Mt. Pisgah Wooden Bridge
I hike quite a bit on a mountain called Mt. Pisgah. Mt Pisgah is a non-profit organization that maintains over 275 acres with over 16 miles of trails on a shoestring, donated budget.
At a fund raising event I was talking with the manager and offered to volunteer to help on some limited project. Brad said there was an upcoming bridge project and I anticipated a 20 ft. bridge over one of the small streams. "I think I can do that," I responded.
Well, here is the bridge project, all 86ft. of it. I worked almost continuously six to seven days per week for six weeks with one to three volunteers on building the bridge. It was built on a grassy slope (somewhat short of ideal conditions).
How do you build a bridge? Well, you find a long place, hopefully one something flat like concrete. Oh, well grassy slope will have to do. We started with some of the nicest donated fir lumber. The logs were milled at a steam driven mill in Philomath, Oregon. I missed this part, unfortunately.
When I arrived on the scene there were a set of engineer/architect plans and a pile of lumber and that was it. Having a construction background I pitched in and we started sorting the lumber.
Side one is done here ready to start stacking on side two. We started by laying out the bottom strait and level and worked from there propping up pieces to compensate for the slope. The top pieces had a slight radius cut on them to give it the trestle effect. Here you can see the pieces as they were attached and the reinforcing blocks at the joints.
Wonder how those holes were drilled so uniformly. As you can see, I am a fan of jigs. You can usually make them quickly for repeatable events and produce professional results.
One of the challenges of the Pisgah project was drilling sets of four holes at the joints. There were from one to four 3/4" bolt holes that had to be drilled through five layers of 4x material (18") using a 13/16 bridge auger. The proposal was to drill them individually but I knew that with my drilling the likelihood of them coming out straight was pretty unlikely. I proposed making a jig. My fellow workers thought it was a waste of time and the idea was met with some resistance. It was Friday afternoon and no work had begun so I decided to build the jigs on my own that weekend.
The Jig is made of three layers of 3/4" plywood laminated together. You can see four 1" x 6" black pipe nipples sticking up. These are anchored from below to inset pipe flanges that bolted from the back side. This meant that I had to re-tap the tapered threads from the back side of the flange so that the pipe nipples could stick up through the jig. As a bearing I found that 3/4" PVC pipe would just drive tightly into the 1" black nipple. This meant the drill bit was guided by the close fitting plastic. (see picture where I am drilling the first hole).
The nipple shortened up the depth the auger went in to about 12" deep. We could drill all the holes, remove the jig and bore through the rest of the way. With 12" of straight hole the rest went right through. It amazed me how accurate the holes came out. Some of the joints were only 10 1/2" thick and we could bore through completely using the jig.
In addition I made a 1/2", 4 bolt jig for the upper sections of the bridge.
You can see in this picture how the second side is built on top of the first side. All holes were drilled and all pieces had to be pinned in place and then labelled before they were loaded and sent off to be pressure treated.click here
I thought I might provide some tools and techniques I picked up in building the Pisgah Bridge. When you bolt beams together the bolt doesn't give you much contact with the material. For the Pisgah Bridge we used rings to get better contact. The pictures above show how the rings are cut (Milwaukee Hole Hawg and special bit). You can see how they are matched both sides. This is 1920's technology, by the way.
The bit was specially made costing over $900. Fortunately we borrowed it. I sharpened the drill bit three times. It had the oddest configuration of cutting tips. Even more fortunate it had directions and a setting block with it which made it easy to reinstall the sharpened blade.
In operation you really had to watch it because it had a lot of torque. The end of the bit was a guide that kept it straight in the previously drilled 3/4" bolt hole. There were an incredible amount of rings to drill. And later we found out when the lumber returned from pressure treating we had to ream them out again.
Rob was one of the more skilled volunteers. I was always glad to see him show up. Here he is shown below drilling the holes.click here
After the pressure treating, the pieces were delivered at a more remote site to be assembled. When we pressure treated the wood it swelled and we had to go back and ream all of the ring holes. We were away from power here and used a generator.click here
In the assembly picture you can see in a more detailed way how the joints went together with the rings installed. The joint pictured had eight rings with sixteen grooves cut into the matching pieces. My welding clamps came in handy here. Pressure treating caused a bit of twisting and slight changes which made it a bit harder to assemble.click here
The pressure treating took longer than we thought because it had to be a specially run batch and the wood had to be then kiln dried to prevent the chemicals from leaching into the pond. Back from pressure treating the bridge was assembled adjacent to the pond it was to span. This was a more remote area with a steeper slope. Notice the color change with the pressure treating. I am in the background.
When we sent the pieces off to be pressure treated they came back randomized. Most of the beams had to be carried in over two hundred feet to be assembled.click here
Here we are attaching the shoes (3/8" steel) to the ends of the bridge. Oregon winter was setting in and we worked frantically to finish this part. Fortunately, a friend of mine returned from his months in Europe to help out (on top). I drew the bottom position here attaching the shoes on the ends of the bridge trying to get the holes to line up.
Here we are out in the rough. Notice the cribbing supporting the stacked sides. The other end was flat with the ground. We periodically had to level the whole thing as the cribbing was slowly sinking in the mud of the encroaching Oregon winter rains. Notice also our numbering system. Fortunately we had a good marking system that we etched into each piece with a small Dremel tool.
Our next job to start this summer (June) is to have a crane lift the two sides, stabilize it and attach the cross bracing and decking. We are currently working out details on this.click here
My most recent contribution was two weeks ago when I built this jig to mount angle brackets to the decking. We decided to use angle brackets on the underside to attach the decking rather than bolting down through to minimize water penetration and dry rot.