HOMESTEAD HANDBOOK: HOW TO BUILD A SIMPLE LOG BRIDGE

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WHAT WILL IT HOLD?

The toughest question we had to face in working up this article was also the most important one: How much weight will a simple log bridge hold?

There's no way we can answer that question for you. You are responsible for the bridge you build — after all, you're the one who's going to be crossing it. So don't cuss at us if a loaded manure-spreader falls clear through your 33 foot poplar span.

Still, we can share the numbers we came up with for ourselves. (As far as we know, no currently available book gives a bridge span table for plain log structures.) However, if you use this ` predesign" information for anything, you should check the data, assumptions, and calculating methods for yourself.

First off, a lot will depend on the load you plan to place on the structure. What are you going to takeover it? The family pickup truck? A tractor with a front-end loader? A bulldozer? A silage truck filled with ten tons of corn? Equally obvious, you'd better be sure the bridge will be on your private property and intended solely for your personal use. If you plan to build one on a public road — no matter how far in the backwoods — you'll have to bring in a county or state road official before you can even start.

In the following chart, we tried to indicate the log diameter needed to hold a concrete truck loaded with eight yards of concrete (a total weightof around 65,000 pounds) over spansof 10 to 24 feet. We derived our formulas principally from Tedd Benson's Building the Timber Frame House (Charles Scribner's Sons, 1980). Since Benson's calculations were for square or rectangular green beams, we incorporated an extra factor of1.4 (the ratio of a circle's diameter to the depth of the largest square inside it) into our calculations to compensate for the fact that we were using round green logs. That correction factor allowed us to transpose log diameter for beam depth in his equations.

We made another important assumption, too. If the structure is designed with two logs under each wheel track and the truck is centered as it goes over the bridge, each of the four logs will have to bear only one-quarter of the total load, or 16,250 pounds.

The important things to consider for calculations were the maximum imposed load a log would have to bear (we'll call this M load) and the maximum design resistance of a log (we'll label this M log). We derived our equations from Benson's formulas for a two-point load — the front and back wheels of the truck — distributed equally across a beam.

Here's what we got:

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