Introduction: A Cantilevered Foot Bridge Out of Logs - With a Hut!
I have the best job in the world. Sometimes I get to work with a dedicated citizen and a group of willing volunteers to make something lasting and beautiful. (And useful!) Come and see this bridge in Petersburg, Alaska!
I acquired a grant to put in a bridge over a stream that was between a park and a mile long trail. The plan had been to install a glue-lam or log bridge across a 50 foot wide part of the stream. Building a log bridge is actually pretty easy if you have big timber to work with. But a citizen approached me and asked if he could design and guide the construction of a cantilever bridge with a small hut on it. Shortly afterward a Boy Scout was looking for an Eagle Scout project... and here is the process.
Step 1: Design
Working with local Forest Service engineers the designer took his vision for the project and ensured it was matched up with the right diameter and species of log. The engineer did calculations and accounted for humans and heavy snow weight to ensure that the volume of concrete and fill used for the cantilever would be adequate.
We were able to use a significant rock to perch the hut on and as the fulcrum point for the long cantilever. However, after calculations regarding the volume of water and high tide we had to raise and enlarge this point.
This bridge consists of two cantilevered wings meeting but not touching, in midstream. The concept is of a glorified log crossing. This bridge is not handicap accessible; in fact it is intended to bounce a bit, just like a log, before one crosses to the other side. The moss covered Troll Booth along the way can seat four people.
My vision is to encourage the pedestrian to pay attention and to enjoy a playful transition from one side of the creek to another. the cozy Troll Booth will make one want to pause, listen and watch. The view of the narrowing walkway ahead, and of the wings not touching, and the slight bounce of the bridge will alert ones' senses. As one steps onto the other wing, the crux is passed. Making the turn and reaching the stability of the other bank will complete the transition across the gateway.
Some people may not want to cross the bridge, because it requires paying attention, just like the trail ahead. And that's ok. There is a little here for everyone; it's more than just a bridge. Going part way and just sitting in the Troll Booth offers its' own rewards. As in life, it's a rough trail ahead, and we must pay attention as we cross each bridge along the way.
Step 2: Center Foundation
The hut will be located above the large rock in the stream. But we wanted to make the rock really suit our design. A form was built around the rock and upstream. Bolts were positioned for straps to attach to the logs. Rebar was tied throughout and a few holes were drilled into the rock with a carbide masonry bit and a Hilty hammer drill.
A ramp was made with 4x4 posts and 2 foot wide strips of plywood. We then packed the concrete into the site with wheelbarrows. Prodding into the concrete often and tapping the form sides to get a good pour.
Then we removed the form after ample time to cure.
Step 3: Foundation Slabs
Forms were made to pour slabs that could be carried to the site. These slabs ensured that the bridge abutment was on solid ground, had adequate rise, and gave space for gravel fill. A specific bolt pattern was devised to connect the slabs using flat stock steel. The steel pieces were created after the slabs were dry and in place allowing for a little bit of variance.
Bolts were set in the forms along with rebar and all of it tied together with wire. Forms were tapped with a hammer and prodded well with spare rebar to ensure minimal air pockets.
Step 4: Assemble the Foundation
Using a small excavator we cut down to the specified depth and installed the foundation pieces on each side of the stream.
Moving large slabs of concrete is easier with the right equipment! Keeping things level at the beginning makes things much easier in the end. Look at all of the photos and you'll see that the log pair that is supported in the center has only three slabs composing the foundation. The unsupported cantilever has a much more complex design. One cross piece forms a toe of the abutment. On top of that, the two side plates sit. And on top of them are the two points for the logs to attach to as well as a plate to help make sure adequate fill supports the cantilever. All but the rear plate are strapped and bolted together with steel stock.
Step 5: Bring the Logs
At one point our plan was to use galvanized steel I beams. Then we considered telephone poles. Finally we settled on locally harvested yellow cedar. We picked them out of a sort yard and had them delivered.
Originally we planned to carry the logs to the site with lots of people. Then we planned to roll them in on plywood and pipes. Finally we decided that an all-terrain forklift would be best. The extendable boom, side to side tilt, and heavy load capability made it much safer and easier to install. Rent was just about $100 an hour for the unit and each log was placed within a few inches of where it needed to be.
At the spot where the logs would meet (but not connect) we assembled a stout crib of lumber. The logs would need to rest on this until they were bolted down and the back fill placed.
The smaller logs for the shorter (but not supported in the center) cantilever were brought close with the forklift and then placed by volunteers. Then all logs were marked for notches, rolled over, and cut with a chainsaw.
Step 6: Notch for the Straps
Notches were cut with a chainsaw and chisels to connect the foundation to the logs. Steel angle iron was used to bolt the logs to the foundation.
Aluminum flashing was used to protect the top side of the logs from weathering.
Step 7: Decking
Treated decking was selected and specifically spaced wider than on a home deck. The intent was that pine cones and other debris would fall through easier.
Deck boards were placed abutting each other and longer where the hut was placed so a dropped lip balm wouldn't end up in the stream.
Angle cuts the length of the deck boards were made at the ends of the cantilevers so that each board had solid contact with the logs.
Each board was installed with two lag bolts and washers. The deck was countersunk and then impact drills were used to drive the lags.
Afterward all edging was routed with a rounded bit to smooth edges.
Step 8: Fill
Using manpower and wheelbarrows gravel was packed to the site and fill was dumped and compacted. Sills were made with leftover deck boards to help keep the gravel where it needed to be and to make steps to get up and down at the bridge abutments.
Step 9: Install Hut
This design incorporates a "sod" roof of locally harvested mosses as well as integrated benches. The entire structure except the plywood on the roof is constructed of red cedar. Most of the pieces were fabricated in a wood shop and then carried out and installed.
A waterproof membrane was attached to plywood and then old fishing net was draped over the roof to provide the moss with a stable foundation.
The bridge has considerable spring to it - it has a few inches of travel just walking on it normally. Bouncing on it will flex it (the section that isn't supported in the center) about 8 or 9 inches.
Total project cost: just over $10,000 not including donations and a few hundred hours of volunteer labor.
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