GUEST BLOGGER: John Seaman - Building A Custom Girder Bridge

January 17th, 2008 in Guest Bloggers, Tips by Dave 1 Comment

Guest blogger and OGaugeWatch reader John Seaman submitted this article about making a scratch built girder bridge.

For those that buy the Atlas O Double Track Pratt Truss Bridge Kit but don’t use the Atlas decking/track that comes with it like me, you can use the deck/track from the kit to create a custom girder bridge. I built a 24” girder bridge. Here’s how:

Click on the link below to read on

What you will need:

3 12” sections of 1/4” K&S square brass tubing
2 12” sections of 1/8” K&S round brass tubing
Plastic Window Frame Side
1 piece of 1” x 1” Aluminum Angle sized to fit the area to be spanned
Small sheet metal screws
Small screws included in Atlas O Double Track Pratt Bridge
Paint in a color of your choosing for railings and ‘girder’.
Tools: Ruler, Dremel with Cut-Off wheel, razor saw, needle nose pliers, precision screw driver, 1/8” drill bit, razor blade knife, small metal files, fine grit sandpaper or Dremel buffing wheel, 140 watt soldering iron, solder, small weights to hold brass pieces in place during soldering, epoxy for joining metal to plastic

You can purchase the aluminum angle at any hardware store. I found the
plastic window frame in a dumpster at a construction site but this too
can be purchased at your local hardware store. I have a band saw so I used it to cut my deck/track and the window frame but I’ve suggested other methods in the steps that follow.

Cut the bridge decking to the desired length using the razor saw.
Using a Dremel with a cut-off wheel, cut 5 pieces of the square brass tubing to a length of 5 1/4” with a 45 degree angle on each end. These will be used for the railing supports and will fit into the spaces on the underside of the decking.
Cut 10 pieces of the square brass tubing to a length of 1/2” with a 45 degree angle on one end to match up with the supports from step 1. These pieces will be used as the railing uprights.
Using the 1/8” drill bit, carefully drill a hole in the top of each railing upright. You may have to file out the holes a bit in order for the 1/8” round brass tubing to fit through the holes.
Using the fine sandpaper or Dremel buffing wheel attachment, clean the 45 degree angle cuts of all the brass pieces cut in steps 1 and 2 to prepare for soldering.
Solder the railing uprights to the railing supports using the 140 watt soldering iron and solder. Use small weights to hold the pieces in place while soldering. Clean up the finished joints with a metal file, fine sandpaper and/or buffing wheel.
Paint the assembled railing supports at this time.
Lay the plastic window frame on the workbench. You will need to cut approximately 2 inches out of the center of the window frame leaving you with 2 pieces of the same size. I used my band saw for cutting but you can use a razor knife or the Dremel cut-off wheel instead. The length of the pieces should be an inch or two longer than the bridge decking so you can trim for a custom fit later. If desired, you can paint both pieces at this point.
The window frame pieces are secured to the decking using the small screws that come with the Atlas O double track Pratt bridge. Use the pre-drilled holes in the decking. Lay the window frame pieces overtop of the decking and mark the location of the screw holes. The distance between the two window frame pieces is dictated by the width of the aluminum angle piece that will be used to support the bridge (1” in my case).
Attach the assembled railing supports to the decking using the epoxy then screw the window frame pieces to the decking.
Thread the small 1/8” round tubing through the railing uprights and trim ends for an exact fit. You can paint the railings at this point.
Measure and cut the aluminum angle piece to the length of the area to be spanned plus an extra inch on each side for attaching it to your benchwork.
After the aluminum is installed, place the completed girder bridge over the aluminum, trim the window frame pieces for an exact fit then fasten the bridge to the aluminum using with small sheet metal screws.
Walkway boards could be added using balsa wood or some other wood.