Sky-N-Sun - Bridge Design

Suspension Bridge Design Considerations and Math
Suspenders

For the suspenders and several other small items I used #9 wire. In the old bridge, the suspenders were attached to the joists by laying the wire on top of the joists and bent down to the bottom, then back up through a hole drilled in the joist. Routing the wire through a hole kept them from sliding off of the end of the joist, but drops of rain water would get trapped in the hole and eventually rust the wire in two. For the new bridge, instead of going back up through a hole, the wire goes all the way across the bottom and up the opposite side of the joist. A fence staple is driven into the bottom of the joist spanning the two sections of wire to prevent the loop of wire from sliding off of the end of the joist. Drive the staple in snuggly, but not tight enough to deform the wire.

My research showed that #9 wire is 0.11443 inch diameter and weighs 489.6 lbs/cubic foot, so the formula I used was:

Wire Weight = length * 0.0052

To provide handrails, and prevent anyone from slipping off the side of the deck, I used standard woven wire fencing. After the decking is completed, stretch the fencing from tower to tower and positioned between the deck and suspenders. Stretch the fencing tightly to make it a more steady hand rail. This also make it self supporting, so you don't need to consider it's weight when calculating the load on the cables.

I used #11 wire to make clips, and attached the bottom strand of the fence to the joists. I also attached the fence to the suspenders at the middle and top of the fence with nylon ty-raps. Unfortunately, the movement of the bridge cut the nylon during the first year of use, so I recomend using a bit of wire instead. When wrapping wire, always do the wrapping and twisting on the outside of the bridge so there is no intrusion into the walkway. Also tuck the ends under to protect hands from sharp edges

Steps

The standard slope for comfortable steps is 7 inches down and 11 inches out. Make the treads 12 to 14 inches deep. Besides being more comfortable for larger feet, it will decrease the "fear factor". People get nervous if they can see streight down to the ground between the steps.

To avoid ground rot of the steps, I wanted to use prefabricated steel steps. I was fortunate enough to find some steel steps in a salvage yard, but they were not long enough to reach the deck level of my bridge. I used them as a base and bolted a section of wooden steps to them to get the length I required. Since the lower half of my steps were steel, I buried them in the ground up to the first step to stabilize the steps.

If I had used wooden steps, I would have poured a small foundation for them to rest on. If you rest the wooden steps on the soil, expect to replace them in two or three years.