How to Build a Log Bridge

Instructions and diagrams for constructing a simple log bridge.

| May/June 1984

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    Rick Compton and Hoy Gross build a log bridge at the MOTHER EARTH NEWS Ecovillage.
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    Calculate the log diameter needed to hold the load you plan to place on the structure. 
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    Rearranging this new equation to get a minimum diameter, we find that: 
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    Calculations. 
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    Theoretical log diameters necessary to hold a quarter of a 65,000 pound concrete truck for 10- to 24-foot-long bridges.
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  • How to Build a Bridge Support Graph

"I tell you what. I'd just about bet my little pickup against a pocketknife that this thing'll hold up a cement truck."

That was Rick Compton's wager about the 14-foot log bridge he and Hoy Gross were finishing out at the MOTHER EARTH NEWS Ecovillage. And if you're curious to see what took place when we did drive one of those massive vehicles over Rick's completed structure, see the Image Gallery. Go ahead . . . we'll wait.

Pretty impressive, huh? Well, don't let Compton's success go to your head. We're going to tell you everything we can about how to build a log bridge. A simple,"no suspension or trusses just lay down some logs and flooring" bridge . . . the kind of farmstead structure that's intended for getting you over drainage ditches or small streams. In doing so, we'll call upon wisdom gleaned from Rick's considerable experience, and all the relevant research and calculations we could muster. But let's get one thing clear right now: We don't guarantee that any bridge you build will bear up to a load like a concrete truck. So if you start to drive the front wheels of an extremely heavy vehicle over your stream-spanner and you hear something begin to crack . . . BACK UP QUICK!

Main Tools and Steps 

An untrussed bridge is simplicity itself. Step one: Build a good base on each side of your waterway. Step two: Lay some stringer logs across the span. Step three: Add flooring. (Mind you, there are a few extra touches and tricks of the trade we'll be telling you about.) The main tools you'll need are equally basic: a tractor or pickup truck with a chain for pulling logs . . . a chain saw . . . an ax . . . a bark peeler and adz (if you don't have these, you can use your ax instead) . . . a chalk line . . . a level . . . a couple of scrap 1 X 8's or similar boards . . . some roofing cement . . . a shovel . . . a couple of peaveys . . . and a heavy hammer (see Fig. 1). You'll also need a helper with a stout back (bridge building is definitely a two-person job).



Of course, you'll need some logs, as well. Around these parts, folks generally use the amazingly durable black locust. But plenty of other woodsassuming they've got more heartwood then sapwoodcan do the job. Bald cypress (old growth), Arizona cypress, catalpa, cedar, chestnut, juniper, mesquite, several of the oaks (bur, chestnut, gambel, Oregon white, post, and white), redwood, Osage orange, and Pacific yew are all quite decay-resistant (especially the last two). You say you don't have any of those? Well, bald cypress (young growth), Douglas fir, honey locust, western larch, swamp chestnut oak, eastern white, longleaf and slash pines, and tamarack are all moderately durable. However, if you're stuck using alder, ash, aspen, beech, birch, buckeye, butternut, cottonwood, elm, hemlock, hickory, magnolia, maple, red or black oak, most pines, poplars, spruces, sweet gum, or true firs, you'd best inspect your bridge logs often!

Once you've cut the best and straightest logs you can findthey should also be at least a foot in diameter (not counting the bark) and have no knots in their middle halvesyou can haul them to your bridge site. (If you bevel the underside of each log a bit first, though, the log will have less tendency to jam in the ground when you're pulling it.) Then you'll be ready to prepare a resting place for those timbers.

netmedic
6/9/2016 9:14:49 PM

8 pages and the only images are of an axe and couple of other tools. Talk about "a picture is worth a thousand words". It's a decent article spoilt by lack of drawings.







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