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Bridge building with trees and branches

 
E Cochran
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Wasn't quite sure where to put this post but this seems appropriate.

Anyway we have 5 1/2 acres of land with a creek running diagonally across it. We want to build a foot bridge so that we can access the other side without creating another driveway and point of ingress for passerbys on the road. Our land is basically the shape of Utah and the creek divides it pretty much in half. The north side has a house and barn, goat yard, chicken house, etc. It's been lived on before. The south side is completely raw as far as we know.

So the creek is deep, 12 - 15 feet (it varies some) and over 25 feet wide inside bank to inside bank where we want to put the bridge. To have supports on solid relatively flat ground we're talking 35+ feet in span.

My problem is I don't have trees that are long enough to make a complete span of the creek and buying something that long would be astronomically out of our price range. So what I want to do is engineer some design or system that allows me to use my shorter trees and still span the creek. BUT ... I've never built a bridge before. (I am an architect though so I have some knowledge of engineering.) So I'm hoping someone on here has some knowledge to share with me on the subject or can point me in the direction I need to go.

Thanks!
 
Kyrt Ryder
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What sort of time frame are we looking at here? Different options take different time to mature.

EDIT: also, what sort of weight is the bridge intended to support? If it's only for foot traffic [including goats] it would require far less strength than if it's intended to convey vehicles.
 
E Cochran
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It is only a foot bridge for people/goats/dogs/cats/chickens.

We don't need to grow anything. We have a ton of woodlands. I need to know how to put trees together to make the spans. We're not in a huge-ish rush to make it but we do want to build on it over the winter.
 
Michelle Bisson
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Are you wanting to build to withstand floods?
 
Anne Miller
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First, you should pick wood that will last the longest so you don't have to rebuild the bridge, wood like durable black locust.  Osage orange is quite decay-resistant. You can also use other woods like cypress, catalpa, cedar, chestnut, juniper, mesquite, and oak.  Douglas fir and honey locust are moderately durable.  If none of these are available use what you have.

You will need to strip the bark off. You would want to make a "L" shape notch at the end of the logs you will be joining together then use a metal straps to secure them together.  Does this make since?

Before completing the top of the bridge you would need to anchor your poles to the ground.  Maybe a post on either side of the pole in concrete.

 
E Cochran
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Well, ideally it won't wash away with the first small flood, yes. But since this creek has flooded once in 25 years, it would be prudent to design some kind of arched bridge that perhaps allows for water flow through it? I don't know. I've literally never dealt with a bridge design before.
 
Casie Becker
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Understand this is from a person with no experience in this, but what about a series of platforms that float on the creeks surface? It might even be possible to design it so that in a flood event there's a controlled break so that the whole assembly would swing to the side for easy retrieving and reinstalling. I don't know, it might be crazy, but it's still better than my first thought of a ferry.
 
E Cochran
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Anne Miller wrote:

You will need to strip the bark off. You would want to make a "L" shape notch at the end of the logs you will be joining together then use a metal straps to secure them together.  Does this make since?

Before completing the top of the bridge you would need to anchor your poles to the ground.  Maybe a post on either side of the pole in concrete.



Yes, I understand the concept. Thank you! Will this kind of joining be strong enough for a people bridge? Does it need reinforced by more structure? How big of diameter are you thinking? Would you lay the "L" notch sideways? Are you talking metal straps as in rings that can be cinched down or rigid metal bars?

I was considering putting in a concrete buttress(?) ... like a low concrete footing with anchors to attach the poles. Posts would be good too though. I'll want handrails.

 
E Cochran
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Casie Becker wrote:Understand this is from a person with no experience in this, but what about a series of platforms that float on the creeks surface? It might even be possible to design it so that in a flood event there's a controlled break so that the whole assembly would swing to the side for easy retrieving and reinstalling. I don't know, it might be crazy, but it's still better than my first thought of a ferry.


That would be great except the creek doesn't always have water in it or it is very low. I'm grinning at the idea of a ferry. Maybe I could build a flying car that operates on pullies to move back and forth ... but then I'm not sure how the goats would work with that. Lol.
 
