Dig stoke, who else likes to play in the dirt???

[toast2266]

Here's a question for you guys:

We need to replace a bridge on a popular existing trail. Span is about 12', and the area is cliffy, so there's no real way to reroute around it. The forest in that area is shit - there's no worthwhile natural material to be harvested on site. Old bridge is made out of shitty natural materials, which is why it's being replaced. The bridge is a 15 minute (steep) uphill walk from the nearest place we can get a vehicle.

Original plan was to carry in some big beams, but after carrying in some smaller beams for another project in the area, I've decided that it's not particularly feasible to bring in beams that are big enough to cover that span. So I'm looking at carrying in some pressure treated 4x4's and building a pair of 4x12 laminated beams on site (thinking we'd epoxy them and then bolt them together). My gut tells me it'd be fine, but the internet tells me its a horrible idea and everyone will die.

Any ideas / experience / predictions of # of fatalities?

[jackstraw]

I don't think you'll need epoxy. The thru-bolts will be fine.

But spanning the 12' with 4'ers maybe problematic. Can you set posts anywhere along the 12' span...like under the butt end joints ideally.

[toast2266]

I don't think you'll need epoxy. The thru-bolts will be fine.

But spanning the 12' with 4'ers maybe problematic. Can you set posts anywhere along the 12' span...like under the butt end joints ideally.

We potentially could set some posts, but I wouldn't really trust them. The area is super rocky, so any posts we set aren't really going to be particularly well dug into the ground - they'd kinda just be wedged into some rocks. Which is probably better than nothing, but I'd still like the beams to be able to carry the load unsupported.

We could also bring in 4x6's and stack them horizontally to create a 6x12 (or 6x whatever). I was mostly gravitating towards 4x4's because they're cheaper and easier to come by.

[evdog]

12' isn't that long of a span. Why not use 2x10s as stringers? If the bridge doesn't need to support horses that should be plenty strong.

[toast2266]

12' isn't that long of a span. Why not use 2x10s as stringers? If the bridge doesn't need to support horses that should be plenty strong.

Think that'd be enough? I'm not a construction / engineer type, so I don't have a great concept of what's actually needed for this kind of situation. My take has always been to just massively overbuild everything in lieu of engineering.

But yeah, doesn't need to support horses. Just bikes and foot traffic.

[zion zig zag]

I wouldn’t do single 2x10’s for stiringers of that span.

[toast2266]

I wouldn’t do single 2x10’s for stiringers of that span.

We use 2 x 12's for the decking. What about bolting a couple of those together to make 4x12's? Think that'd do it?

[zion zig zag]

I’m not an engineer, but I think that’d give you a better margin. Is this on fed land? They have to meet different standards but you can find plans online.


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[toast2266]

I’m not an engineer, but I think that’d give you a better margin. Is this on fed land? They have to meet different standards but you can find plans online.


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State land, so no specific design requirements. But that's a good thought; I'll look into what the feds say for standards.

[jackstraw]

We potentially could set some posts, but I wouldn't really trust them. The area is super rocky, so any posts we set aren't really going to be particularly well dug into the ground - they'd kinda just be wedged into some rocks. Which is probably better than nothing, but I'd still like the beams to be able to carry the load unsupported.

We could also bring in 4x6's and stack them horizontally to create a 6x12 (or 6x whatever). I was mostly gravitating towards 4x4's because they're cheaper and easier to come by.

Yeah, I've built stuff on rocky areas like that before. As long as it's braced with a 45 and metal ties to the joist/beams it will suffice imo. It's more an extra brace for flex and if it happened to fail that would hold it from a catastrophic failure...possibly. Ideally, you drill into the rock, but I probably wouldn't for a mtn bike bridge.

[evdog]

Not an engineer either but 2x12s would definitely do the trick, and even moreso with two bolted together. As a bonus they could be carried in separately and assembled on site. I'll see if I can find pics of any of the bridges on my local trails. I don't recall any that would have been more than 2x8 stringers.

We did have a volunteer who was a civil engineer who designed some bridges for the city that were interesting. A layer of two parallel 2x12s as decking and another layer below, with 2 or 3 2x4s on end in between to space the layers out in the middle. The layers are bolted together at each end which puts tension on the whole thing. It has a bit of bounce to it but is very solid. Span is 20 or 24ft I think. We have another one of similar design that is probably 12 ft.

[toast2266]

Nice, thanks for the input, all. I did find some forest service specs for bridges. They're calling for 4x12's for a 12' span. Although that's for a single, rough cut beam. They don't mention anything about bolting together 2x12's, but given our situation, I'm thinking that'll be fine.

