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I'm interested in building a small access bridge across a creek on my farm. This bridge will be approximately 5m long and 3m wide and my tractor is about 4000 pounds. I read somewhere a long time ago that properly maintained, wood bridge supports can outlast steel bridge supports. I have to admit I spend most of my time "putting out fires" so sometimes maintenance is not high on the list of priorities on the place so once it's installed, I might not be coming back for a lot of maintenance. I'm building the bridge in a small bend in the creek so the bridge will basically be built on dry land and then dirt filled in around it to reroute the creek through the bridge. I'll have 6 supports for the bridge with 2 at each end and 2 in the middle. I'm in the planning stages and trying to decide whether to use wood telephone poles for the supports or steel columns. I have a steel structure I purchased at my local scrap yard that will basically be the frame that sits on top of the poles and then I'll attach wood cross members and wood runners the length of the bridge. All the support posts will be in the water. I know it's a pretty open ended question, but would wooden poles or steel beams last longer? When I say "steel beams" they probably won't be I beams or anything engineered of a certain type of steel, it'll be whatever I can find at the local scrap yard with varying amounts of rust. I have a natural tendency to over-engineer things so I'm not really worried about the bridge holding up initially, I'm more concerned about the longevity of the supports, how much maintenance they'll need and whether or not they'll outlast me at 56 years old. I just don't want to be 75 years old knee deep in water trying to repair something that I might have gotten an extra 10 years out of if I had made a different choice originally. :) Thank you for your time!

Pat

P.S. I suppose I could also consider railroad ties. I don't know if they're still using creosote in those or if that's been outlawed in the U.S. and I don't know if that's allowed in running water. I also don't know what the preservative is in the telephone poles I'd use as I'd buy them used off FB or Craigslist. Again, I'm more concerned about the long term maintenance and I'll do the additional research on the rest when I start leaning one way or the other. :)

P.P.S. I guess I could also bury some of those sonotubes and pour concrete down them, but this is a fairly remote location, would require trucking in bags of concrete from Home Depot or the like and would be pretty labor intensive. I do have an electric mixer and a generator to run it though, so that is an option.

Bridge Drawing

Bridge Steel Frame

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    $\begingroup$ Seems to me if you're very concerned about the long term maintenance, the worst part to have to repair someday would be the foundation. So I'd go with concrete for that, and maybe add some fiber to the concrete mix. Fiber is fairly cheap to add, and will go a long way towards making it last that extra decade you're after. If you do concrete, you'll want to clear out any loose sediment in the stream bed and have it sit on something that isn't going so scour out, like competent rock. Might install vertical dowels in the rock. If there's no rock, might have to do a deep foundation into the bed. $\endgroup$
    – Rick
    Aug 17, 2023 at 17:47
  • $\begingroup$ Ever give consideration to a culvert? Both precast culvert and Corrugate Steel Plate can have life expectancies of 75 years. These would both be engineered products, and by pass the the scrap heap option. Precast option you would need to prepare the bedding and drop in the culvert and back fill. With the corrugated steel plates, you they will deliver all the material to you site and your can assemble it yourself, then simply backfill. As for rerouting your watercourse, you know your situation better, but you may need approval to do that. $\endgroup$
    – Forward Ed
    Aug 19, 2023 at 5:18
  • $\begingroup$ take a look at government auction sites and see if they are selling any surplus modular bridge panels. You should be able to span 5 m pretty easy. Alternatively you also may be able to get away with something like recycling on old semi trailer chassis. $\endgroup$
    – Forward Ed
    Aug 19, 2023 at 5:21
  • $\begingroup$ My apologies for just now getting back to my question and I appreciate the responses! Putting out fires as usual..... :( All sound like viable options, I just have very limited access to the area and currently getting a large truck in there to deliver something is out of the question. I'll take some pictures today and try to come up with a sketch of what I'm trying to do and upload it this evening. $\endgroup$
    – Currie
    Aug 28, 2023 at 13:10

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Depending on the maximum seasonal speed and volume of the current in the creek and if applicable, the frost line, you need to make reinforced concrete foundations.

The depth and size of the foundation have to do with the quality of the soil, and you can try to search to see if there has been a soil report near your lot. In the absence of data, you can expect approximately 400–300 lbs. per square foot of bearing if the soil is compact sand and silt. So for your case as a very rough illustration:

$$4000lbs.*1.5 (safety factor)/ (2*400)=7.5 sq.ft$$ The foundation should be oval and elongated along the flow of water.

Basically, your foundation should do two things, provide bearing and protect the superstructure from water damage!

The superstructure can be made of lumber or steel. Both have to be built with proper bolts or fasteners, and steel needs to be covered by rustproofing.

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  • $\begingroup$ I apologize, my engineering knowledge is pretty slim. :) Are you saying I need 7.5 sq ft of concrete actually sitting on/under the ground to support my 4000 pounds? So if I've got 2 concrete columns on each end and 2 in the middle, I divide 7.5 sq ft by 6 and come up with a column base sitting on the ground having a diameter of approximately 1.26 ft? I apologize if my understanding is way off. I'm not dumb, but I am pretty ignorant. :) I think our frost line is about 6" and the seasonal speed and volume of the creek is pretty small. We do have storms sometimes that picks it up though. $\endgroup$
    – Currie
    Aug 28, 2023 at 13:22
  • $\begingroup$ no! Each concrete foundation should be roughly 2 feet by 4 feet, the length parallel to the flow. The tp surface of the footing must be higher than the level of high water! $\endgroup$
    – kamran
    Aug 28, 2023 at 15:30

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