Kyrt Ryder
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Actually, it occurs to me that at that span a rope/cable bridge is not unreasonable if the right types of sturdy trees are available on either side of the creek.

Goats and Chickens are both sure-footed enough they shouldn't care if the bridge has a little sway to it. [Emphasis on little sway of course.]
 
Michelle Bisson
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I asked my son who is in the military reserves in Canada since I knew he had some training in building temporary bridges. His response was.

"we mostly do rope bridge or cable bridge for people only"

"One other idea could be a pontoon bridge That float over his Creek"

Since I did not know what a pontoon bridge was, I went to the Wikipedia site:

https://en.wikipedia.org/wiki/Pontoon_bridge


Span between Russellville and Dardanelle, Arkansas, at the time the longest pontoon bridge in the world.[citation needed] (Photo c. 1913–1926)


Robe bridge, cable bridge or pontoon bridge, maybe one of these can work for you.








 
E Cochran
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Kyrt Ryder wrote:Actually, it occurs to me that at that span a rope/cable bridge is not unreasonable if the right types of sturdy trees are available on either side of the creek.


We do have sturdy trees on both sides so I'll look into that, too. It would probably be easier to replace if we had high waters or could even be made to be lifted during flood periods so it doesn't wash away? Any good sources for building one?
 
Kyrt Ryder
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E Cochran wrote:
Kyrt Ryder wrote:Actually, it occurs to me that at that span a rope/cable bridge is not unreasonable if the right types of sturdy trees are available on either side of the creek.


We do have sturdy trees on both sides so I'll look into that, too. It would probably be easier to replace if we had high waters or could even be made to be lifted during flood periods so it doesn't wash away? Any good sources for building one?

I'd just set the default height above expected potential flood levels. Not sure about the resources, it's not something I've ever had to build before.
 
Anne Miller
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E Cochran wrote: Yes, I understand the concept. Thank you! Will this kind of joining be strong enough for a people bridge? Does it need reinforced by more structure? How big of diameter are you thinking? Would you lay the "L" notch sideways? Are you talking metal straps as in rings that can be cinched down or rigid metal bars?

I was considering putting in a concrete buttress(?) ... like a low concrete footing with anchors to attach the poles. Posts would be good too though. I'll want handrails.


I am not sure the diameter, the larger the stronger? I would think your logs would need to be about the same sizes. I was thinking the notch would be top to bottom.  The rigid metal bars would probably be best.

I wonder if there might be some "building bridges" on You Tube?
 
E Cochran
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I just went and did a You-tube search. Will have to adapt to using local materials but I'm thinking it might be easier and longer lasting with a cable suspension bridge. I can still use my trees, just in a different capacity.
 
Anne Miller
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Miles, those are really interesting links!

Here are some permies links with some bridges others here have built.  I thought the pictures were interesting.

https://permies.com/t/60312/Homestead-development-Madhuvan-Eco-Village#512824

https://permies.com/t/57907/Curved-draw-knife-straight#490275

This one has a video:

https://permies.com/t/58332/Cob-Cottage-Company-Port-Orford

This one has some discussion on building bridges:

https://permies.com/t/54646/Small-bridge-ideas#456778

A couple of people in that thread had some good ideas:

" Rescue a telephone pole (is impregnated with interesting stuff so they don't rot) cut in half. Railway ties are great for this. flop across ditch 3' apart. Break down some rescued pallets for crossers - try to get the good hardwood onces."

"4. If you have larger streams to cross, consider building a "Pipe Bridge": Lay 2 or 3 scrap I-Beams across the creek then weld 4 inch diameter scrap gas pipe horizontally across the I-Beams. Space the pipes 1 to 2 inches apart. Pipe bridges last forever and will stand the weight of dump trucks and other heavy equipment. Pipe bridges are standard engineering in developing countries"


 
E Cochran
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Miles Flansburg wrote:
And the one I was looking for!


