Rope Bridge (Full Project!)

Rope Bridge

Problem: Your cat does have places to perch, but they are not accessible.

Solution: Connect them with a rope bridge

 

Difficulty: Medium

Cost: Medium – High

Build Time: 5 – 7 hours

Bridging the Gaps

Other projects such as the shelves and ramp have described some reasons why your cat may appreciate access to elevated places. One key motivation to climb may be to escape threats that will not follow them up there. Staying up high is an easy way to avoid common threats such as dogs, toddlers, or vacuum cleaners. However, their escape may not feel nearly as safe if it has a dead end. If the only way down is back the way they came, then it may feel more like a trap than a safe place.

This rope bridge project is a fun way to expand your cat’s elevated options. It can provide an escape route so their existing perch does not feel like a  trap, and it also serves as a comfortable cat hammock itself.

Engineering 101: Suspension Bridges

Bridges are one of the most highly revered products of engineering. They require clever construction techniques to build, involve beautiful mathematics, and can be the centerpiece of a city through both their form and function. The design, analysis, and construction of bridges fall under a discipline called civil engineering. This is fortunate for the civil engineer because it may be the discipline’s only redeeming quality. Other projects that fall into the civil portfolio include highways (flat strips of concrete!), sewage systems (tubes full of poo!), and dams (a structure that can literally be recreated by rodents). Some civil engineers may try to claim credit for buildings in general, but they know this is a lie. Yes civil engineers may work on buildings, but none of the fun parts. Generations ago a clever con artist invented a profession called “architect”, where they would take all the fun design work for a building while the engineers still did all the math to make it work. This ingeniously resulted in the architect getting all of the credit if a building was a success, and the engineers taking all the blame if it was a failure. This is still a fairly sore spot in the civil engineering community.

Considering the highways and sewage systems in your life, you have probably experienced first-hand how a civil engineer may be prone to failure. With such a track record it may be reasonable to assume they messed up bridges in some way as well. For example, one of the most iconic bridge designs is the suspension bridge, which looks something like this:

The civil engineers claim that the large cables running across the top carry the weight of the road to the columns. This is clearly a poor design. If they were carrying so much weight why would they be allowed to droop like that? Why weren’t they pulled tight? Well believe it or not, but this is actually a case where the civil engineers got one right. An optimally loaded cable will droop just as shown. The images below show cables that are pulled tight with an increasing number of evenly spaced masses suspended from them. You can see that as more masses are added, the shape approaches that of those “drooping” cables on the suspension bridge. Those cables are actually pulled incredibly tight, but they are supporting a large amount of mass that is evenly spread along the length, which gives them their final shape.

For our cat bridge in particular this final shape is important to know before you try to install it. Since the bridge does not form a straight line, the total length will be longer than the span.

To determine the total length needed, we turn to math. For the suspension bridge shown above, where the main cables support a flat roadway beneath it, the cables will form the shape of a parabola. This is a common mathematical shape that can be described by the function f(x)=ax2+bx+c. Our bridge does not have a flat roadway beneath it, the roadway actually follows the path of the support cables. This changes the shape slightly so that it follows a curve known as a catenary. A catenary can be described by the function f(x)=a cosh(x/a), where “cosh” is the hyperbolic cosine.

Calculating the length of our bridge based on an equation like that may seem stressful, but there is no need to worry. We are going to solve our length problem like real civil engineers – by looking up the answer in a table. When a civil engineer has to do math they don’t do it directly, they have some other smart engineers do it for them and create a table like the ones below so they can find the answer immediately. They may point out that this is way more efficient and significantly reduces the chance for error, but that is just an excuse. The real reason for tables is that they hate fun. So here is an efficient yet fun-killing table:

To use the table, first look up the total span your bridge will cover. There are two options for the total length, the tight option will result in a bridge with less droop that moves less as it is walked on. The total vertical drop of these bridges at the center will be 1/12th the bridge span. The loose option will have more droop but will be easier to install. These may also serve as more comfortable hammocks. The total drop of the loose bridges will be 1/6th the bridge span. You are not limited to the values in the table. If your span is in between two values or you want a happy medium between loose and tight, then you can interpolate as needed.

