10-foot BridgeWritten by TJ on July 8th, 2009
Over the past two weeks, I’ve been working on a new bridge. This will be for TexLUG’s display in an upcoming model train show.
The design length of the bridge, measured from center-to-center of the end supports, is 10.46 ft (3.19 m). The actual clear span will be just 1/2 inch, or so, less than this distance. The actual length, in reality, will be just slightly less or more, depending on how the structure settles.
click larger view
I designed the bridge by determining the sizes of all the main members and also the deck and hanger pieces. I modifed my old truss spreadsheet for this design: www.texbrick.com/model_10ftbridge/bridge_10ft.xls
member sizes (click for larger)
I changed a few things about the construction of this bridge as compared to how I normally build trusses. I’ll describe all the fine details later on when I make a formal webpage for this bridge.
I had to correct a few things after I had mostly built the whole bridge. I wished I had realized some of these things from the start:
1. It wasn’t wide enough. The original design would have accomodated 8-wide trains just barely. However, I realized later that we’ll have 12-wide trains (i.e. “natural” 10-wides with extras hanging off the sides; e.g. Tony Sava’s monster Allegheny). I spent a lot of extra time making the portal frames wider.
2. Height. The end portals of the bridge were lower in height in the original design. I had to change up the design (by lengthening some members) to create extra height so that trains passing through would have adequte clearance.
3. Strength. After I had the original structure built, I noticed that the main chords deflected a lot when I loaded the bridge. I experienced the same trouble with the 17-foot bridge I built 4 years ago. So, I doubled-up most of the main truss chords on this new bridge. When Tony’s Allegheny rolls accross it, I want the bridge to stay intact!
Anyway, the bridge is nearly complete, and I hope it will hold up this weekend at the show.