Sky-N-Sun - Bridge Design

Suspension Bridge Design Considerations and Math
Anchors and Cable Routing

The anchors are the counter balance to the pull on the cables by the weight of the bridge. It stands to reason then the simple answer to "How much should the anchors weigh?" would be "If they were hanging streight down, at least the weight of the bridge and any additional load on it." However, if your anchor is buried, the soil around and over it will resist the pull of the cables. The farther from the bridge the anchors are located, the closer you get to pulling them sideways through the ground. In this case, the surface area pushing against the soil is the important factor, and in theory, the weight could be zero.

The composition and water content of the soil will affect the amount of resistance it can supply. You will need a much bigger anchor in wet, sandy soil than in dry clay. Unfortunately, I can't supply any real math to answer this question. You might get some ideas from talking to someone familiar with anchoring in your soil type. Farmers and construction workers are two good choices of people to talk to.

Once the anchor is done, for each side of the bridge attach a section of cable to the cable attachment points. Attach the other end of the cable to a turnbuckle with 18 to 24 inches of adjustment. The main bridge cables will be attached to the other end of the turnbuckle. Plan the cable lengths so that the turnbuckles are between mid-chest and shoulder high to make adjustments easy.

The graphic at the top of this page shows the sequence from one end of the bridge to the other. It is:
Anchor short cable turnbuckle main cable turnbuckle short cable anchor.

Note: If your anchor is in a grassy area, you might consider threading each of the short cables through a length of plastic pipe. The cable can be hard on a string grass cutter. I wish I had thought of this before I attached the cables!