A Cantilevered Foot Bridge Out of Logs - With a Hut!
Intro: 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.
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.
Designer's statement:
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.
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.
Designer's statement:
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.
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.
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.
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.
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.
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.
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.
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.
52 Comments
OhYeahThatGuy81 8 years ago
Wow, that is really amazing!
Congratulations on your success and many to come!
Two Paddles Design 8 years ago
Thanks for sharing, this is well done.
tim_n 14 years ago
clazman 12 years ago
Cantilevers are very interesting, i.e. Husky stadium at the UW in Seattle. It was designed as a cantilever system. To the average person on the street it appears as impossible.
zieak 14 years ago
Ferguson199 13 years ago
vincent7520 13 years ago
o.neill.k 13 years ago
critterfluffy 14 years ago
die_dunkelheit 14 years ago
-chase- 14 years ago
Looks Great Zieak... may i ask
the Bridge designers name? Might as well give him some online credit... ;0)
Did you have a part in the design as well?
How long from start to finish?
Where exactly is this bridge?
After thought - Can one see it on google satilite maps i wonder?
Best time of year to hike the trail to see your Bridge project or should one say "Our" new bridge... ;0)
Again hats off to you and all involved.
zieak 14 years ago
I met with him a few times during the process but only in a supportive role.
The bridge is located here. But online satellite maps are not good enough resolution for my area to see the bridge or to zoom in close enough to even see the stream.
The bridge is only 200 feet from the parking area inside Sandy Beach Park so any time of the year is good!
-chase- 14 years ago
And andother wow - i thought this was in Cali or Washington state somewhere... that's not too far away... Arkansas is great state with a lot of good parks and scenery. I traveled through there and met really nice people along the way.
If you don't mind me asking, What is the name of the Park it is located at? I didn't see it above. I dable in 4x5 photography and am always making note of locations that are interesting. This bridge adds rather than takes away as many structures do and i'm sure the rest of the trail does as well.
How long is the trail that this is on? I mean is it a Day Trail or overnighter - 2-3 days to complete? Or a link to find out more about it?
I only ask about the Designers name because i thought i might see if there was any other cool stuff he designed online. He seems to take in the seemingly lost art of designing with nature instead of against it - if you follow my meaning.
thanks for anwsering my post - and again hats off to all of you involved in this project. I'll take agander at your site this evening - i noticed your link. Seems your involved in some interesting things as you mention in your instructable. Liking what you do always helps in the outcome. ;0) Seems you have found a passion for this sort of thing. Congrats.
-chase-
zieak 14 years ago
Just kidding - people make that mistake with AK all the time.
It is located in Sandy Beach Park. My father is a photographer and took this one of an old cabin chimney that's about 150 feet further along the trail.
The path is one mile long. It isn't developed at all though - just a swath cut through the forest along the beach. But A $26,000 grant will upgrade that path this coming summer.
Dieter hasn't done any other bridges like this. His work primarily involves residential and commercial buildings. If you look up his name online you can read about his mountaineering experiences in the are though!
-chase- 14 years ago
Your Dads pic is real nice -
though i wish he would post it just a tad bigger... i have pretty high rez on my laptop and the wide white border tends to over power the image imo) Though i understand reasons for keeping it small on the web.
- just looked again at it, others and around the site -
The whole site is your dads... and he shoots 4x5 and 5x7 - that's so cool.
The photo your dad took does show that the area has some real character. And with your bridge project adding to the ambiance... I definitlely need to get up that way again. Fartherst i've been is Big Sur in Cali. at this point on the west coast - Montreal, Ca on the east coast.
I like your Ball Field Pond pic on your site - that's a lot of shoveling from what it looks like... glad it wasn't me. ha... ;0)
I'll definitly have to plan for another trip your way - there is much i want to see on the west coast again or for the first time.
thanks for taking the time to answer my questions and the chat.
And thanks for all your hard work on the trail and to all involved.
-chase-
zieak 14 years ago
-chase- 14 years ago
Take care and stay warm!
-chase-
T3ch0b0y 14 years ago
Congratulations on such a great project. I'm jealous.
-Mike
rrm 14 years ago
zieak 14 years ago
Before this bridge, a person with any mobility impairment, loss of sight, or any other of many limitations would find this route (it is 200 feet from a parking lot) completely inaccessible.
My first job in my degree focus after college was providing recreational services for people with disabilities. We took them out cross country and downhill skiing. Ever loaded someone with severe MS into a chairlift? I'm just saying to let you know I'm very well aware of the struggles of people with disabilities. But I'm just as well aware that if there is a will, there is a way. And that this is not a topic I take lightly.