OHHHHHH!! Thank you so much for this. THIS is what I had in my head but I couldn't explain it or remember where I had seen it. I studied this in Non-Western Architecture class many years ago.
 
E Cochran
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Anne Miller wrote:Miles, those are really interesting links!

Here are some permies links with some bridges others here have built.  I thought the pictures were interesting.



Thank you Anne!! Those are interesting. I appreciate the links and ideas!
 
Anne Miller
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I have been trying to think of where I have seen a pontoon bridge.  I think most marinas use that concept.  Some incorporate Styrofoam to make the deck float.  But the pontoons off an old deck boat would work or metal barrels.

I have not been to Broken Bow in many years but where one of the marina was the water was 90 ft deep so they could not use pilings or posts.  The ramps that went out to the docks for the boats was made so it could adjust to the rise and fall of the water.

http://www.beaversbendmarina.com/
 
Glenn Herbert
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The curved arch Chinese bridge is a very cool design though I wonder how you would get the parts in place without being able to support them in the middle of the creek during construction.

I would consider a simple kingpost truss bridge; you could build the two truss elements on the ground, then rope them across and swing them up (somehow... that part needs some engineering). There would not be a joint in the middle of the span aside from the top of the truss.

Splicing logs for a simple span as long as you need would not be practical; they would have to be massive to be strong enough. Truss members for a 35' footbridge could be as little as 4-5" diameter at the small end; you would need members around 25-30' long to have a good height to the trusses.

I would consider the shape and condition of the banks. How high could the creek get in a big flood? You don't have to base the trusses on flat ground if you have solid foundations. How fast would it flow in a big flood? Is erosion an issue?

I have a creek with bed about 25-30' wide in most places, but it has a stony bottom and is 6" to a couple of feet deep most of the year with just a trickle in dry weather. In the biggest floods, though, it can be 8' deep, carry 80' pine trees down from upstream, and move two-ton rocks 20' downstream.

I am curious to know how you can have a creek 25' wide and 12' or more deep; the banks must be extremely steep and I would think must be rocky to maintain the slope. The potential violence of floods is a major consideration in how high and strong you need to make a bridge. Is there any chance that trees could be carried downstream in floods? That increases clearance issues.

A cable suspension bridge may be the most practical if you have strong trees close enough to the banks, or can build piers or supports to make the clear span close to the banks while anchoring to the trees.
 
Casie Becker
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I couldn't think of the correct name for it, but that's how most of the lake businesses function on Lake Travis. Since there can easily be a 30 foot difference in water depth during a year.

Poking around, there are tones of DIY instructions online for floating docks, which made narrow and laid end to end would fit my idea of a bridge. Attach ramps on pivots or hinges at each end and it would raise and lower itself with the water level. Just remember to make the ramp long enough to not be too steep at the lowest level. Your handrails supports could be attached to the side of each platform.
 
E Cochran
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Glenn Herbert wrote:The curved arch Chinese bridge is a very cool design though I wonder how you would get the parts in place without being able to support them in the middle of the creek during construction.

I am curious to know how you can have a creek 25' wide and 12' or more deep; the banks must be extremely steep and I would think must be rocky to maintain the slope. The potential violence of floods is a major consideration in how high and strong you need to make a bridge. Is there any chance that trees could be carried downstream in floods? That increases clearance issues.

A cable suspension bridge may be the most practical if you have strong trees close enough to the banks, or can build piers or supports to make the clear span close to the banks while anchoring to the trees.


A full bore Chinese arch bridge is way more complicated than we could build, for sure. But a simplified version with a few modifications might be possible. Leaning more towards the Rainbow Bridge end of things.

Our creek is deep. The north bank slopes slightly and has good vegetation coverage - reeds, grasses, vines, trees. It is also lower than the south bank. The south bank is just plain old steep. Huge tree roots and heavy clay soil are the only thing keeping it in place to my knowledge. We've found exactly one rock on the whole place. That said ... this creek doesn't run year round. When we bought the place in August the creek had about 18 inches of water in it. By the end of September it was dry. We had a couple of 1 inch rains one week and ended up with about 8 inches in the creek, which due to lack of moisture since, is now dry again. One neighbor said he's seen it out of it's banks once in 25 years. But the channel that we have to bridge is 12 to 15' deep along the whole length.

