion of the bridge, I started by attaching two wood abut- ments to the plywood base. Next, I made a bridge deck from .040″ styrene, spanning from one abutment to the oth- er, and cemented Rix No. 105 highway overpass beams to the underside of the deck, splicing them at the midpoint. I used the flat-sided beams along the edge that abuts the arch. The basic bridge assembly was completed by glu- ing the deck and flat beams to the side of the arch so the top of the deck was level with the top edge of the arch sheet. Because the prototype is well above the natural ground level in the area, the bridge deck has a vertical curve. The construction methods below accommo- date this with some extra steps as the price. If the road is level or nearly so across the bridge, one could make the entire deck out of styrene and assemble the balustrade to it before working on the piers for the beam section. I fabricated square columns and caps for the beam bridge’s piers using pine strips and blocks cut on a table saw. Square styrene tube and sheet may be substituted if you don’t have the equipment to cut wood to size. The piers should be made just high enough that they support the beams but don’t lift the bases of the arch off the ply- wood support. I glued the bases, columns and caps together with white glue and set them aside to dry. I began the two balustrades by cut- ting and splicing Rix No. 104 early highway overpass railing sections to make longer parts. As the balustrade on the arch side doesn’t match the newer beam bridge side, the cuts dif- fered. I assembled and glued the rail- ing sections against an .060″×.040″ styrene curb strip for strength. When
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The sides and arch interior were assembled on a plywood sub-roadbed between the wood blocks that will support the deck (top). The more distant risers will support the roadway at the abutments. The bridge has been turned over (above) to show the beams attached to the arch. Note that the arch’s balustrade has been attached and extensions were added to the bottom of the arch sides to improve clearances. The author later de- cided that these were not necessary. The wings have been built and permanently at- tached to the arch (page 49, top). The curve visible in the beam bridge was formed in place by fastening the hardboard roadway over it while gluing its balustrade in place. Once the bridge was painted and placed on the layout (page 49, bottom), it was weath- ered. The black smudge marks were made with soot from a candle flame.
they had dried, I fitted the
curb/balustrade assemblies to the top edge of the arch and to the styrene deck in the beam section but did not glue them.
I made a one-piece roadway for the bridge and its approaches by cutting ¹/₈″ hardboard to fit between the curb portions of the balustrades. I fastened the hardboard down onto the wood blocks inside the arch ends and the
beam bridge’s abutments using small flat-head wood screws, countersinking the holes in the hardboard so the screw heads would not interfere with paving the road later. I made the abutments lower than the arch and columns sup- porting the beams so the hardboard roadway forced the ends of the beam bridge section into a gentle curve downwards. This also bends them over the piers on either side of the tracks.
APRIL 2014
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