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Equismith 12-21-2009 08:29 AM

Nubee intro and question

I just joined this forum to seek advice on building a 16' plus bridge over a creek. I ride mountian bikes. As a result, I volunteer with trail work. We need to span a creek that frequently floods. The span will be at least 16'. There is risk that the underlying dirt bed/bank will possibly erode away over time. That's what has caused the need for this construction project. The old bridge is now sinking due to erosion. It has been in place for nearly 20 years. We're placing the new bridge farther up creek where the banks are more stable.

We must hand carry the wood for the project into the woods about 3/4 of a mile. 16' 4x6 are heavy, and sometimes I work on these projects alone or with one other person. Would alternately marrying two 2x6x12s be acceptable to span this distance? The 2x6x12s are not only cheaper, but less weight to carry in that far. This is on public land in the city watershed. We have built several small wood projects, but this one will be the longest span yet.

Any tips? Advice?

Liability is also a concern so if we do need to go the 4x6 route, then so be it. It'll just mean more breaks on the haul trips.

RickyBobby 12-21-2009 09:32 AM

Seems that there should be some pilings/posts for this project. I am guessing you would want it above the creek a few feet so the flooding would not go over it?

Don't think you would really want to just span the creek with the supports directly on the ground.

And since you mention the liablity issue, probably the best route would be talking to an engineer/code department, etc.

You could always just build a sweet ramp on either side!!!!:thumbup:

Equismith 12-21-2009 01:38 PM

The joists will be attached to posts. Either 4x4s 30" deep or 4x6s. Ramps on either end will be present. Maybe 6' long ramps at that. But not to jump the creek, although it would be fun:thumbsup:.

Our last project on this trail was closer to the trail head. We built a boardwalk across a sloppy wet area that is always muddy in the winter.

RickyBobby 12-21-2009 01:54 PM

I would think that 4x4 would be sufficient, but again, not sure what your code is calling for. The span is probably the biggest factor in this equation. Obviously need to make sure that that is addressed appropriately.

Again, depending on where you are, 30" may not be deep enough, especially since it is pretty soggy. In the northeast standard practice is 48" due to the freeze/thaw issue. Fence posts not being set deep enough look pretty bad when the ground heaves, it could prove disastrous for a bridge.

Seems like a very labor intensive project. A good amount of digging at least.

GOod luck

Thurman 12-21-2009 09:13 PM

I was asked to help with a similar project a few years back, it was a pedestrian foot-bridge four (4) feet wide, over a wet-weather stream. Transporting the materials was a problem, such as you have. No power near the area for the foot-bridge either. I decided to build the bridge off-site, as it would be built on-site. The same materials, the same dimensions, etc. We allowed for our support post to be buried and built up from that. We made the bridge in two pieces, and so they could be bolted to the support post, so they could be bolted together on site, and both ramps were built to be bolted to the bridge. Each section could be transported by four, healthy grown men but we used six for safety. Using manual labor and hand tools, this project worked out just fine. And--it's still there. Maybe an idea for your project, David

Equismith 12-22-2009 05:56 AM

Tell me about it...I built one bridge over a wet weather creek/drainage ditch that comes off a large power line R.O.W. That was a 14' project. I pre-cut everything at home but hand carried the materials in. I was able to drive close to the work area but still had to go about 100 yards in. It took me about 11 trips back and forth. The two 14' 4x6s were the worst.

The problem here is getting that many to help out. On weekend workdays we have about 40-50 folks show up. But the wood projects are usually done during the week. You can cut alot of trail with 40-50 people using hand tools. The wood projects take longer and result in less foot per person average on trail building. That's why we do them on the weekdays.

I guess my main concern is scabbing, marrying, sandwiching (whatever you want to call it) alternating 12' 2x6s. Taking a 12' 2x6 and a 4' 2x6 matched up to another set, but with the joints staggered, to create, in a sense, a 16' 4x6. With the joint being closer to the ends and solid material in the center I hoped that it would be as strong as the solid 4x6. Mountain bikes carry weight well across a span, better than a human foot. The tire roll is less pounding than a running or walking foot. Although the bridge will be open to hikers and runners.

Thanks for the help guys.

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