cable suspended bridge

[mattp]

I'm working on a treehouse project where we plan to build a cabin 80 feet up a tree that is in a ravine, so that access will be via a cable-suspended bridge from the bank on the edge of the ravine. Right now I have ropes in the tree at about 100 feet and at the 80 foot level, and I have set a tyrolean traverse.

 

I've built ropes courses in the past, and who among us hasn't built a treehouse, but our bridge span will be about 75 feet and I have no experience with something of this magnitude. Anybody here? I'd love to solicit free or even maybe paid advice.

[DPS]

My wife is a bridge engineer (she built the new red bridge over the NF Snoqualmie on the way to Little/Mt Si). She said suspension bridges are a whole different ball of wax. Good luck.

[JasonG]

These guys were probably the best around, but unfortunately closed their doors after the death of their founder....http://www.sahale.com/

 

Some of the staff have continued on as these guys: http://seattlebridgebuilders.com/

 

Certainly not free, but they would give you the straight story and are very helpful. Sounds like a cool project!!

[mattp]

Thanks, Jason. I've looked at that site and they have some of the most helpful information I've found in Google searching. DPS: our bridge won't have to carry cars. I think we can stretch some cable across the void and it'll "work" but we may have to guy it out to trees or other anchor points off to the side of mid-span in order to make it comfortable.

 

My initial research indicates that we won't have a problem with loading, but our challenges will have more to do with the fact that trees move around in the wind and we don't want to kill the trees that we intend to use for anchors at either end.

 

And the logistics and design of this thing are quite a challenge. I've hiked over a lot of suspension bridges and 75 feet is medium small in comparison to bridges you encounter on lots of popular hiking trails but it is still pretty big for us.

[genepires]

In the movies, heros always fall through the floor slats and the vines that span the canyons always fray and snap when you get to within 10 ft of the other side. Good luck figuring out how to make it do that.

[111]

whatever you end up with, you might consider running 3 guy lines to the ground from where the bridge touches the tree. It will keep the tree from swaying as much in the wind and thus straining the bridge connections.

[mattp]

111: Do you have ideas about how we might "secure" the tree without damaging it? The treehouse people are all pretty consistent about wanting to let the tree be a tree and I'm not sure swaying in the wind is necessarily an essential part of that but having anchor points on the tree where we pull in three directions to anchor it might be pretty harsh as compared to having a suspended bridge that is not stretched too tight.

 

I have not scoped it out completely but I think that at 80 feet we are not half way up the target tree and it is in a thick forest and while I could feel it moving on a day with relatively little wind when I was last up there I don't think it moves a whole lot at that level.

[Rad]

But Matt, the suspension bridges you encounter hiking usually are anchored in concrete pilings in the ground, not in trees. Maybe some rain forest canopy camps can provide you with some models.

[sobo]

Let me get this straight, matt... are you considering the tree in the ravine (the one with the planned treehouse in it) is going to be one of your two backstays? Or is it to be one of two towers? Does this bridge completely span the ravine and the treehouse is a "dropping off" point? I'm having a difficult time imagining this bridge in my mind from what you've described. If you're planning on using the "treehouse tree" itself for a backstay/anchor and the terminus of the span, I think you'll find that you're in for some serious stability issues in the long run.

[mattp]

Yup, Sobo, we are planning to use the treehouse tree itself as a "backstay." The bridge we envision is not, according to the Seattle Bridge website, a "suspension bridge." We envision a "negative camber suspended bridge" or something like that. The difference is that we will not have two bridge towers and "backstay" anchors. We are looking at simply building a glorified version of a tyrolean traverse - something that is commonly called a "Burma bridge" in ropes course discussion.

 

The target tree is pretty big. It is a 100+ year old cedar and, while there have been some sluffs nearby, we figure that it is good for another 100 years,. We are more worried about the stability of the proposed anchor trees on the bank.

 

If we were to try to build a bridge over the ravine our span would be more like 300 feet.

[mattp]

Right you are, Rad. We do not think it necessary to build concrete towers and we are envisioning something a lot less involved than a true suspension bridge. But the movement and growth associated with trees presents a significant set of challenges.

 

As I said: I've build ropes courses before. Those projects were located lower down in the trees where there was even less movement and, in some cases, we only set them up for temporary use. Based on that experience, I'm thinking that we can use technology more like that for the tree-canopy zip line experience than an automobile bridge over a river.

 

One where I once facilitated a group challenge event was in a clearing where the modern Little Si climbing area access trail for the Woods and British Aisles forks off the Little Si trail. A friend of mine had a "ring jump" where the participants would climb about 30 or 40 feet up a tree and then they'd be asked to jump and grab a ring that was suspended in space about 6 feet away. They had a full body harness and a belay from a cable stretched above, but it was a very exciting event.

 

The combination of a tree-house and a bridge to the ground presents some unique challenges. It'll be pretty cool if we can build it, though.

[Rad]

I look forward to seeing it and hope that keeping the tree healthy is your top priority.

[111]

111: having anchor points on the tree where we pull in three directions to anchor it might be pretty harsh as compared to having a suspended bridge that is not stretched too tight.

 

 

good point, I guess as long as you take movement into account with designing it...

 

[mattp]

For sure, Rad. Our motivation for this project stems from our excitement about these magnificent trees that escaped logging when the area was cut 100 years ago (this one may not have been big enough to bother with). We hope to be able to take people who otherwise would not climb trees to visit the woods, not at canopy height, perhaps, but not from the ground. The bridge will travel past a big leafed maple that is moss clad and has flowers growing in it and the destination level in our primary tree feels like you are up high in the forest.

