Polydron Play: Bridges Part 5: Swing Bridges / Ponts Tournants

So I'm going to set basic truss bridges aside for a little while and move on to bridges that move. Maybe I'll come back to the trusses later, but even so, we haven't left them completely behind since they are one of the fundamental bridge structures.

The Swing Bridge in the Polydron Bridge kit uses a Pratt Truss structure and looks like this:

For mine, I chose a Warren Truss just to show what we can do and mine looks like this:

Now the most difficult thing in this bridge is trying to make a central pivot without specialized pieces, and I'll admit that I did have to use two pieces that not everyone may have.

In the Bridges kit the pivot is made from two square pieces with holes in the middle through which passes a strut and which has an added pulley piece attached in order to keep the strut from slipping through. It looks like this:

That's a lot of specialized pieces. For mine, I used two squares with gimbals which come in some of the Polydron tubs.

I also gently removed one of the gimbals from one of the squares. You have to be careful in order not to break them:

When you set these two squares on top of each other, they pivot. Here's a view of my pivot system with part of the truss structure pulled aside:

If you have triangle pivot joints, you could probably work out a system as well. I know I have in the past by making a roadbed and all of the truss structure out of only equilateral triangle pieces and by making triangular shaped plinths. However I couldn't find a way to keep the bridge from leaning because none of my Polydron pieces would fit on the ends of the bridge, as I explain here at the bottom of the page.

If you don't have any special pieces, you can always make the bridge and plinths in order to demonstrate the bridge without having any real pivot piece. I'd love to hear if anyone has any ideas about other ways to make the pivot.

The next thing I had to do was modify the plinths. I made two for the end plinths and one with a gimbal square for the central plinth. Here are each of them with their accompanying flat layout:

To make my version of this bridge you need:

3 modified plinths:
     36 equilateral triangles
     26 squares
     1 gimbal square without the gimbal

Warren truss bridge:
     42 equilateral triangles
     22 squares or 10 solid squares and 12 Frameworks squares
     1 square with gimbal

Here's the flat layout of the bridge:

For the Pratt truss version from the Polydron kit, but using my pivot setup and my plinths, you need:

3 modified plinths:
      36 equilateral triangles
      26 squares
      1 gimbal square without the gimbal

Pratt truss bridge:
     36 right angle triangles
     22 squares or 10 solid squares and 12 Frameworks squares
     2 rectangles
     1 square with gimbal

Here's the flat layout of the Polydron kit bridge. You just need to replace the center gray square with the hole in it by a square with a gimbal: