How to Build a Log Bridge

Instructions and diagrams for constructing a simple log bridge.
By the MOTHER EARTH NEWS editors
May/June 1984
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"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.

The Base 

The design of the foundation on which you put your main bridge logs, or stringers, will depend on where you're building. Ideally, you'll have steep banks that rise well above the stream you're crossing and are close enough to each other to give you an acceptably short span. In such a case, dig ditches about four feet long into both sides of the bank to receive your logs. Then lay some good-sizedsay, 6"-thickflat rocks in the troughs and set your stringers on those (Fig. 2). Or, if you don't have the rocks, you can pour cement for these "footings". And try to design all this in such a way that the logs end up a little bit above ground height. That way, you'll have to "ramp" up slightly to the bridge itself, which will help the structure shed rainwater.

If your banks taper too gradually down to the stream for that "best case" solutionor if your span's a bit longer than you'd likeyou can build out or up from the sides of the stream bed using crib construction. Simply assemble a three-sided wall of logs on each creek sidewith the open end stuck in the banksort of as if you were making a little log cabin (Fig. 3). Be sure to notch out half the diameter of each timber where it lies on top of the one underneath so the sides of your crib will fit together. You can do this roughly by eye or make accurate full-scribe, bowl-shaped cuts. (Information on the latter technique should be in any decent log-building book . . . or you could see An Introduction to Log Construction.) Fill in the opening behind each layer of logs with gravel and dirt as you go up. Then stack "riprap"that's local quarry talk for big rocksaround the up- and downstream sides of the crib to help reduce the erosive effects of the stream.

Crib construction is also useful for raising the height of your bridge if you're spanning a stream that gets an unusually high runoff from heavy rains. And how can you tellahead of timeif you have such a flood-prone creek? Watch the stream when it's high, talk to neighbors about how much it rises, or—best of allconsult your local soil conservation service. The folks there should have topographic maps showing the entire watershed, and will likely be capable of giving a very accurate estimate of the peak flows you can expect to see.

By the way, Rick builds his cribs with 90° angles, so each enclosure looks like three sides of a box. Some other folks in our area make their supports with 60° angles (then they look like the top halves of hexagons). And if your soil's too muddy for a wooden crib to hold up, you'll just have to pour a concrete base and use reinforced blockin other words, build a retaining wallto support your bridge.

The third crossing situation you're likely to find is the one Rick recently tackled out at the MOTHER EARTH NEWS Ecovillage: crossing a stream that has very low flow and quite shallow banks. In this instance, you can't lay your logs in ditches (they'd be too low), but you don't want to build up a crib (which would put them so high you'd have to raise the road).

To handle this sort of problem, Compton digs out the streambed as much as he can (to lower the water), and then raises his bridge a bit by setting his stringers on bed logs that run parallel to the banks (Fig. 4). The bed logs are laid in gravel-filled dirtfor drainageand leveled on top. They should also be high enough to keep the bridge a couple of feet above flood level. 

Whichever technique you end up using, you'll probably want to make sure your stringers will lie fairly level. Actually, it won't hurt if you have to drive a little up or down to get across your bridge. But you don't want a structure that's sloped enough to one side to cause a car to start sliding off on an icy night! (One fellow we know did build his bridge with a slight downward lean to the upstream side because he knew it'd get flooded occasionally and wanted that high water to rush over—not push up under—his bridge. And so far, his span has survived the location's seasonal flooding.)

Stringers 

OK, your base is built. Now it's time to get those creek-spanning logs in place. Rick used four stringers for his bridge, setting them in pairs whose centers were 6 1/2 feet apart (the width of a pickup truck's axle). He figures that if his bridge were going to be used solely for cars, one log under each wheel track "would have been God's plenty". On the other hand, he knows loggers who run three big timbers together under the wheel paths of their trucks.

Naturally, other folks have other ways of doing things. Some people around here will use seven logs, all spaced the same distance apart, for their bridges. Why, if you go up to timbering country in Alaska, you can find bridges where a whole bunch of logs were cabled together like a raft and then surfaced with small rocks. And some of those mighty structures are 80 to 100 feet long!

However many logs you use, getting them across the stream probably won't be an easy task. If your banks are steep, you'll do best to get your hauling vehicle on the far side from the log. Lay a heavy plank down the slope of that side and then pull your log until its front end falls onto the board (Fig. 5). You should be able to drag the log up that board without a lot of trouble (especially if you beveled its lower edge like we mentioned earlier).

You say you can't get that tractor to the other side yet? (That's what the bridge is supposed to be for, right?) If so, run a long cable from the log to a snatch block hooked up high on the other side and back around to your pulling vehicle on the near side. Then go ahead and yank away!