Build Instructions

The following tables list the tools and materials required to build this rope bridge. The labels (T1, P1…) are referenced in each step of the instructions.

Step 1: Cut out the planks of wood.

The dimensions can vary based on the placement of the bridge. Recommended size shown is 2″ x 10″ and 0.75″ thick. These can be ripped from a piece of plywood or common board, or you can use a standard stud size instead. To prevent the planks from sagging under the weight of a heavy cat, make sure the grain is parallel to the long dimension.

The amount of planks needed will be based on the total length of the bridge (see table above for reference). It would be a good idea to cut a few extra planks just in case.

There will be a spacer rope in between each plank, so the total length each plank takes up is the width of the plank plus the diameter of the rope (see Step 5).

Tools: T1     Materials: P1

 

Step 2: Sand and paint the planks.

It is especially important to sand down the corners that could pinch together when the bridge flexes.

Tools: T6

Step 3: Drill 1/8″ diameter holes through each plank.

The spacing shown here is for a 10″ wide bridge, adjust as necessary if you used a different width.

Tools: T2

Step 4: Loop the cable through the planks.

Before performing this step take a look at Step 7 on hanging up the bridge. If you want to go with option 2, you will loop the cable through the eye hooks here as well.

Do not trim the ends yet. The extra cable will make installation easier.

Materials: P2

Step 5: Thread in the spacer rope.

Tie off the rope to the cable on one end, then thread through as shown and tie off on the opposite end.

Tools: T5     Materials: P4

Step 6: Connect the cables.

Use two cable clamps to connect the cable. Make sure to include the eye hooks if you plan to go with installation option 2 in step 7.

To install the cable clamps, run the cable through as shown and then pinch the clamps shut. A special tool can be used to do this reliably, or it can be done less expensively with some very large pliers. Once you feel the clamps are securely installed, cut off the excess cable.

Tools: T3, T4     Materials: P3

Step 7: Hang up the bridge.

There are several different ways this step can be approached, two options will be detailed here.

Option 1: Before installing the 4 eye hooks, open them up just enough so that the cable can slip through. You may need large pliers, a vice, or a pry bar to do this. Install the 4 eye hooks in place where the bridge will hang. The distance between the outside edges of the hooks should be the same distance between the holes in the planks. Hang the bridge by slipping the cable into the eye hooks. For tightly pulled bridges this may require the help of another person.

Tools: T2     Materials: P6

Option 2: This option takes a little bit more work, but does not require the eye hooks to be opened up and can be an easier option for installing a tightly pulled bridge (minimal droop). Before looping the cable through the planks, install the eye hooks onto a separate piece of wood. The piece should have enough depth so that the eye hooks can be fully threaded in, a 2×4 will work fine. When looping the cable through the planks, loop it through the eye hooks as well. Use sufficiently long wood screws to attach the bridge in place.

Tools: T1, T2, T6     Materials: P6, P7, P8

Step 8: Wrap the ends in rope to conceal the cable.

There are many different ways this can be achieved, but the goal is to make the ends look cleaner and make sure nothing gets snagged on the cable.

Tools: T5     Materials: P4

Step 9: Hang the rope handrails.

Run the thicker rope 6-8 inches above the bridge to create the handrails. Use the smaller rope to connect the handrails to the bridge. These will not take any load for the bridge and it is not critical what kind of pattern you follow to connect them. These will give your cat more confidence in walking across the bridge, and will allow it to be used as a more effective hammock.

Tools: T5     Materials: P4, P5

Complete!

 

 

 

 

 

DISCLAIMER:

Please use caution when constructing this or any other project from this site. The publisher and author are not responsible for accidents or injuries that may be incurred by humans or cats during or after the building of these projects. It is your responsibility to use tools safely and judge that the completed projects are safe to use.

For additional helpful tips and tricks on how to make these projects efficiently and safely, check out the full book!