Sunday, February 8, 2009

Building a PCEI foot bridge, Part I

This has nothing to do with the PCEI Artist Studio, but hey.

Last Fall, Tom Lamar asked me if I would be interested in building a small bridge across a wetland channel here at the PCEI Nature Center. I told him that I would love to do it, but that I would keep it very simple and rustic. My plan was to mill a couple of stringer logs flat on the top and bottom and span the distance across the small stream. After that, I'd apply a 2x6 Western Red Cedar decking across the stringers.

In late Fall, Jacob Dolence (a PCEI Americorp Volunteer) and I milled a Western Larch and Grand Fir log in this way and we brought them to PCEI. Unfortunately, weather (snow and frozen ground) prevented us from moving the stringer logs into place across the span. However, before the snows came, I did excavate large "foundation" holes on either side of the waterway and fill them with river rock. The stringers will sit on these piles of rock, which in theory will help drain water away from the base of the bridge.

Above are the two milled stringer logs, waiting to be moved into place. The waterway is behind them in this picture. The logs are sitting on Western Red Cedar "footings" that I also milled flat on two sides. These 4" thick rot-resistant Cedar footings will have direct contact with the ground so that the logs don't have to.

The Larch stringer on the left is 9.5" thick and the Grand Fir stringer on the right is 10.5" thick. I had meant to make them the same thickness at the mill, but that didn't work out, so I will have to reduce the thicker log at the ends to make the bridge level.

The first order of business was to move the stringer logs into position across the span. Since I was working solo, I used my Jeep's winch and a snatch block and the base of a nearby tree to drag the logs into place. Fortunately the ground was frozen in the early morning, so I was able to do all of this driving and dragging with very little impact.

After the stringer logs were in place, I raised the ends up using a large high-lift jack and put them on temporary supports made of timber framing scraps and 4x4s that I found in one of the PCEI toolsheds.

With the stringers in place and raised, I worked on leveling the cedar footings on each end of the span. The bridge needs to be longitudinally level but also level across its width.

I then reduced the ends of the thicker Grand Fir to 9.5" by using a circular saw, a chisel, and a mallet:

In order to prevent the logs from shifting and slipping off of the Cedar footings, I decided to peg the stringers to the footings. Here, you can see me using an auger to make a shallow peg hole in what will be the underside of the log. I purposely made the peg hole extra wide so that the 1" Locust peg would have some flexibility and make assembly a lot easier, since the log will have to contact the Cedar footer at exactly the right place.

During all of the activities, Mama Cat would come by and check out the progress:

Here you can see the shallow peg holes on what will be the underside of the stringers:

I drilled peg holes in the Cedar footing and put the 1" Locust pegs in:

I then flipped the stringers over, removed the temporary supports and teased the pegs holes on the underside of the stringers into position on top of the locust pegs.

Here are various pictures of the bridge so far:

The next (and final) step is to put 5' wide Cedar decking onto the bridge. That should go quickly and look fantastic.

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