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I have to build a bridge across a creek on my property. The creek banks are 3 feet above the creek and 14 feet wide. I need to be able to get a 2500 pound tractor over it. I was planning on bolting together three 2x10-18' to form a beam and then construct 4 of them. The bridge will be 5 feet wide and decked with 2x10s also. Will this construction hold the tractor? I tend over engineer and don't have an issue with this construction will work and is overkill.

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    Why bolt together dimensional lumber instead of getting LVLs or similar? The cost difference likely isn't huge once you include structural bolts and labor. Plus I suspect you need to special order dimensional lumber that size.. why not order an engineered product instead?
    – Matthew
    May 29, 2020 at 3:06

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I just ran some structural checks and everything passes if you use Douglas Fir #1 planks for the deck. Anything lower quality will not work structurally.

Beams are fine as is, just make sure they're bolted/screwed together well. Also, make sure that the wheels on either side are centered between the two beams. Don't drive it directly over a single beam. This way each beam is taking 1/4 of the weight. You do not have something that's "over engineered", it's close.

Regarding the soil, the further away you can get from the soil drop off the better. If you can get 20' long beams, do it. I also recommend placing a continuous 2x10 under the end of the beams to act as your foundation, otherwise you may rip up the soil there. enter image description here

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    Did you account for “impact loading”? What does that do to the stresses?
    – Lee Sam
    Apr 29, 2020 at 4:13
  • What would happen if I used 5 beams instead of 4. Does it make sense also to use 2X12s. Additionally I planned on going with 20' lengths after I posted. Thanks
    – Walt
    Apr 29, 2020 at 13:50
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    Impact loading isn't directly taken into account in my calculations. However I am designing for a live load here, so there's so strength reduction that goes along with that. I also idealized the loading as 2 point loads, not 4 because sometimes objects like this are much heavier on one end versus the other.
    – represton
    Apr 29, 2020 at 13:55
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    @Walt Regarding your question of adding a beam, how far are the wheels on the same axle spaced apart from one another? Using 2x12s for the planks would be a much better idea, this would allow you to use Doug Fir #2 instead. I don't think it'd make much of a difference for the beams unless your wheel spacing doesn't correspond to the diagram above.
    – represton
    Apr 29, 2020 at 13:58
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    @Walt Why not add two planks on top, both parallel to the beams, and spaced so that the tires fit between them? Safer structurally and no chance of running off the side.
    – Phil Freedenberg
    Sep 27, 2020 at 23:53

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