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  1. #1
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    Do bridges have to be level?

    I'm curious about building a bridge for a proposed section at our Salisbury project in Pennsylvania. This section will run through a pretty cool area that is sloped and has some elevation gain on either side of the creek we need to cross.

    Anyhow, we want to run a bridge across this creek. One side has some great big rocks to anchor the bridge on, but the other side doesn't. It is a bit lower too. The run is about 12 or so feet. The side without the rocks is 2-3 feet lower.My question is, can the bridge that crosses something have a downward slope? This would save us from building up a rock foundation on the lower side. Our trail system gets ridden about 3/4 of the time in one direction , and for thi section that would woek with a downhill bridge.

    Any ideas? I know there are tons of bridges in the videos dropping down from a big boulder but I've never seem one that crosses a creek.

  2. #2
    backwoods and backwards
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    Shouldn't be a problem.
    Moonbeam's compost-powered hybrid generates a respectable 32 fruit flies per rotting banana peel.

  3. #3
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  4. #4
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    might only be a problem if it's multi use. The finished surface on dimensional lumber can be quite slick if there is dew or moisture. Rough sawn wood or some type of metal lath may be needed to reduce accidents, though falling on a cheese grater wouldn't be good either.

    Nice log bridge though.
    Michael Vitti
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  5. #5
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    We have a forest service built bridge at one of our trail systems that is a site to see. The only thing I can figure is that they had money budgeted for the trail system and this was the last bridge to be built, so they threw in all the chips. It's about 100' long. You start off heading downhill slightly and end up climbing for about 25'. There is then a left climbing turn built into the bridge which requires you to almost be in granny. It crazy. Anyway, it's lined with 1/2" hardware cloth (wire mesh) and is ride-able rain or shine. I know home depot carries it in 2'x25' roles 1/2" is the size of the gaps in the weave. I'll post a pick of the bridge next time I make it down that way.

  6. #6
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    2-3 feet will make for quite a pitch spanned over just 12 feet. Too much i would say.

    What would make sense is to just anchor it to the rock, build it high on the other side. Harvest a ton of rock/soil and build an approach to the side that's got a 3 foot drop. And make it into a 10 inch drop with a nice tranny, so climbers can still pop onto it, and descenders can wheelie drop off the bridge and maintain flow.

    i would say keep the bridge level, or keep the grade to a minimum.

    That's my vote.
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  7. #7
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    What is the difficulty level of the trail and average rider? The bridge in the post above sounds like fun for me, but would be too intimidating on a beginner level trail. My wife also says a bridge with an incline or descent sounds terrifying.

  8. #8
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    I think the bridge needs to fit in with the trail. On our easy trails we have 2 foot wide bridges. On our black diamond trails we have narrow bridges,one is 50ft long and less that 1 foot wide. We have some that are flat and some that are steep,but the steep ones are made from split cedar and have lots of grip. I think I have some photos if you want to see them.h

  9. #9
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    I may have stated the length incorrectly . Maybe more like 15 ft.

    This will be a trail that runs downhill through a little ravine almost, but a real wide and mild ravine. Its quite rocky in some spots. The trail will cross the creek a few times. To avoid a swampy area it has to cross where I want to do the bridge. The bank is rocky and maybe 4ft above the creek on one side and a fairly steep dirt hill on the other side. We'll benchcut into this hill, but it doesn't have the large rock structure to support the bridge on this side. The ground is maybe 1 foot above the creek and then it hits a steep bank maybe 6-7 feet away from the creek. I'll look for my pics.

    The trail here would be among most difficult in our system.
    Last edited by Guitarswheelies; 10-25-2008 at 11:54 PM.

  10. #10
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    Here's the area for the bridge. The perspective isn't very good in this shot-its a little bigger than it looks in the photo.


    salscreek55.peg

    Last edited by Guitarswheelies; 10-26-2008 at 03:58 AM.

  11. #11
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    In general this area has lots of rocks. It will be a fairly short additional loop to our existing 9.5 mile system. There is a road 100' to the right of the bridge so it would see some multi use from energetic hikers and hunters.


    salscreek3


    salscreek6


    salscreek4


  12. #12
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    You've got tons of rock available right there to harvest and use to make the bridge even. With all those deciduous leafy trees, combined with the rain, you got potential for a slimed slippery bridge guaranteed each season change.

    If you're looking for a crossing with a techy challenge, get creative with all that rock.

    Or if you're looking for something that doesn't involve much work, make a flat bridge that's got 2 surface sections, in other words a tiered bridge with a step in the middle of it. Or two smaller bridges using that rock inbetween the two for a step.

    Lotta possibilities, but a bridge with alot of slope that's slimy, is lame in my book.

    I
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  13. #13
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    Find a narrower section and build a rock bridge! It's a bit more work, but you've got tons of rock available right there. Rock bridges will never rot away and the material if free! If built correctly it will allow water to run through without getting clogged with leaves, making them maintainance free, forever!!!!!

    On our trail system, as long as the rock is available we always prefer to build water crossings with rock.

