Sky-N-Sun - Bridge Design

Suspension Bridge Design Considerations and Math
Towers

tower Towers provide the high points for the cables, support for the deck, and a place to attach the steps or ramps that connect the deck to the ground.

The total height of the tower is sum of three factors.

The height of the deck above the foundation. This will depend on what you are crossing. In my case, I wanted to not only be higher that any major flood water, but also have some clearance for floating debris.
The desired height of the cables above the deck at their lowest point. Since I wanted them to be above the hand rails, this had to be four feet or more.
The sag of the cables from the top of the saddles to their lowest point. This sag has to be enough to keep the cable tension within its capability. Generally, as sag increases the cable size can decrease or the span can become larger but the cost of the tower increases.

The picture at the right shows one of the towers we built. It consists of two sections of telephone pole with boards bolted into notches front and back at the top and at deck level, and cross braces bolted on below deck level. A split telephone pole section is nailed across the top. The saddles are sections of galvanized stove pipe split open and nailed to the cap directly above the vertical poles. We placed a tablespoon of axle grease on the saddles to allow the cables to slide as they expand and contract with the seasons.

Notching the poles for the horizontal boards not only makes the tower look more "finished", but by making the notches very tight, the tower is rigid even before the cross bracing is added.

The cross bracing is to keep the tower from leaning side to side while the angle the cable makes as it leaves either side of the saddle will keep it from leaning forward or back. In addition, the completed bridge will have fencing stretched on each side of the deck from tower to tower. That will keep the towers from leaning away from each other, and we added some #9 wire from the top of the tower to the anchor to keep them from leaning toward each other. That bit of wire probably isn't needed, but it made me feel better. :-))

The tower is wider at the base by about four inches. This amount adds to the tower's stability but keeps the downward force of the cables within the poles so that strength is not compromised.

The picture was taken just after we had draped the cables over the towers. If you look carefully you can see the cables, and the rope we used to pull them across the creek. At this point, the towers are still held vertical by the ropes tied to near by trees and the anchor.

Foundations

The bridge we had lost in the flood had a two foot by two foot foundation that was only a few inches thick for each leg of the towers, and we had fought problems with uneven sinking causing the bridge to lean. I considered the following as I decided on what the foundations would be.

The foundations needed to support the full force of the cables as well as the weight of the tower
In our part of the country the frost level is about 15 inches down
We wanted the top of the foundation to be above ground level to protect the bottom of the wooden tower from ground rot.
A single foundation for each tower should provide better stability

It may have been overkill, but we poured concrete foundations that were three feet thick, three feet wide, and eight feet long. The foundations were reinforced with re-bar and also had two pieces of re-bar extending out the top that fitted into holes drilled in the bottom of the tower to keep the tower from sliding on the foundation.