In Rick's case, the banks and water were so low that he and Hoy were able to just work each log across with peaveys! To pivot a log, Rick would hook a peavey into one end and then start rolling, while on the other end Hoy stuck his tool in the ground under the opposite sidelike a steep rampto hold that end in place (Fig. 6). Other times, one man would lever one end up a bit by wedging the tool just off center under one end and lifting. And when there wasn't any easier way out, they'd both hook their peaveys into opposite sides of one end, lift, and then drag the log a short ways.

After all four logs were in place and the two Ecovillage crew members had taken a short breather14" X 14' green locust logs are heavy!—they started to flatten the rounds' tops to produce a good, level flooring surface. The first job here was getting the bark off the tops of the logs. (Some folks feel you should peel all the logs' bark off, since the rounds'll dry faster that way and dry wood is stronger than green.) This job can be done with an ax, but it goes a lot faster with a homemade bark peeler (Fig. 1). Rick's is simply an old truck spring that's been sharpened on one end and welded at a 30° angle to a piece of pipe. The whole thing's attached to an old shovel handle.

Once the stringers were skinned, Rick and Hoy focused on making sure the flattened tops would all be even and level, sloping neither to the upstream nor downstream side of the bridge. To do so, they first nailed a spare 1 X 8 board onto the log ends at one bank. This plank was carefully leveled and set so its top edge marked the desired cut-as-little-as-possible trim line on the stringers (Fig. 7). They then nailed another 1 X 8 on the other end, making sure that it, too, was level and the same height above the bed log on its side as the first board was on its side.

It was then a simple matter to snap an accurate chalk line on both edges of each stringer by pulling that colored cord from top to top of the 1 X 8's (Fig. 8). One word of advice, though: Do your best to draw that chalked string out horizontally when you're snapping it, or the line it marks may be off level.

With that done, Rick cranked up a chain saw and scored each log by cutting level groovesacross the stringerrunning from one chalk line to the other (Fig. 9). He made those "slices" every two inches or so down the log . . . but if you're sawing a wood that's softer than locust, you can space the cuts farther apart. Then he started knocking out the in-between wood sections. Although Compton mainly employed the back of an ax for this job (Fig. 10), he says, "Whop 'em, chop 'em, adz 'em, ax 'em. Just get them off somehow."

Hoy then followed with an adz to further flatten out the log tops. An ax does work for this sort of large-scale chiseling, but the curved-blade adz is the more efficient tool. Indeed, Rick claims, "My grandfather could walk up one side of a poplar log with his adz, turn it over, cut back down the other side, and have it all squared up. He did have a pretty hefty adz, though." (Compton recommends using an adz in such a way that it carves "where the grain runs up out of the wood, not dives down into it".)

Rick and Hoy frequently checked the depth of their cutting efforts, either with a level or by setting the edge of a spare 1 X 8 across all four logs to make sure there weren't low or high spots in any one timber. Once they got fairly close to being done, Rick "sanded" all the tops smooth . . . with the chain saw! (See Fig. 11.) "Smoothing a board with a saw ain't all that tricky if you have a fast-turning saw," he says. (Be sure you know how to handle a saw before trying this . . . and watch out for kickback!) "Just lay it level on your wood, rev the engine up and let it 'float itself'without putting much downward pressure on it—over the top. And try to keep the saw from getting at a right angle to the grain as you work it back and forth, or it'll tend to dig down in."

Using that "delicate" finishing tool, our chief bridge-builder soon had all four stringers level, even with each other, and smooth enough to dine on. Once he was satisfied with the surfaces, he got some rebar, a portable electric generator, a 1/2" drill, and a 16"-long 9/16" bit. He used all this to bore a pegging hole into each log end where it rested on the bed log. Then he drove a rebar section (cut to length with a hacksaw) down in place. Such a method, while a fine way to secure your stringer logs, is dependent on the somewhat rare luxury of having electric power available at your bridge site. Just in case you don't, you can secure runners to a bed log or crib by using a brace, a bit, and a lot of labor . . . by driving spikes into the base alongside the runners . . . or by notching the stringers out some on their undersides before you level their tops.

Flooring 

Once the stringers are laid, leveled, and secured, you can finally start making your bridge look like a bridge . . . by flooring it. But before you start nailing planks down, check carefully to make sure the flooring will be squared up. If you don't, by the time you reach the last board on the far side of the bridge, you may find your floor so askew that one end of your final plank fits over its stringer logs while the other falls off!

To determine his bridge's squareness, Rick used the 1 X 8's he'd nailed to the two ends of the stringersthey helped mark the leveling cuts, remember?and two strings tacked 12 feet apart (the intended width of the flooring) at the ends of those boards. He knew that once the shape formed by those strings and 1 X 8's was a true rectangle, he'd be all lined up for nailing. Ah, but how can you tell if the four sides really make a rectangle? Well, you do it by using that piece of old carpentry lore . . . seeing if the two diagonals across the box's inside are of equal length (Fig. 12). All Rick had to do, then, was to shift one or the other of those 1 X 8's slightly sideways until his diagonals did indeed match. After that was done, he took off one string, leaving the other as a guide for placing the ends of his floorboards.

With all that figuring out of the way, Rick and Hoy could get down to hammering their 3"-thick oak boards in place with the 6"60 penny—nails they had on hand (2" boards and/or 5" nails would also work). First off, though, they slapped a coating of roofing cement on the stringers at those points where a board would lie. That tar layer helps keep water from pooling between the wood surfaces and making the bridge rot faster. Then between planks, they stick a piece of 1/8" masonite on end to serve as a temporary spacer (Fig. 13). The gap thus created would enlarge to about 1/2"the desired water-shedding spacingonce the green boards they used had dried.

Now it was time to pull out a three-pound hammer and get to nailing (Fig. 14). While brawny Rick Compton can accurately drive a 60-penny nail through oak and locust in as few as four whacksor even two when he's using the back of a splitting maul—many builders may find the going a bit more laborious. If you, like some of us, occasionally bend some nails, just bring to mind Rick's lighthearted words of advice concerning this mishap: "The instructions said, 'Hit the nail on the head!' "

Finishing the Log Bridge 

When the hammering was all done, Rick trimmed both ends of the boards by marking his cuts with the chalk line and then running down those lines with a chain saw (Fig. 15). Next, he and Hoy removed the 1 X 8's at the bridge's ends, trimmed those log ends even with the flooring, and permanently nailed extra flooring boards in their stead (Fig. 16). These planks closed off the ends of the structure so that, when the two men filled in the road up to the bridge, less dirt and rock would get shoved down under the crossway. The pair then heaped in rocks, gravel, and dirt at the ends"ramping" the road up slightly to the bridgeand packed it all down with a tractor.

Their job was done! In four days, Rick and Hoy had built a log bridge to be proud of.

The stream-crosser Compton and Gross constructed is simple and functional. If for aesthetic or safety reasons you want to add handrails to your bridge, you can build them easily enough out of 2 X 4's. Your arm supports will be a lot sturdier, though, if you plan ahead by leaving extended boards at regular intervals along your bridge when you're flooring. That way, you can easily add outside braces for the railing (Fig. 17).

Life Expectancy

It took four days' work to build our bridge . . . but how many years before wear and tear destroy it? There are two answers to that one. The flooring should go first. Rick's best guess is that oak boards can be expected to last from five to ten years before they'll need to be replaced. ("Mother Nature'll control that for you.") Just remember: The more steps you take during construction to help water stay off the boards, the longer they'll last. In fact, we made the mistake of ignoring that rule ourselves. On one bridge out at the Ecovillage, we laid unprotected "wheel track" boards right on top of the flooring. The wood rotted so quickly where the board layers met that we had to replace that bridge top after just three years!

The life expectancy of the stringers will vary widely, depending on the type of wood you use. "Good locust logs might even last 50 years," Rick projects, "but some other woods . . . well, that's your guess." In other words, when that bridge starts creaking as you drive over it . . . when the nails come out easy . . . or when the logs seem "punky" if you poke around underneath with a screwdriver, it's time to replace the stringers.

Whatever you do, don't take your bridge for granted until it's so far gone that somebody has an accident. Of course, if you go to all the work of cutting and hauling the logs . . . building up the base . . . laying and flattening the stringers . . . nailing on all the flooring . . . and trimming up the sides and end of your own log bridge, you'll probably never take the structure for granted. You'll be too busy appreciating it . . . every time you drive across. 

Bearing the Load  

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 chart (see Image Gallery), we tried to indicate the log diameter needed to hold a concrete truck loaded with eight yards of concrete (a total weight of around 65,000 pounds) over spans of 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 of 1.4 (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 (see Image Gallery) from Benson's formulas for a two-point load—the front and back wheels of the truck—distributed equally across a beam. 

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

black locust 3,600 psi*

white and northern red oak 2,200 psi**

Douglas fir and southern pine 2,100 psi**

eastern and Sitka spruce and other pines 1,400 psi*

*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 of 3.5 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 a chart (see Image Gallery) for theoretical log diameters necessary to hold a quarter of a 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! "I 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 of any 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 of fertilizer 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 worn out 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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