HOMESTEAD HANDBOOK: HOW TO BUILD A SIMPLE LOG BRIDGE

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Rearranging this new equation to get a minimum diameter, we find that

The values of f for the woods we examined
are the following:

These numbers were derived by correlating ones given in Agricultural Yearbook of Standards — 1983 for other woods with those in Wood Handbook: Wood as an Engineering Material (United States Department of Agriculture, 1974).

"These numbers come from Agricultural Yearbook of Standards-1983.

All the f values incorporate a factor of3.5 to allow for the fact that you're using green and probably not quite perfect (so-called clear) logs.

By putting all these numbers together, we came up with the accompanying chart for theoretical log diameters necessary to hold a quar ter ofa 65,000-pound concrete truck for 10- to 24-foot-long bridges. (Note: Since the distance between the front and first set of back wheels on most concrete trucks is 12 feet, a bridge under 12 feet long will bear only the front or back wheels at one time, giving it about half the load calculated here.)

And just what does all this mean in plain English? Well, that's hard to say. Rick Compton admits that he's never done any such calculating for the bridges he's built! just don't mess with anything but black locust that's at least twelve inches in diameter," he says. "Besides, you can bet the driver ofany dump or concrete truck is going to get out, look underneath, and then tell you right quick if he'll drive across your bridge or not, no matter how you build it!

"Still, I have talked with lots of truck drivers who've gone over plenty of log bridges. I've done it myself with corn silage and ten-ton loads offertilizer any number of times. I even have a friend who braced poles under the center of his bridge to shore it up a bit to convince a cement-truck driver to go on over. "

On the cautionary side, Compton adds, "I've also seen a tractor fall through a bridge a time or two. That was always when crossing wornout structures, though, that needed to be replaced. In general, it's surprising what a sturdy log bridge will hold. Of course, if you want to build a big, long bridge — 30, 40, 50 feet — you'd best go hire yourself an engineer. "

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