Engineers of the OB needed (bridge advice)

I'm not an engineer but I would have 4, 2 on either side.

6k is a lot of weight for 8"

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how did you arrive at 6k lbs?

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The weight of my tractor, FEL, and rotary cutter

I doubt there is a span table for a piling/round post because they aren’t conventionally used for that. I couldn’t even find a span table for a 8x8 timber which is odd. There is a very rough rule of thumb to use 1” of thickness and width for every foot you are spanning, which would be a true 12x12 timber, but that’s not with a 6k rolling load. 8” piles are way light.

Spent my career in construction and Eng, and I have multiple bridges on my land in MS, and also low water crossings which support loaded log trucks (80k lb). Bridges that I drive my tractors over, 1 has 18”x24” beams with 3x12 decking and the other is an 18 wheeler float trailer.

If it was mine, I’d use 18x12 (18” in vertical) minimum with 3x12 decking, all treated at .80 pcf. And make sure the bridge is in a straight section of the creek, or it will wash out. Broken concrete or rip rap on the upstream side of the bridge will help it not wash but it still should be in a straight run of the creek.

Some guys I know bought an old flat bed semi trailer that was in rough shape. Cut the axels off and drug it across the creek they were crossing with a tractor and FEL.

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There was actually an Auburn masters thesis done for just what you want….

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da Vinci invented the self supporting bridge in the 15th century



OP, get a flat trailer and use that. It should last forever or close to it

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See if you can find a used flat rack shipping “container”

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We have a few bridges like this in the woods. None are only 2.
Dig in 24inches. Sill plates on each end. Three 8x8s laying on gravel parralel to the ditch. Four 8x8s on top running across the span. 3x12 lumber on top. Use bolts or large spikes to attach these cross boards... Which will also greatly increase the beam load capacity.

It will support 6k easy.

I would add significant blocking between the beams just to avoid any racking.



Edit: not an engineer. I had a german grandfather with a penchant for tasking his young and teen grandsons with constructing projects like these. In his words, an engineer is for when you care about material cost. We built a wooden bridge that was able to support a john deere 550 dozer routinely.

I believe the oldest of us was 16 at the time.

Using an old trailer or container is going to save you time, money, and doing it twice. If you insist on building it, the foundation on each end will be what makes or breaks the project over time. If it moves or twists at the ends, it won't matter what size timber you use. You avoid this problem with the old trailer because its already engineered to support the weight in its own design.