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  1. #1
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    Building bridges

    I'm helping make a new trail system. We have flagged and cut a 3 mile loop. We now have to build 3 bridges to cross ditches.

    They all are pretty much 16-18 feet across.

    We have gotten old telephone poles donated to use. Currently we are planning a build day. We are looking at using 1x6's.

    A couple items for debate have been how wide? Some are thinking 30 inches wide, others 46 inches. Which would be better?

    Next is spacing of the 1x6's? Orginally was thinking 1 inch gap. now it has been said 2.5 inch gap.

    I personally think 2.5 inch is too much. These bridges would be used my walkers, or even kids walking bikes over.

    Lastly any kind of covering on the surface?

    Thanks for information.
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  2. #2
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    Several thoughts based on our experience in NW Washington. First, old telephone poles should make great stringers, but they will be heavy, so plan on very well-attended work parties to emplace them. It would be good if you can set them on rocks or masonry footings (even a gravel-filled cinder block at each end would be better than nothing). Second, how wide depends on how high the tread will be above ground at the deepest point of a crossing. Some building code requirements for residential decks, for example, start requiring handrails if the deck is as little as 18" above grade; others kick in at 30". From this I conclude that if the tread isn't very high off the ground, it doesn't need to be too wide. 32" is good, because so much tread material is available in 8' (96") lengths, so there's no waste. If tread height above grade is much higher, I'd go with 48" or even 60".
    Third, even at 32" widths, 1x6 is going to be flexy. I'd go 2X6 or 2X8. Fourth, the gap depends on how multi-use the trails will be. I did a couple boardwalks where we used 1.5" gaps (easy to be consistent; just stick a 2X4 in there for a spacer while you fasten the tread in place) but found out afterward from hiking pet owners that the gap was very scary for most dogs. So, next time I'm going to try 1/2" and see if that works better.
    Fifth, lay down a strip of heavy tar paper over the stringers before installing the tread; that will extend the service live of the stringers by quite a bit. Also at each end, tarpaper over the ends of the stringers and install a treated lumber backboard to stabilize them and protect them from moisture when you fill in the approach ramps from the tread boards to the trail grade. Sixth, if you use treated lumber for the tread, assume it will be slippery for the first few weeks, esp. if the work is done in the fall or winter. Once the "green" wood dries out (even if it keeps getting wet from rain) it won't be crazy slippery (from one who has slipped and fallen in front of witnesses riding across a just-completed boardwalk...)
    Hope this helps, will be very interested to see what others say.

  3. #3
    Zach Attack
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    I love bridges and have been obsessed with them lately. I think narrow surface trail is sweet but what you hang on this ribbon of dirt is what makes the trail special. Bridges have a lot of potential for a big experiential impact and is often where people may pause on the trail.
    I have been building Stone Arched Bridges. Both because they last fricken a long time, there are usually lots of rock at creeks and they look cool. Material cost is very inexpensive. This 5ft long span cost around $15.


    Here is a larger 45fter, $1400


    This one is at the bottom of a big run, it is a sweet jump!


    I am building ferrocement bridges now. Pretty slick. Working on final touches to a 30ft span 180 bridge. The concrete deck is a banked turn that arcs 180 deg as it flys over. Will dig up some photos of this one....
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  4. #4
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    Zachi, I have been on 2 of your bridges and they are indeed very cool and a great place to stop. Thanks for going the extra step to make them special.

  5. #5
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    I'd say 3ft for width would be good for multi purpose use especially if its high off the ground. If you're going that wide without a middle stringer 1x6 might be too flexy. I've used PT 1x4 (actually 5/4x4) and had real good success. Spacing I think 2 inches would be ok, 1.5 might be better. Precut everything and just lay it out and see what the building party thinks.
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  6. #6
    pedal fatass!
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    Way to make the rest of us humble builders look bad, Zachi Beautiful stuff! I was supposed be around Nevada City this weekend for some riding & camping but injury put rest to those plans.

