I need to get a tractor across a small stream on our property. What can you suggest? Thanks
–Stranded in Horseshoe
Dear Stranded –
I faced a similar situation a year or so ago. We had just installed a wood furnace to heat our greenhouse and a corner of our woodlot was across a stream that we did not want to drive through repeatedly. A medium-sized locust tree fell into our orchard during a wind storm and we decided to put that asset to use in solving our firewood problem.
Locust is amazingly rot resistant – an important feature for a bridge, particularly for the parts that will be in contact with the soil. Treated wood is the standard approach to this problem but many of us would rather not be around those chemicals. We have locust fence posts on our land that appear to have lasted close to 100 years.
I searched the internet for ideas on designs and one of the first hits was an article originally written in your community of Horseshoe NC at the Mother Earth News Ecovillage. Mother Earth News changed ownership since its founding in Western North Carolina, but in the late 70s and early 80s it hosted an Ecovillage that offered classes and demonstrations. Many of our early organic leaders are veterans of that facility and their work continues to be helpful.
The problem faced by the Ecovillage staff (Rick Compton and Hoy Gross) was transporting a loaded cement truck across a small creek. I am confident that your tractor weighs less than the 65,000 pounds that their cement truck weighed. They used four 14” diameter logs. A very useful chart provided in article’s photo gallery above is a graph of the strength of various types of logs with various distances between the bridge piers.
Source: The Mother Earth News (see link above)
They placed the four logs so that two would support each set of wheels and applied a deck of two inch boards. Here is their proof of concept demonstration.
They do not mention in the article if they had a structural engineer review their plans but I found the photo above convincing. If others will use your bridge, it probably makes sense to have a qualified professional review your plans.
My locust tree was 8 inches at the top – much less than the 14 inches that they used. Their graph indicates that even those smaller logs will come close to supporting that huge truck across my shorter 12 foot span. We used locust logs on each stream bank perpendicular to the span of the bridge as log supports at each end of the bridge with shallow V-shaped notches to hold the logs in position. A large lag bolt tied the four bridge logs to the support logs. Our bridge deck is the branches of the locust tree and we placed boards to over those branches to make the ride a little smoother. The whole project took about a day with our crew helping put the logs in place and securing the deck.
This graphic is from http://www.pioneeringprojects.org/projects/bridges.htm Doomsday preppers also have an interest in bridge building including some fairly interesting drawbridge designs. Here is a link to a fairly comprehensive review of improvised bridges from around the world, including the photograph to the right.
As you probably realize, our streams rise from time to time and wooden bridges resemble wooden rafts that may be headed downstream in extreme weather events. Consider that situation and how to manage it. One source suggests cables to upstream trees which may allow the bridge to rise during extreme high flows and then settle back in roughly the same location. If your bridge moves during a flood, your bridge materials may threaten other structures downstream so the neighborly approach is probably to plan to keep it in place.
If your bridge building involves altering a “navigable water of the US” a Corps of Engineers dredge and fill permit may be required.
As with all farmer-to-farmer advice offered by OGS, please consult qualified professionals before making decisions that may affect safety or welfare of you, your farm crew or the public. Be sure and let us know how it goes.
Ask Tom © 2013 Tom Elmore & Organic Growers School
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Author: Tom Elmore
Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.