As with most of my “operations”, there is generally a higher plan if you will. If you recall, the summer project was focused on “parkifying” the area around a stream in our back lot. There was actually another follow on effort to establish a means to get myself and ATV across the stream so I can enjoy the other 7 acres of my lot. Focusing on the brush cleanup around the stream allowed me to take a step back and find the best possible placement to construct a bridge. With the brush and thorns cleared out I could judge the flood lines and pick the highest banks. All of this prework put me in a good position to start the fall project of constructing the bridge.
From a requirements perspective, it needed to span the river without any permanent supports in the actual stream due to not wanting to deal with any DNR permits or restrictions. If you are curious, you may own the property, but they own the rights to the waterways and any flow changes or obstructions need to be cleared through them. Secondly, it needed to be wide enough and strong enough to support my ATV. That is my workhorse and I already have a few trails over there that I currently enjoy those few times of the year that are dry enough to drive through the stream bed. With these few requirements, I set about planning the design. The sticking point was what construction material to use. A few options included a semi-truck bed, steel construction beams, cables, concrete and lastly lumber. The first two options were ruled out after unsuccessfully trying to figure out how to get those materials down to the stream and the steel beams added some significant cash to the effort. The cable option would probably be the cheapest, but figured the ATV traversal would be a little hair raising. The concrete option looked intriguing. There would be plenty of water for mixing, but the inability to get a concrete truck down there would mean hauling a staggering number of concrete bags. With those options crossed off, I was left with the treated lumber option.
With that decision out of the way, the next task was to set the design. Fortunately, my oldest brother (Dan) is well versed in the engineering design field and spent a long time helping me figure out the best approach to spanning the stream. By far, the trickiest part was compensating for no mid-stream supports. This means the structure has to be able to support a hefty downward force and distribute that weight out to the bank supports. With arcs being a tad difficult construct out of lumber, we went with the next best geometric shape, the triangle. I will take a short cut here and simply state we made a LOT of design changes before we were done and actually still considering adding some additional support. Oh, and credit should also be given to the John’s Hopkins Online Bridge Designer website. Although this site does not give rotational forces, it does a very good job of showing how the downward forces will distribute over the triangle legs. Thanks to this program we actually decided to double the amount of triangles and actually removed the upward triangle legs at the post ends.
Next up.. the prep work – hit the jump to check out some pictures of project