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Ross Crawford / projects / archbr

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Double-arch suspension bridge

The idea for this obviously came from the Sydney harbour bridge in Sydney, Australia. I did the arch design in about 1980, when technic was still fairly young, as was I, and I only had enough beams / connectors to build about half of one arch 8?(

Now that I’m older, and have a slightly larger income 8?), I decided to build something a bit closer to the real thing. But as I was sorting through my childhood Lego, I found the original design as I’d drawn it 20 years ago, so I thought it’d be nostalgic to build it, you might say “as a pre-cursor” to the main project. Here’s a photo of the “design”.

Design

Notice, nothing about the bridge deck, what happens in the middle, and useful stuff like that! Anyway, Here’s some pictures of the finished bridge:



Profile of the bridge. You can see the arch isn’t quite smooth at the centre, but it’s pretty close.	Train crossing the bridge, from above.	The entire layout. Not quite up to rtlToronto standards...	Train passing under the bridge. The club car only clears by about 1 plate.	Coming over the bridge. You can see the extra light I added to my loco.

It’s all done with beams & friction pins - 68 1x16s and 68 1x12s. Oh, and there’s some plates holding it all together. Apologies for the colours - I was unable to use any black beams because of my other bridge project. The two sides are 10 studs apart, leaving plenty of clearance for all those 8-wide trains...

Also, there’s nothing stopping the base of the arch from spreading - except the arch itself. As the train rolls over it, the centre drops less than 1mm. However, the arch does distort, and the points at 1/4 & 3/4 length drop by about 1.5mm. The track drops by a bit more because it’s only suspended by string, and there’s nothing stiffening it.

The length of the arch span is 170cm, less than half the length of my next bridge!

Update

Well, I wasn’t really happy with the stability as it was, so I did a bit of adjustment. First I added some baseplates to hold the distance between the bases constant. Then I replaced the string holding the deck with beams. The bridge deck now deflects less than 1mm, even with the S/Line freight train crossing it! Here’s the pics:

Site last modified November 4, 2004 Home

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