[evdog]

Here's the bridge I mentioned above. Cool design. It's not easy to see given the angle, but the 2x4s on end in the middle give it some rigidity like a truss, but it still has some bounce if you jump on it. This is in a city park and was signed off by the rangers. It has to be a breakaway design so it is chained to a tree on the near side. Every time there is a storm it gets dislodged and has to be moved back into place. Heavy bastard.

[jackstraw]

The 2x4s are perpendicular? Where the bolts are seen from the top? Anything parallel also?

[evdog]

Yeah, the 2x4s are upright in between, perpendicular to the 2x12s. So the 2x12s have 4" space between them except at each end. You can kind of see one of the 2x4s poking out in the middle. I guess they would be where the bolts are. The nearest bolts at bottom of the pic would be holding the upper and lower 2x12s together.

[jackstraw]

Right on. Those spans of 4' are only supported by 1 1/2" on the flat. I wouldn't recommend that for anything to high or with high consequences if it failed.

On edge is obviously much stronger. This guy did some math -

a flat 2x4 has about 20% the strength as a standing one
a flat 2x6 has about 7% the strength as a standing one
a flat 2x8 has about 4% the strength as a standing one

Also the type/quality of wood depends as well - https://courses.cit.cornell.edu/arch...7.1/index.html

[thommy21]

The span for the Bridge we put in on Baxter’s Gulch was 22’. We hand carried in each LVL. Similar distance/time you are talking. About 5 people per LVL. 3, 22’ long, 3 1/2” x 11 1/4” LVL’s. They are overkill, but that’s a long span and has to withstand spring runoff that comes in from pretty high up. 3 1/2 is (2) 1 3/4” LVL’s bolted together. So 6 pieces, made 3 beams. https://m.facebook.com/cbmba.org/pho...pe=3&source=54

[thommy21]

One of the side supports broke but was a big year snow wise. Pretty easy fix. No Horse traffic. They just wade the stream to the right of the bridge. We sloped the sides for them so it’s not a big drop on either side.

[thommy21]

[jamal]

Sometimes a trail just needs to be nice and wide and smooth:



One of our more popular and easy trails has seen a lot of re-routing work over the last few years, mostly by the forest service and montana conservation corps. Recently, the FS went in and re-routed one of the last troublesome sections, where you went straight down a meadow. There was no drainage, so it was just like 3 bumpy, eroded ruts for a couple hundred yards, and actually a large portion of the original trail was basically that. The new re-route wasn't that great to ride though due to some overly tight and off camber corners. I'd actually only been down it once since the work. Yesterday we went in and made it a little more bike friendly. I still want to go in there and do more work on that lower section though as the original finishing work, I think by the MCC, was a little shoddy and it's lumpy as hell.

[JerseyDave]

Sometimes a trail just needs to be nice and wide and smooth:

No. No it doesn't. Not for beginners, not for kids. If a trail is smooth, why does it also need to be wide? Always have to give the proper high five to anyone who volunteers...but why make such a thing for a Mountain Bike? Bikes=more and more capable. Trails=less and less technical. Why?

[jackstraw]

No. No it doesn't. Not for beginners, not for kids. If a trail is smooth, why does it also need to be wide? Always have to give the proper high five to anyone who volunteers...but why make such a thing for a Mountain Bike? Bikes=more and more capable. Trails=less and less technical. Why?

Dave, sometimes it's like someone pissed in your cheerios. That trail if perfect for a beginner. Not all beginners are kids. Some are 50+ women or men that don't want to risk falling. Get over yourself man.

[jamal]

This corner was only a few months old and was already fucked because you came into it too fast and it was flat/off camber and sent you wide at the exit. We slowed down the entrance and gave it some inslope so now people will actually stay on the trail. It will grow in and narrow to an established line. Before it was just going to continue to get eroded wider on the exit.

[toast2266]

Hell, I like super wide advanced trails too. Some of my favorite trails of all time are 20' wide and have a bazillion lines through every section. I can ride it 100 times and never ride it the same, and never get bored with it.

*which is most definitely not to say that all trails should be wide.

[jackstraw]

Hell, I like super wide advanced trails too. Some of my favorite trails of all time are 20' wide and have a bazillion lines through every section. I can ride it 100 times and never ride it the same, and never get bored with it.

*which is most definitely not to say that all trails should be wide.

Because...mtn biking is fun! Pretty simple really eh?