As much as I'd like to build with only our trees and branches, etc ... a cable bridge will probably be more practical for our needs. But I'm appreciating all the feedback and ideas.
 
E Cochran
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Casie Becker wrote:I couldn't think of the correct name for it, but that's how most of the lake businesses function on Lake Travis. Since there can easily be a 30 foot difference in water depth during a year.

Poking around, there are tones of DIY instructions online for floating docks, which made narrow and laid end to end would fit my idea of a bridge. Attach ramps on pivots or hinges at each end and it would raise and lower itself with the water level. Just remember to make the ramp long enough to not be too steep at the lowest level. Your handrails supports could be attached to the side of each platform.


Pontoon bridges, floating bridges, won't work for us because we don't have a continuous flow of water or a stable depth of water. Apparently there is water in it about 10 months of the year and then it's mostly dry the other two months. We just bought it in August so we aren't completely familiar with it's ebb and flow yet.
 
Michelle Bisson
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Since it is so soon since you purchased your property, you'll want to observe it very closely for a period of time to see how it reacts over time and weather events. 

I thought that your creek had deep flowing water year round. 

How much energy & resources you put into building a bridge will be based upon what you want to do on the other side of your land.

 
E Cochran
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True. Observation is essential for understanding of the environment.

We plan to build our home on the other side of the creek since it isn't in the floodplain. The north side is almost entirely in the flood plain. The south side is not ... but the north side has the driveway already in place and we don't want another access point on our property for cars. We're building a parking area close inside the gate and plan to use foot power or a small gator to traverse otherwise.
 
Michelle Bisson
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ok,  we are slowly getting the picture.

Having a foot bridge to cross the creek because your future house will be on the other side since it is on higher ground means that whatever you build it has to be sturdy and can withstand flood events. Otherwise, you could be stranded in a flood event that washes out your bridge and have no way to cross.  I assume that you could still walk to the road on some pathway in the event of a washout of the bridge.

Keep us posted once you start building as we are now curious about this special project.

We once looked at some property that you had to cross a private bridge and realized that we did not want our chalet on the other side.
 
Glenn Herbert
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Hmm... I don't know if this is important to you, but if your house is sited in a location where fire trucks or other emergency vehicles cannot get close to it, you might have difficulty getting insurance, or it might be considerably more expensive. You would not be getting concrete delivered for sure

On the up side, since there is not deep water all year, building the bridge is no more complex than reaching fairly high from the middle of the bed when it is dry. I was contemplating the idea of rafts to support lifting gear in the middle of the creek...

Is the 12-15' depth from the banks, or what is the maximum depth from the north and south banks/plains to the bottom?
 
E Cochran
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Both good points to consider when building on the other side of the creek ... access during a flood and emergency services. Not so worried about getting a concrete truck in as we are building ourselves with earthbags/strawbale/cordwood/whole trees. The goal is have it as fire/flood/wind/earthquake proof as is possible. Where we're building is out of the flood plain. Since most of our property is in a 100 year flood plain which requires permits, etc ... we've chosen to build up on a hill that is out of the flood plain and not subject to permits. My husband is now considering putting a drive along the fence line to access that house.

But we still want a bridge ...

The north side bank plain is 11' - 12' above creek bottom. That's the best measurement we have right now. The south side is a bit higher at the bridge site. So it's all totally doable with a bit of support to build from the center I think. If we go the route of the Rainbow bridge, the structure doesn't require bridge abutments contrary to what my mind wants to think. And I think we can build the long spans, put them in place with a series of pullies, and add the cross bits. However, for ease of time, we may just go the route of a cable bridge. So much to do ... so little time.
 
Michelle Bisson
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Do take lots of pictures and share them with us.
 
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