Cat Bridge With K'nex

After reading Catification by Jackson Galaxy of the TV show "My Cat from Hell," I decided to make a cat bridge out of K'nex! Catification is about "Designing a Happy and Stylish Home for Your Cat (and You!)." And K'nex certainly fits my definition of stylish ;). I'm also excited to point out a number of other awesome instructables that successfully catify living spaces with cat walls, activity walls, trees, hammocks and more, albeit it not with K'nex.

Usually K'nex constructions don't have to support a ten pound cat, but therein lies the challenge and fun. Follow along to learn some structural engineering, play with K'nex, and amuse your cat!

NOTE: The bridge I'm going to focus on is only one part of the cat trail you can see in the attached panorama photo, but the principles easily apply to any other structures you may wish to make depending on the space you are filling or bridging.

Step 1: Materials

Parts:

1. K'nex
2. Angle Brackets
3. Screws
4. Cardboard

Tools:

1. Drill
2. Box Cutter

For K'nex, you'll need quite a lot of pieces, depending on how long of a bridge you wish to make. My collection comes primarily from two large sets I had as a kid. These were the ball machine and the roller coaster, whose box photos are attached. The ball machine is actually available anew in a 20th anniversary version, at least on amazon where I looked briefly.

I used some nicer looking decorative shelving brackets I found at Home Depot.

Step 2: Make the Sides

Begin by making the sides of the bridge. It is crucial for load-bearing structures that diagonal pieces be used so that the smallest unit shapes are triangles, not squares. Squares can deform into rhombuses merely by bending the weak connectors, but triangles cannot deform unless the bars themselves bend, which requires much more force.

I'm only showing one small section, but as the two cute green K'nex ellipses indicate, this should be repeated for as long as your bridge will be. Mine has eighteen repeating units.

Step 3: Add Crossbars

Here's where the bridge becomes 3D. I'm using an orange piece that is the same length as the classic red piece (and the newer green piece). It is important that the cross bar be the same length as the red bar in the repeating unit, because this will allow us to use diagonal braces in the next step.

The orange pieces are also a good choice because they have a small constraint that prevents pieces snapped on near the ends from sliding toward the middle. These pieces were originally used in a roller coaster set where the two rails of the track need to maintain a fixed distance from one another.

Step 4: Diagonal Bracing

Now, if we look at our structure from the top, we see a problem. The fundamental shapes are rectangles, not triangles. This means that this structure won't resist bending in the sideways direction. To address this, add diagonal braces by connecting X-shapes to the crossbars, as shown.

You might think this is unimportant, since the cat will be pressing down from above, not pushing from the side. However, that would be a grave mistake. When loading is applied from above, part of the structure will be in tension, and the other part will be in compression. (In other words, the cat makes the bridge want to bend into a smiley face. The upper lip gets compressed, the lower stretched out). The compressed side of the structure, if not diagonally braced as seen from above, will buckle. Instead of straight lines, the two upper sides of the bridge will become zig-zags. If you don't believe me, skip this step and try it out with the bridge only slightly off the ground. Its fun to watch!

Step 5: Diagonal Bracing on Bottom

Now you might think the bottom of the bridge will be in tension, thus no buckling, so no bracing needed, right?

Correct, as long as your bridge is mounted at the ends. I mounted mine a bit away from the ends however, so when the cat passes the mount, the loading reverses. In other words, when the cat is past the mount, she makes the bridge want to bend into a frown, not a smiley.

If you're not mounting right at the end, add bracing on the bottom too. No need for a full X-brace since we're not distributing the weight of the cat like on top, just a zig-zag will do.

Step 6: A Crucial Pitfall

Some of the K'nex snobs may be wondering why I'm not using the inherently three dimensional connector pieces, The purple and blue joints that can be snapped into each other to connect diagonal beams in two different planes.

The reason for this is that these joints are only good in compression. If a load is pulling the joint apart, that is to say if the joint is in tension, it will come apart slightly, leading to a very large deformation when a cat is to be supported. You can use these joints if they will be in compression, but think very carefully about how the applied force will be distributed.

Step 7: Cardboard Layer

I cut strips of cardboard to the same length as the top diagonal bracing. This layer is important for distributing the cat's weight and providing a better surface than K'nex for him/her to walk along. You can keep the cardboard in place with a simple K'nex clamp like the ones I used.

If you have access to a machine shop, I recommend a band saw with a guide for doing the cardboard. Its crazy fast, and you can do 10 sheets of cardboard stacked at a time. Just put a few scrap pieces of cardboard through first to get out all the metal chips. Of course a box-cutter and patience will work just as well!

Step 8: Connect Bridge to Mounts

You'll have to do something different depending on the mount you choose, but you can see what I did in the attached photo. Basically just replace the bars you had at first with a more dense set of bars and connectors that closely surround the bracket. This will take some finagling, but once you're done, the bridge is ready to go!