  14. #14
    beer thief
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    I agree with Skookum & jmitchell - build as much as you can out of rock and if you need to resort to wood, don't exceed 8%, or about 1' drop in 12' of run.

    I am a structural engineer and have designed and built a number of bridges. From a structural standpoint they don't need to be dead level, but from a user's perspective they should be somewhat close.

  15. #15
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    The bridge at an incline is not that bad....that's a foot in 5 feet. You can always do a 50/50 compromise with a rock build up at the low end, although a Wheelie Drop off the end and all those rocks doesn't leave you with a very good LZ. This would be tough as beginner to intermediate level trail although a straight intermediate trail would be OK. Rough cedar or similar would be acceptable. we just did one with cut lumber and used a Paint coating with a coarse sand mix in it and it's been OK, but your application looks like cedar planks would be the better call.
    Last edited by mtb777; 10-29-2008 at 12:20 PM.
    IF YOU CAN READ THIS, YOU'RE NOT RIDING (or building)!

  16. #16
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    The step idea above is a good one. You could also split the difference with a reduced grade and steps at the low end.

    You can raise the low end up on posts or rocks and then build steps to transition the dropoff.

    You want to make it rideable both ways and you probably have to make it doable for hikers too.
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  17. #17
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    wow. nice stuff and nice pictures! thanks for all of your ideas. I showed this little section to a few guys who I ride and build with and they thought it would take 2 years to build it! I don' think so.

    I like the idea about doing a rock bridge but that would be a big project. Obviously , throwing down two runners and nailing some slats down would be quicker easier.

    I'll post a better pic in a few days

  18. #18
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    We have this on our trail system.

    It was made from leftover trees after the property was logged and cut on site with a chainsaw.
    It's a little slick when wet, and I'm sure will get slicker as it ages.
    Will proably require some type of traction in the near future.


    Quote Originally Posted by Guitarswheelies
    I like the idea about doing a rock bridge but that would be a big project. Obviously , throwing down two runners and nailing some slats down would be quicker easier.
    In my experience the best solution is rarely the easiest.

  19. #19
    dog's best friend
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    Info on Aluminum trail bridges

    Our local multi use trail group is planning to replace a rotting wooden bridge crossing a 30' span at the trailhead. Anyone have some good leads on aluminum trail bridges? Non motorized use only.

  20. #20
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    Quote Originally Posted by telebiker
    Our local multi use trail group is planning to replace a rotting wooden bridge crossing a 30' span at the trailhead. Anyone have some good leads on aluminum trail bridges? Non motorized use only.
    This is probably a bit out of most people's abilities to build, but check out what these guys did:
    http://www.pinehillpark.org/php/Pine...k_Welcome.html

    Friends of mine rode there last weekend and said the bridge was amazing.

  21. #21
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    Quote Originally Posted by jmitchell13
    This is probably a bit out of most people's abilities to build, but check out what these guys did:
    http://www.pinehillpark.org/php/Pine...k_Welcome.html

    Friends of mine rode there last weekend and said the bridge was amazing.
    Wow that's a cool design. Lots of bags of concrete hauled in.

  22. #22
    beer thief
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    Not a big fan of aluminum for structural applications. We've built these 50' and 25' spans with steel stringers and wood deck & rails.

    How accessible is your bridge site? That's a key piece of design for longer spans.
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  23. #23
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    Quote Originally Posted by Guitarswheelies
    a few guys who I ride and build with and they thought it would take 2 years to build it! I don' think so.
    No it could be done in a weekend if everything lines up perfect, what's key is you have a couple of guys to help. The more help you have the skies the limit. Especially doing something challenging like this, it's so much easier when you have feedback between more people if a problem comes up, as well as sharing the workload when you got your game plan down.

    Figure out what to do, get all the materials lined up (get a little more than you think you will need), get all the tools you need, create a plan, be flexible if you need to adjust, and do it.

    Quote Originally Posted by Guitarswheelies
    I like the idea about doing a rock bridge but that would be a big project. Obviously , throwing down two runners and nailing some slats down would be quicker easier.
    Yah but rock is forever.

    A trail builder really buzzed this idea by me when we were conversing and it really struck as true. In any construction project you can get 2 out of 3 things. Quality, Cheap, or Fast. Most always you only get to pick any 2 out of those three options.

    Rock work can be slow, but you don't have to buy anything, except for better tools to work the rock (rock bars, rock sled, buckets). Also consider how long of a haul it is to bring in all the wood materials, when it looks like all the rock you need is there.

    If you haven't worked with rock, it's slower work that involves good planning, or big muscles/strong back, and sometimes paitence. But if you build that all out of rock, you will have something that will be there exactly how you left it, for your great grandkids to use.
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  24. #24
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    Does anyone have some more pictures of rock work on bridges?

  25. #25
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    So I know you are in PA, but whats the land management agency? You will want to be careful with that aspect of a structure. With our local forest district, we have pretty much carte blanche to construct with rock, but any other structure requires permitting, a process thats pretty time consuming. One of the forest districts north of here is requiring aluminum stringers on all their bridges.

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