Sky-N-Sun - Bridge Design

Suspension Bridge Design Considerations and Math

There are many types of bridges, but for long spans the suspension bridge is the best choice. The idea is simple: string a couple of cables over some high points, and attach a deck to them. As with any project though, the fun is in the details!

There were two major designs for suspension foot bridges in the area at the time I began this project. One design had the deck setting on top of the cables. The other had the deck suspended below the cables with equal length suspenders. In both designs, the deck follows the sag of the cables.

The decking on top of the cables has several advantages. The first, of course, is that the towers only need to be as tall as the sag of the cable. This makes their construction less expensive. Secondly, if the deck is only setting on the cables and not attached, flood waters may take the deck without destroying the towers or cables.

The deck suspended below the cables design has two advantages that ultimately led to my selection of this design. With this design, the suspenders can also be used to stabilize the hand rails, making the bridge at least seem safer. The second advantage is that by adjusting the length of the suspenders, the deck is no longer forced to mimic the sag of the cables.

I had experimented with adjusting the suspender length on the old bridge and discovered that a level deck was possible. I also discovered that using the "by guess and by gosh" method of adjusting the suspenders would result in, at best, a wavy deck that was only sort of level. After many, many passes back and forth across the bridge adjusting the suspenders, I realized that I would probably cause stress failures in the suspenders by un-wrapping and re-wrapping them repeatedly long before I got a truely level deck.

I wanted a flat, level deck on the new bridge so I selected this design and began searching for a way to calculate the right length for the suspenders. Searching the internet eventually led me to a formula that describes the sag of the cables, and I'll try to explain its use later in the Catenary section of these pages.

On the next few pages I will discuss the design considerations for the major components of the bridge. Suspender Length and Load Bearing calculations will be discussed on the Catenary page. The "Results" pages contain data from this project and other projects I've done since then.