    To the OP: I wouldn't use anything less than a 2x4 at anything over 1/2 inch gaps. 2x6 or 8's are even better to reduce flex. We have a couple of wooden dowels we use to check the gaps quickly. Larger gaps can ride choppy. On the width, I'd say 36-48" for a span of that size. If you use 2x4's, you may find that they flex in the middle if the width is 3 feet or more, so you may want to screw a stringer down the center on the bottom.

    The phone poles should be great, and if you can get pressure treated wood for the decking, do so. If the bridges are to be straight & level, traction might not be as big a concern, but if they're on an angle or will be ridden when moist, you might consider covering the deck in chicken wire, with 1" or less mesh. Also, as was mentioned above, you should check to see what kind of regulations may apply to your bridges with regard to height & width. For example, the City of San Luis Obispo has required us to fit railings to to any free-standing elevated structure over 36" off the ground.
    Last edited by M5Tucker; 07-14-2012 at 12:17 PM.
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  7. #7
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    If you expect dogs on the trail you have to space to not trap a paw and break Fido's leg. 2.5" seems a little wide.

  8. #8
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    We are going to use 2x6's, that was my mistake. PT also. They will also be straight as well.

    Current debate on spacing is with a 2.5 inch spacing the local horse riders would not want to cross them. I think this is valid, but I think the spacing is a little much.

    The trails are built for MTB, but the overall area is for everyone and everything. We will have signs that will state no horses on bike trails. We have no problem with hikers using the trails, but we all know what horse traffic can do to a trail. The horses have plenty of fire roads and we have been trying to stay away from those.

    So far the first 4 bridges we have to build will be for entering or exiting a trail, not in the middle of the trail.

    I'm attaching photo's of 3 of the ares we are going to be crossing.
    This would be the first one


    The second one


    and third


    Also I was told we will have concrete bases for the poles to rest on and keep them from sinking.
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  9. #9
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    Those are pretty small areas to cross. Here's what we came up with at a similar crossing. Some people prefer the stream crossing when the water level is low and the temps are warm. Building bridges-imageuploadedbytapatalk1342306068.732983.jpg
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  10. #10
    Single(Pivot)and Happy
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    Many dogs will have issues with 1.5"-2.5" spacing. Kids have little feet also. I have no idea if hikers/walkers in your area push strollers but stoller wheels will have issues.

    You mention "mainly mountain bike trail" but also say the "overall area is for everyone and everything". Unless the agreement to construct the trail is specified as "mountain biking only", you are building a "shared-use trail" and if this trail building is sanctioned, I have to believe there are specific guidelines that you must conform with.
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  11. #11
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    JMHZ, you didn't mention horses in your original post. I'm thinking even 2x lumber may not be enough. See if you can find rough-cut cedar 3X in your area, or go PT 4X if equestrians are really going to frequent these trails. A beefy (4X12+) down the middle might be ok with 2x8 or 2x10 tread, but that feels light-duty to me where half-ton animals are concerned. Good luck, post pics of what you build.

  12. #12
    gran jefe
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    mmm, go with 5/4 lumber and put up "no horses" signs. you want for them to look at the bridge and for it to be obvious to them that the horse will crash through. maybe even make a graphic of the horse crashing thru. then go with spacing narrow enough not to hurt fido or a kid.

    or, if you make a sturdier bridge that might be attractive to horse riders, put a metal doorframe type structure in the middle that a rider/horse will not fit under.

    when you install the boards, nail them in with the bark side up. well, where the bark used to be. boards naturally cup over time.