[Whatcomboy]

Take a trip to Vancouver to Capilano Park and check out the bridges that span big douglas fir trees. It's a fun place if a bit touristy. Probably cost a lot of coin to build. Sounds like fun.

http://www.capbridge.com/treetops.php

[mattp]

That Capilano bridge looks AWESOME!

[AlpineK]

whatever you end up with, you might consider running 3 guy lines to the ground from where the bridge touches the tree. It will keep the tree from swaying as much in the wind and thus straining the bridge connections.

 

Trees flex to compensate for forces like wind. The entire tree needs to move with the wind. If you keep the tree from swaying to an arbitrary point, risks of failure above your arbitrary point will rise. Placing a tree house in the target zone for a failure and increasing risks isn't the best idea.

[num1mc]

I would think that the engineering challenges and legal responsibilities of building a bridge, suspension or otherwise, 100' long and 80' high for use by humans requires the assistance of a professional.

 

I don't believe the OP would suggest that individuals get the answers to difficult legal questions with severe ramifications off a board like CC. Likewise, failure of this bridge would have huge consequences.

 

There are big differences between building a ropes course and a suspension bridge. Ropes courses are of short duration, and generally offer a belay or other secondary safety device.

 

The scale of this project is what makes everything so daunting

[max]

The military produced a boatload of books on field engineering. They outline the basic dimensions and methods for using basic materials and construction techniques to build no-frills things like bridges, spar pole derricks, bridge abutments, foundations, etc. It is the military, and I assume there is some "quick-and-dirty" involved, but you might check it out (particularly the spar pole idea....)

 

Good luck!

[sobo]

Yup, Sobo, we are planning to use the treehouse tree itself as a "backstay." The bridge we envision is not, according to the Seattle Bridge website, a "suspension bridge." We envision a "negative camber suspended bridge" or something like that. The difference is that we will not have two bridge towers and "backstay" anchors. We are looking at simply building a glorified version of a tyrolean traverse - something that is commonly called a "Burma bridge" in ropes course discussion.

 

The target tree is pretty big. It is a 100+ year old cedar and, while there have been some sluffs nearby, we figure that it is good for another 100 years,. We are more worried about the stability of the proposed anchor trees on the bank.

 

If we were to try to build a bridge over the ravine our span would be more like 300 feet.

AH! Now I get it. I had the definition of a true suspension bridge in my head, and let my civil engineering career get in the way of my rational mind. I was thinking you were planning to build a true suspension bridge (like Tacoma Narrows, Golden Gate, Brooklyn, Verrazano, etc.), where the deck is suspended from above by suspender cables hung from the main (near-parabolic) cables, which is a very different structure than what you are planning, which is a simple suspension bridge, also known as a rope bridge, where the deck is supported by the main cables themselves and form a catenary curve. With the curve sagging, the result is a nagative camber, which is why Seattle Bridge is calling your bridge concept a negative camber catenary suspended bridge.

[G-spotter]

I would think that the engineering challenges and legal responsibilities of building a bridge, suspension or otherwise, 100' long and 80' high for use by humans requires the assistance of a professional.

 

I don't believe the OP would suggest that individuals get the answers to difficult legal questions with severe ramifications off a board like CC. Likewise, failure of this bridge would have huge consequences.

 

There are big differences between building a ropes course and a suspension bridge. Ropes courses are of short duration, and generally offer a belay or other secondary safety device.

 

The scale of this project is what makes everything so daunting

 

+1

 

Tacoma Narrows 2.0

[sobo]

Matt-

Camp Prime Time near White Pass is a charitable organization benefitting terminally ill children that I have been involved with over the years, donating sweat and civil engineering talent. They recently completed an ADA-compliant treehouse project. The treehouse is not anchored to the trees, but "floats" on supports that allows the tree to continue growing without restriction. I don't remember the name of the treehouse consultant that donated his time to the design, but I could easily find that out for you if you're interested. It was a USFS requirement that the tree not be damaged or killed (Camp PT is on USFS property).

 

Scroll to the bottom of the linked page for construction photos.

[Stefan]

you may have already seen this site.

this guy has paid work parties.

 

It is literally a hotel made of treehouses in Cave Junction, Oregon. I encourage you to visit...I have been there and it is WAY COOL, and they have done exactly what you are asking about, but between trees...with cables. The cables walkways are somewhat slack and move with the trees as the trees sway.

 

http://www.treehouses.com/

 

 

[Stefan]

look at the cable suspension on this one:

http://www.treehouses.com/treehouse/treesort/forestreeint.htm

 

Edited September 20, 2011 by Stefan

[num1mc]

Matt,

 

After visiting www.treehouses.com , I suggest that you either hire a PE or SE to design your bridge, or devise another means of access. You are incapable of designing this structure yourself.

 

In 2008, a suspension bridge at the Treehouse Hotel failed, pitching a family from Kirkland off and to the ground. Because the owner is a scum sucking asshole who designed the bridge himself, has no permits or building inspections, Josephine County ended up party to the suit because they allowed continued use of these non-inspected structures. So now the tax payers need to pony up $1.2 million.

 

I don't know what fiduciary responsibility Mr Treehouse was held to. But now Josephine County is shutting him down, and he is aghast at the county for doing so. Which is reasonable, because his fuck ups have only cost the county $1.2 million, he should be allowed to continue until he costs them another $1.2 M.

 

This shows how serious engineering and construction mistakes can be. And if this family had been 80' in the air, instead of at a more modest height, they would all be dead

 

you may have already seen this site.

this guy has paid work parties.

 

It is literally a hotel made of treehouses in Cave Junction, Oregon. I encourage you to visit...I have been there and it is WAY COOL, and they have done exactly what you are asking about, but between trees...with cables. The cables walkways are somewhat slack and move with the trees as the trees sway.

 

http://www.treehouses.com/