cheap to build bridge to access woodlot ?

On the utility poles it might depend on what they have. Some utility poles are not that big and I'd hate to build something and fall through with a load on. Been there; done that.

Why not just make your own with the trees you already have? Then lay your 2x decking on that. You also can lay more than 2 poles; double them up. It is cheap but you may have to replace the poles after maybe 10 years or more.

semi trailer

I always think culverts are better. Why not get a larger diameter one that would not clog as easily. They are cheaper than trailers, probably a couple hundred, you only need a 10 foot wide one. To me dirt and gravel are cheap to put over it and if done properly could last for decades. A bridge would be cool but more issues.

Looks like there is a typo in the value for the span. Please tell us the correct span.

In addition, please tell us something about the soil/banks and what you are willing to do for abutments.

Culvert will channel the water past the bridge. Just about anything else will obstruct the water, and water always wins, sooner or later.

I'd locate a suitably graded approach, excavate it down to hard pan, back fill with suitable riprap, now you have a drivable underwater bridge. Made one threw our swamp that way but didn't have a 6' bank. Now life is good for us log draggers

Thank you for the additional info. The US Forest Products Laboratory's Wood Handbook has equations that can be used for a log bridge with decking.

I built a simple log bridge for a 24 foot span. I used two treated utility poles with 15" diameter at the small end and placed them under the tire treads. 2" x 10" (full dimension) rough sawn softwood decking. The decking is undersized and I should have used at least one more utility pole in the center. I'm careful where I drive. Deflection is less than an inch at a total live load of approx 3,500 pounds (communicated to the bridge through the four tractor tires). Abutments are nothing special and that is a weak point of this bridge. I used gravel for the two approaches. The decking on this bridge will rot sooner than later....

Used poles, in excellent condition, cost me $100 total for the two. Decking was $225. Galvanized pole barn spikes, $25 or so.

I am not a civil engineer and do not give engineering advice. YMMV.

savageactor7 said:

I'd locate a suitably graded approach, excavate it down to hard pan, back fill with suitable riprap, now you have a drivable underwater bridge. Made one threw our swamp that way but didn't have a 6' bank. Now life is good for us log draggers

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Be careful doing this, in many states you need special permits (yes, even on private property) to make a drive-through stream crossing. Not getting the proper permits can get you in a lot of trouble and cost a lot of money, especially if it has been determined that driving through the stream somehow contaminated it. I would advised against doing this if at all possible. The only time I've seen this as a practical method is if you have a really wide, shallow stream where a bridge is just going to be too costly.

My dad is a drainage contractor, he knows the ins and outs of a lot of that stuff and it's rubbed off on me a little.

We (Snowmobile and ATV Clubs) build bridges all the time . . . and most of these bridges are designed to carry the weight of pretty heavy equipment (typically the weight of a groomer -- a Ford Ranger, snow drag and of course the weight of the snow) . . . most of the time we've used old telephone poles (or similarly sized cedar trees) as stringers for a distance that you've mentioned.