Our Suspension Bridges

History - page 3

I spent the summer and part of the fall of 1997 doing research on the net and playing with what information I could gather about suspension bridge design. I wrote and re-wrote several versions of programs to help with the design of suspension bridges, until one day I had one that seemed to work. If you are interested in what the program actually did and how, don't forget to see the design considerations and math pages.

First we had to find a new place to construct the bridge. The flood had washed out the banks at the old location, making it too close to the road to allow for a good anchor system. We picked a new location and did our survey of the spot using a piece of tubing taped to a carpenter's level for an improvised transit, a 1000 foot ball of string and tape measure combined with a little trig. to be sure we got the angles right. We tied colored tape to tobacco sticks for survey stakes, sometimes lashing two sticks together to get enough height to be seen. High tech. it was not. Cheap but reasonably accurate was what we wanted. During our survey we noted the terrain elevations so that we could be sure that this new bridge would be at least three feet higher than the high water mark of the flood. We measured every thing twice, then traded jobs and measured twice again with fresh eyes.

Crude as our tools were, the extra patience and double-checking paid off. After the construction was complete we estimate we were off in a distance of 80 feet by less than an inch horizontally and less than 3/4 inch vertically. A thousand foot of string and some trig. can make up for a lot of equipment inadequacies.

The foundations for the towers and the anchors were poured concrete, approximately 3 foot by 3 foot by 8 foot. In addition to the reinforcing bars in the concrete, the anchors contained two old turnbuckles affixed to the re-bar with an eye sticking out of the concrete as a cable attachment point. The foundations had two re-bars sticking out the top about 6 inches. The tower poles were drilled to stand over the re-bar pins.

In this picture you can see the trenches we dug to the anchor for the cables. Behind me, you can see the foundation for the east tower.

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