  13. #13
    pedal fatass!
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    Agreed, the equestrian element changes the decking requirements. I'd think that 4x's would be necessary. Building & marking it to be obviously unfit for for horses is one approach, but if it's a real multi-use system you can't afford to piss off the equestrians. That said, those ditches look like a horse could simply ride through them. If you do decide to build to include horses, try to get some financial support for the materials from the local equestrian user group. They can be strong allies, and they have the money, I promise.
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  14. #14
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    I would run a 2x6 for the center of the bridge span and then 2x6 on each side and laminate a 2x8 to each of the side 2x6's. Solves the flex of the stringers, and the 2x8 will cover your but ends for cleaner look on the sides and stiffen the main beams. Then just cut some 2x4x16" blocks and attach down each side on a layout, with spacing and then attach a 2x4 "top rail" down that. This way, if the bridges are slippery, and someone slips, there is an edge to catch your tire, boot, etc.....been there, done that, lol. If you decide to use the Telepoles, run three, one down the center, and this should accommodate, the equines. Good luck.

  15. #15
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    If equestrians will be using this bridge, the bridge must be engineered to handle the live loads, deflection and also the vibration of multiple animals.

    A bridge designed without engineering is a liability. Also, without an understanding of basic horse behavior and characteristics, you will build a bridge that will not be used by equestrians for safety reasons. If the purpose for the bridge is to manage trail user movement and minimize habitat destruction, building a feature that cannot be used by all trail user groups, in my opinion, is a failed project

    Partnering with your equestrian community can lead to many benefits for all trail users. Most issues between trail user groups stem from the lack of understanding and lack of dialog between groups.
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  16. #16
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    We gap our boards 1/4 inch for water and dirt to get through, wider why?

  17. #17
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    around here with the amount of mud/soft soil we get 1/4 gaps would clog up real fast. We generally just us a 2x4 on it's thin side to get a consistent gap. It also uses less material.

  18. #18
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    I use a piece of 5/4 for the gap between deck boards. 24" or narrower I deck with 5/4, wider 2x8s
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  19. #19
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    These bridges are not meant for horses. We do have horses in the area and they have the fire roads. Since cutting the first trails we have had a couple horses check out some of our trails and we have put up temp bike only signs which seem to have worked.

    So we are not building the bridges for horses. Bikes and people, mostly bikes.

    I personally like the 1 inch gap on the boards, but others think the 2.5 inch gap will help scare the horses away. Hopefully the bridges will scare them off regardless.
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  20. #20
    gran jefe
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    build all the bridges as teeter-totter ttf's. end of horse problem.

  21. #21
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    M5Tucker mentioned chicken wire for traction. I prefer not to use it on trails that dogs might use. It's tough on their feet.
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  22. #22
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    Quote Originally Posted by JMHZ2401 View Post
    These bridges are not meant for horses. We do have horses in the area and they have the fire roads. Since cutting the first trails we have had a couple horses check out some of our trails and we have put up temp bike only signs which seem to have worked.

    So we are not building the bridges for horses. Bikes and people, mostly bikes.

    I personally like the 1 inch gap on the boards, but others think the 2.5 inch gap will help scare the horses away. Hopefully the bridges will scare them off regardless.
    post signs with bridge's max load, maybe that'll deter some of the equines
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  23. #23
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    Quote Originally Posted by Trail Ninja View Post
    M5Tucker mentioned chicken wire for traction. I prefer not to use it on trails that dogs might use. It's tough on their feet.
    Good point. I'm not a dog guy so I didn't even think of it. That said, after having seen the marks of numerous bike crashes on some slippery bare bridges here, I think for short spans it's probably ok for them if you use a tight mesh and secure it well, and a number of folks I know ride (and have helped build) the area in question with their dogs regularly. I think that larger decking gaps (1"+) would be a bigger problem for man's best friend, and in that case I wouldn't use the wire because it might easily catch a paw in a gap.
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  24. #24
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    Strongly recommend against chicken wire. It doesn't wear well, could even puncture tires. If you use treated lumber, once it dries out it should be ok. If you're really concerned about traction, after it dries out apply some non-skid paint or stain (there are lots of products out there, some you mix special sand into whatever paint you want, etc).

  25. #25
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    I think it depends on the region of the country. Here is the PNW we use a lot of chicken wire and it holds up great if you install it right. We havent had any issues with dogs or flats. The best stuff to use is the coated type.

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