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How to secure a simple plank bridge?

2497 Views 5 Replies 5 Participants Last post by aero901, Jul 25, 2021

SWriverstone

Discussion Starter · Jul 16, 2021

I'm interested in building a few simple plank bridges-to span some steep V-shaped stream crossings that are a few feet wide and to level out the trail a bit. I'm not sure if there is another name for these than "plank bridge," but I'm talking really simple: a couple of 8-foot 2"x12" pressure-treated planks attached by some cross-braces beneath. That's it.

Building the plank bridges is pretty straightforward. What I'm wondering about is the best way to secure them (so they don't flop around)? They'd be fairly heavy so I'm wondering if they'd stay in place fine under their own weight? Alternately, is there any safe, reliable method for anchoring both ends to the ground?

If there was enough rock, I'd consider just building a higher turnpike across the stream by throwing more rock directly into the stream to ride over. But the plank bridge seems like a better solution.

ALSO: the same trail has several places where there is a short, very steep climb (with little room to build any momentum because you're coming out of a sharp corner) and a BIG root right at the crest of the climb (the classic "flowbusting" scenario). By big I mean 6-8" high. What's the best way to get rid of this obstacle? (Improve flow!) I'm guessing it would be to just build a short ramp with rock on the downhill side...but checking to see if anyone has other options?

Thanks!
Scott

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127.0.0.1

· #2 · Jul 16, 2021

I collect rock and make deep slots on each end of bank, dig down, lay in a rock chamber,
with a big rock for the base....stick plank in it. big, half buried flat stones for approach ramp...

ride from dirt to plank is flat and seamless and
on sides lay in rock that pops in above plank to hold it snug
not enough rock ? I use stoneboats and will fetch from a mile
away if I have to...

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crank1979

· #3 · Jul 17, 2021

SWriverstone said:
I'm interested in building a few simple plank bridges-to span some steep V-shaped stream crossings that are a few feet wide and to level out the trail a bit. I'm not sure if there is another name for these than "plank bridge," but I'm talking really simple: a couple of 8-foot 2"x12" pressure-treated planks attached by some cross-braces beneath. That's it.

Building the plank bridges is pretty straightforward. What I'm wondering about is the best way to secure them (so they don't flop around)? They'd be fairly heavy so I'm wondering if they'd stay in place fine under their own weight? Alternately, is there any safe, reliable method for anchoring both ends to the ground?

If there was enough rock, I'd consider just building a higher turnpike across the stream by throwing more rock directly into the stream to ride over. But the plank bridge seems like a better solution.

ALSO: the same trail has several places where there is a short, very steep climb (with little room to build any momentum because you're coming out of a sharp corner) and a BIG root right at the crest of the climb (the classic "flowbusting" scenario). By big I mean 6-8" high. What's the best way to get rid of this obstacle? (Improve flow!) I'm guessing it would be to just build a short ramp with rock on the downhill side...but checking to see if anyone has other options?

Thanks!
Scott

If you can dig holes for posts or piers at the end of each bridge you could secure the ladder bridge to the piers. Or create your own abutments to secure the bridge to. Whatever is allowable in the area and will be the best/easiest solution.

For the pinch climbs, in my area this FRP grating is getting a good amount of use. FRP Moulded Grating | Grating FRP Australia

It has plenty of grip and is easier to secure. It's being used directly on the ground or as a raised tread section.

The roots are probably going to either get cut out by someone, or the erosion on either side will become deep enough the roots might be broken by riding over them. I'd be building ramps over them if they are that bad and there is no better trail reroute option.

That's all based on not seeing the specific sites, sorry.

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JustMtnB44

· #4 · Jul 18, 2021

For bridges up to 12 feet long, it's easier and stronger to use pressure treated 4x6's instead of 2x12's as stringers. You don't need any cross bracing and they will last longer too. Then use more pieces of 4x6 to build a pier for each end to sit on. Anchor it all together with large spike nails. You could put vertical posts under this pier but in a V shaped gully the bridge can't really go anywhere. Then use a few rocks at each end of the bridge to keep dirt away from the cut ends of the wood. I've attached the basic plans I use for simple bridges like this. Note this drawing calls out 2x4 decking boards, but I usually use 2x6's for decking. Do not use 5/4 deck boards, they do not last very long.

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SWriverstone

Discussion Starter · #5 · Jul 23, 2021

Thanks all for the great info! I've been busy so haven't been able to focus on this but I will soon. This particular location is USACE property, but I'm just going to do it. I'm 100% certain if it's done well they aren't going to go in and tear it out just because I didn't authorize via proper channels. The trail gets very little maintenance generally—and very little traffic too.

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aero901

· #6 · Jul 25, 2021

For smaller bridges we bury a treated 6x6 at each abutment and then set the beams on top and screw them together with either metal brackets or +4" timber screws in a "toe-nailed" fashion. Blocking is added to the ends of the beams to keep dirt from caving in between them. The 6x6 can be dug so the beams are flush with grade or the top of the 6x6 can be set at grade level and a dirt ramp built up to the level of the bridge decking. Any lumber touching dirt is treated for direct ground contact or burial (yes there are different types of treated wood).