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10-foot Bridge Success

Wednesday, July 15th, 2009

The bridge made it through our train show (see photo gallery)! It held up great. I was quite surprised to find that no member or cross bracking loosened noticeably. Usually, on trusses like this, a piece of cross bracing will come loose at some point due to vibrations from the trains.

packed up and ready for the show (click for larger)

I packed up the model by breaking it into four sections, and then piggybacked two section pairs together.

at the show (click for larger)

TexLUG’s layout consisted of three 8-foot long tables on either end of the bridge. One of our members (Matt S.) brought two 2.5-foot small truss bridges, and we used those on one side of the layout.

inside view (click for larger)

Super Chief crossing (click for larger)

The bridge did endure some heavy loads. At one point, we had two very long trains on the main loop, and they managed to end up on the bridge at the same time. Both trains were longer than the bridge :-)

Tony’s Allegheny (click for larger)

Their were a few accidental human contacts, but it survived. The worse was when my 4-year old son was underneath and suddenly popped up. He buckled a section of deck with his head. I managed to press it all back together quickly before the train came back around :-)

10-foot Bridge

Wednesday, 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

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:

member sizes (click for larger)

deck members (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.