# Building bridges



## JMHZ2401 (Mar 10, 2011)

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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## TFitz (Jun 21, 2008)

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.


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## zachi (Jul 25, 2006)

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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## pixy (Nov 8, 2005)

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. :thumbsup:


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## pascale27 (Aug 26, 2011)

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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## M5Tucker (Aug 8, 2011)

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.


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## faceplant72 (Oct 25, 2009)

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.


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## JMHZ2401 (Mar 10, 2011)

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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## pascale27 (Aug 26, 2011)

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.


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## Boulder Pilot (Jan 23, 2004)

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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## TFitz (Jun 21, 2008)

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.


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## Bill in Houston (Nov 26, 2011)

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.


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## M5Tucker (Aug 8, 2011)

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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## ghglenn (Jan 26, 2012)

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.


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## Boulder Pilot (Jan 23, 2004)

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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## leeboh (Aug 5, 2011)

We gap our boards 1/4 inch for water and dirt to get through, wider why?


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## Mr. Lynch (Jun 11, 2010)

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.


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## hado_pv (May 26, 2006)

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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## JMHZ2401 (Mar 10, 2011)

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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## Bill in Houston (Nov 26, 2011)

build all the bridges as teeter-totter ttf's. end of horse problem.


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## Trail Ninja (Sep 25, 2008)

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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## pascale27 (Aug 26, 2011)

JMHZ2401 said:


> 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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## M5Tucker (Aug 8, 2011)

Trail Ninja said:


> 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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## TFitz (Jun 21, 2008)

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).


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## Mr. Lynch (Jun 11, 2010)

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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## JMHZ2401 (Mar 10, 2011)

We have already decided against chicken wire, because of the chance for tire punctures down the road. We have talked about tar paper, and will address after they are built and it becomes a issue.


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## ghglenn (Jan 26, 2012)

I just rode the 24hrs of Adrenaline in Canmore. The Nordic Centre had many wood bridges and boardwalks for us to ride. They used concrete wire and sandwiched the edges of the wire between wood. I would suppose they also use nail-on staples to hole it down. This is much heavier gauge, but does a great job for traction, as it was wet at times on the ride.

If you go to their photos page you can scroll through and find some shots of what I'm talking about.https://www.facebook.com/pages/24-Hours-of-Adrenalin-Canmore/109625625787403


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## ajd245246 (Sep 1, 2008)

From what I've read in here it looks like you have most, if not all of your answers. 2.5" sounds way too big for the conditions you have mentioned. 1-1.5" is definitely the largest you want to go. As for the grip issue, you CAN use chicken wire if you apply it correctly. It also depends how much traffic the trails will see, more traffic=more wear and tear on the wire. Your other alternative is the textured paint/cement as mentioned earlier. (a better option no doubt)


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## traildoc (Mar 5, 2007)

JMHZ2401 said:


> 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.


You never know when an equestrian might be from a foreign country that doesn't understand a "NO HORSES" sign, so you better make the tread out of 8X8's, then you won't need a stringer, if you use a 36' width.


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## pascale27 (Aug 26, 2011)

traildoc said:


> You never know when an equestrian might be from a foreign country that doesn't understand a "NO HORSES" sign, so you better make the tread out of 8X8's, then you won't need a stringer, if you use a 36' width.


Sign can include picture of horse with the standard red circle with a line thru it. That pretty much would cover everyone except blind equestrians


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## Boulder Pilot (Jan 23, 2004)

I'm just curious. Who owns the property you are building these trails/bridges?


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## Bill in Houston (Nov 26, 2011)

pascale27 said:


> Sign can include picture of horse with the standard red circle with a line thru it. That pretty much would cover everyone except blind equestrians


I find that anyone who wants to ignore any sign on any trail will do so, regardless of mode of transport, language spoken, or physical abilities.


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## pascale27 (Aug 26, 2011)

True but atleast the builders "should" be covered if a horsey has an accident on the bridge.


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## bedwards1000 (May 31, 2011)

On the bridge we just built we used rough sawn 2x6. The rough sawn gives you an extra 1/2" of thickness and it has better traction than finished lumber. It can be cheaper too since it saves a step. Just make sure it doesn't have ground contact. You could probably use a PT at each end if that's an issue. We used 3/4" gap. We had a gauge/tool that was a 3/4" board that had had 2 pieces of scrap nailed to it to set the distance from the stringer, the decking and set the gap.


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## Mr. Lynch (Jun 11, 2010)

I love the rough sawn stuff. A few local warehouses get it at braces for the deliveries in trailers, and they usually cut it up and throw it out. I have them save it and I pick up 100+ boards every few months. They usually pay people OT to come in on a Saturday to cut it up and toss it in the dumpster, so it is win win for everyone. I use if for most the decking on the structures I build but I do stain it first.


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## woodway (Dec 27, 2005)

Just finished a bridge here in the PNW and we reclaimed downed cedar and axe split the decking boards. Traction is great dry or wet and cedar lasts a long time. I've used the rough sawn cedar as well, and will sometimes score it a bit more with a chainsaw for extra traction.


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## Bill in Houston (Nov 26, 2011)

that has a really nice look, woodway.


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## donwatts (Aug 1, 2006)

For traction try epoxy based paint topped with sand


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## JMHZ2401 (Mar 10, 2011)

Boulder Pilot said:


> I'm just curious. Who owns the property you are building these trails/bridges?


County property.


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## traildoc (Mar 5, 2007)

I helped with this recent bridge project in Downieville, CA


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## mtbikernc69 (Mar 23, 2004)

2x6 or 2x8 decking. Coated deck screws. I use play sand mixed with exterior latex floor paint for traction if using smooth PT decking. On the Cedar, Western Red Cedar is very rot resistant. Eastern Red Cedar is not.


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## pascale27 (Aug 26, 2011)

traildoc said:


> I helped with this recent bridge project in Downieville, CA


Is that pressure treated plywood ?


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## snug dug (Jul 7, 2012)

I've been building bridges lately (on public land) using a USFS engineers plan. Depending on length and width, we use 2x12's glue laminated and nailed together in doubles, triples or quadruples. The quads are effing heavy! Depending on application, equestrian or bike, we deck them with 4 inch rough cut or 2 inch rough cut treated pine.

Will post pics as soon as my post count is higher!


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## Ridnparadise (Dec 14, 2007)

traildoc said:


> I helped with this recent bridge project in Downieville, CA


Where's the pic of him riding the white skinny past the guy in purple?


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## zachi (Jul 25, 2006)

M5Tucker said:


> 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.


Hey Tucker,

Sorry to hear about your injury.

That wood banked turn at the fasta project is some awesome action.

Hope you can make it up again sometime.

z


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## JMHZ2401 (Mar 10, 2011)

I wanted to update this. We got the bridges built. Turned out pretty good I think. We might have to make a few small adjustments. Here is a couple pictures, and some of these were taken while we were still doing the work. I have not been back to the area since this day due to weather, and not having a bike to ride right now.

The first bridge into the trail.









The second bridge on the trail









Third bridge on the trail.









The last bridge ciming out of the trail.









The same last bridge but while on it.









Also the last bridge looking back into the trail.









We were able to build 4 bridges in about 4 hours. This included cutting the boards.


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## snug dug (Jul 7, 2012)

snug dug said:


> I've been building bridges lately (on public land) using a USFS engineers plan. Depending on length and width, we use 2x12's glue laminated and nailed together in doubles, triples or quadruples. The quads are effing heavy! Depending on application, equestrian or bike, we deck them with 4 inch rough cut or 2 inch rough cut treated pine.
> 
> Will post pics as soon as my post count is higher!


Here's a 20 foot long by 8 foot wide equestrian bridge. We build all our bridges like this and narrow the width depending on use.
glue laminated and nailed 2x12 stringers and 8x8 sleepers









installing blocking, 1/4 inch angle plates visible that lock the stringers down









stream bank armoring, done first!









getting ready to deck 4"x8"x8' rough cut decking









decked, handrails up, face plates on, just need to ramp up with dirt, rock and stone


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## TFitz (Jun 21, 2008)

Those turned out really very nice! Are the stringers treated? Either way they're beefy enough, they should last awhile.


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## snug dug (Jul 7, 2012)

TFitz said:


> Those turned out really very nice! Are the stringers treated? Either way they're beefy enough, they should last awhile.


Definitely treated. I don't know how much difference it makes in the long run, but we ask our suppliers to provide materials that at a minimum meet UC4B use category, which apply to these conditions: Ground contact or fresh water. Critical components or difficult replacement.


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## snug dug (Jul 7, 2012)

*a few more*

Here's a 4 foot wide by 20 foot long bridge built the same way as the 8 foot wide by 20 foot long.









Here's a hybrid bridge/puncheon









Here's a puncheon


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## Bill in Houston (Nov 26, 2011)

i don't think that railroad trestles are built quite so sturdily. amazing work.


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## hey_poolboy (Jul 16, 2012)

Never built bridges for mtb before, but after spending most of my life around horses I would say the best way to keep horses off the bridge is to space the slats far enough apart that you can see through them. Horses are not very likely to cross something they can see through w/o a significant amount of training. 

Sent from my DROID RAZR using Tapatalk 2


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## forrestvt (May 20, 2007)

Dang! Why dont u just pave those damned "trails"! What are you using a mtn bike for? Get my road bike out for those.....


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## Trail Ninja (Sep 25, 2008)

forrestvt said:


> Dang! Why dont u just pave those damned "trails"! What are you using a mtn bike for? Get my road bike out for those.....


Can we see pictures of the ones you built? Just so we know how to do it right?


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## zrm (Oct 11, 2006)

woodway said:


> Just finished a bridge here in the PNW and we reclaimed downed cedar and axe split the decking boards. Traction is great dry or wet and cedar lasts a long time. I've used the rough sawn cedar as well, and will sometimes score it a bit more with a chainsaw for extra traction.


Why is that there? The trail doesn't look like it's particularly prone to poor drainage.


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## Mr. Lynch (Jun 11, 2010)

That trail is a low flat swampy area 9 months out of the year. The spot the bridge is on is usually really thick mud at best and deep standing water at its worst. It is a poor draining area with no real slope to divert the water away.


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## pascale27 (Aug 26, 2011)

Mr. Lynch said:


> That trail is a low flat swampy area 9 months out of the year. The spot the bridge is on is usually really thick mud at best and deep standing water at its worst. It is a poor draining area with no real slope to divert the water away.


Looks great.


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## woodway (Dec 27, 2005)

zrm said:


> Why is that there? The trail doesn't look like it's particularly prone to poor drainage.


What Mr. Lynch said. Notice the same curved tree on the right side in both photos.


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## pascale27 (Aug 26, 2011)

woodway said:


> What Mr. Lynch said. Notice the same curved tree on the right side in both photos.


The before shot makes the after shot even better. Great work.


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## robbiexor (Aug 22, 2011)

I would enjoy riding the after shot in the before setting!


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## Mr. Lynch (Jun 11, 2010)

In a few weeks it will be possible! It doesnt take much to flood out that spot.


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## radair (Dec 19, 2002)

We've built a fair number, from bog bridges (puncheons) up to 50' span steel stringers.


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## leeboh (Aug 5, 2011)

Does that Harpoon IPA help with the build?


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## woodway (Dec 27, 2005)

robbiexor said:


> I would enjoy riding the after shot in the before setting!


You can do it right now! (thanks jalm111 for the photo)


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## Scarsandtears (Sep 18, 2010)

woodway said:


> You can do it right now! (thanks jalm111 for the photo)


Very, very cool!


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## gcappy (Jul 1, 2012)

I am going to turn my back yard into a short trail system this year. I have about 6 acres available. Mostly hard woods but there is a beaver dam a quarter mile from me that has backed up a low section of my land. There is a 150' section of water and marsh I have to cross. I t may be a couple feet deep in places. My plan is to set log pylons every 20 ' and cut trees for the stringers and deck. Does this make sense? I want to do this as inexpensively as possible. It will only be used for mnt. biking and dog walking.
thanks


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## danielcoyle (Nov 28, 2009)

Anyone have any resources that give a sense of how long different types of woods (harvested from the surrounding dead trees) used as stringers will last without protection (rocks underneath etc.)? Have used oak stringers and am loathe to go back and get rock under them as there is little good rock available and no good trail access for carting it in.


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## danielcoyle (Nov 28, 2009)

Amazing bridge. Crazy really. Do you have resources you can share about how one is made? Do you create a template to lay it out on? Seems like it would be simple but a lot of hard work for that price.


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## zachi (Jul 25, 2006)

Awesome bridges!

Just a shout out... PT and Plastic are crap. PT is really toxic to use for trail user exposure. In Calif, new PT is called ACQ, lots of PT still used has arsenic in it and one sliver can result in amputation of a finger in a couple days. Took 11 years of thousands of amputations prior to doing something about it. Even though illegal to have exposed PT for building homes, it was commonly used for bridges etc... crazy.

Plastic, or Trex stuff is horrible. Grows mold like crazy, degrades with sunlight and is toxic if it burns.

We use local cedar due to its resistance to rot. Like redwood, it is brittle so you must utilize different span calcs. We also use lots of rock like photos I have provided.

There is a killer time lapse of a stone faced culvert bridge we built last week at our FB site Forest Trails Alliance | Facebook

Here are a couple concrete bridges we are working on


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## singlesprocket (Jun 9, 2004)

very cool, like those concrete bridges... gives me a few ideas...



zachi said:


> Awesome bridges!
> 
> Just a shout out... PT and Plastic are crap. PT is really toxic to use for trail user exposure. In Calif, new PT is called ACQ, lots of PT still used has arsenic in it and one sliver can result in amputation of a finger in a couple days. Took 11 years of thousands of amputations prior to doing something about it. Even though illegal to have exposed PT for building homes, it was commonly used for bridges etc... crazy.
> 
> ...


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