Polydron Play

Monday, October 6, 2014

Bridges Part 3: The Pratt Truss / Ponts a Treillis Pratt

The bridge I'm going to show this time is a Pratt truss which is very similar to the Howe as far as the Polydron pieces are concerned.

In the kit, it looks like this:

My Frameworks version looks like this:

If you happen to have solid square pieces, it's nice to use them for the deck as I have in the above picture, but it's not absolutely necessary. However if you have only Frameworks pieces, you might want to add a piece of stiff cardboard on top of the deck in order for the children to drive their toy cars on the bridge.

For this bridge you need:

2 plinths (see previous post here)

20 equilateral triangles

8 squares

Bridge

20 right angle triangles

12 squares or 6 frameworks squares and 6 full squares

Here's the flat layout of the above bridge:

Here's the flat layout using only Frameworks pieces:

In the Polydron Bridges kit they add rectangles at each end of the bridge for stability because without them the bridge will lean.

Because my plinth construction is slightly different, you can't use rectangles, so I've added squares instead:

Pictures of real bridges:

In Iowa

In North Dakota

Links:

Truss bridges on Wikipedia

Pratt trusses on Garrett's Bridges

En Francais:

Ponts a treillis sur Wikipedia

Ponts a treillis sur Gramme

Bridges Part 2: The Howe Truss / Ponts a treillis Howe

First off, I am not an engineering expert, so I'm only showing you what I've learned myself through using Polydrons.

I'm going to start with the truss bridges as they are the most basic and that's what you start with in the Polydron Bridges kit. I've color coded the pieces in order to make it visually easier to understand, but when you make your own bridges, of course you can use any color combinations you like.

The type of truss bridge I'm going to show in this post is the Howe Truss and in the kit it looks like this:

My Frameworks version looks like this:

For this bridge you need:

2 plinths (see previous post here)

20 equilateral triangles

8 squares

Bridge

20 right angle triangles

12 squares or 6 frameworks squares and 6 full squares

Here's the flat layout of the above bridge:

Here's the flat layout using only Frameworks pieces:

In the Polydron Bridges kit they add rectangles at each end of the bridge for stability because without them the bridge will lean.

Because my plinth construction is slightly different, you can't use rectangles, so I've added squares instead:

Of course if you have the pieces available, you can try making longer bridges and explore what kind of requirements you need in order to keep the bridge from buckling (more plinths, etc.)

Pictures of real bridges:

- In Manitoba

- In British Columbia

Links:

Truss bridges on Wikipedia

Howe trusses on Garrett's Bridges

En Francais:

Ponts a treillis sur Wikipedia

Ponts a treillis sur Gramme

Bridges Part 1: Plinths

A few years ago I was able to pick up a Polydron Bridge kit off of E-Bay for a reasonable price. A number of the bridges in the kit can be built using regular Polydron Frameworks pièces. I'm going to try and show you what I've done as well as some of the changes I've made in order to do without the special pièces found in the kit.

The foundational element of the bridges is the plinth which is the part of the bridge which anchors into the ground and gives support to the whole structure. In the Bridges kit it looks like this:

It's fairly easy to make one out of Frameworks pièces or even regular solid pièces. Mine looks like this:

I left an empty space on two opposite sides of the plinth in order to be able to attach the bridge easily and keep it level.

You need at least two plinths for most of the bridges.

For each plinth you need:
4 squares
10 equilateral triangles

Here is the flat layout for a plinth: