A few years ago on this board there was a guy that put a bridge over the same type of small waterway. I recall he got a small county bridge and moved it to his property!
That's pretty much what the previous owners of our farm did: they salvaged I-beams and decking timbers from a bridge the county was replacing. They built footers, unfortunately out of concrete block, welded up the I-beams, unfortunately not square, set the I-beam frame on the footers and set the deck on top of that. It all collapsed into the creek a few years after we moved in.
Contacted the "bridge guy" at the regional highway shed up the road. He originally recommended the culvert approach, but this bridge is right in front of our house and carries our driveway. I wanted it to look good, as well as carry heavy loads (lime trucks, concrete trucks, whatever). I came up with a plan and the highway guy helped me spec the footers for it.
First step, remove the old deck and frame intact, the crane came, lifted it up, and swung it 90 degrees into the paddock where it waited until it was time to put it back:
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Second step, carefully measure the old frame and pour footers to match:
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The footers were poured in several stages: bottom of first footer, bottom of second and top of first, top of second. You can see why- the stream had to be diverted and the footer excavation pumped out. BTW, the PVC pipe is for the telephone line.
Finally, the crane came back and the frame and deck were put back on. Bullet-sweating during the project was caused by first, will my forms hold (they did, but for something like this they CANNOT be too sturdy- concrete delivery driver regaled me with great stories of DIY frames giving way in the middle of the pour), second was did I lay out the footers properly to accommodate that irregular frame? Worked out OK.
Since I put it in it has had 12 ton/axle over it, and now it looks like it has been there forever. Even the highway guys came to look and congratulated me on it, so I ended up pretty pleased. Took about 2 1/2 months, and the hardest part was working the concrete when it was poured, pretty hard physical labor.
Anyway this is one approach to think of! I'll take a pic today if you want to see the finished (weathered) product.