Planning a bridge

If it were me. I'd go with one of two options.
A simple timber bridge or a transverse laminated deck.
Simple timber bridge would be made up of 12" butt timber piling driver to the point of refusal. Use three on each end. You can then use a single cap. 12x12 or a double cap bolted to the sides. Say three 2x12's nailed together then through bolt with a length of all thread rod. I'd go 3/4 to 1".
now for a simple timber bridge I'd use three 2x12x16 nailed together every 16". Then use a double 2x8 deck.
For the transverse deck. Use 2x12x16 set on edge across the entire width of the bridge. Nail together and insert 5/8 rod every 2'. Use an og washer and nut on rod. Shoot me an email and I'll send drawings. rich@creativebridgedesigns.com

We had to abandon the project due to my husband developing severe back problems. Thanks anyway.

Have you thought about using an old flatbed railroad car.
I helped a friend pour the pilings on each side of the creek and then we used a trackhoe to hang the car across the span.
Over the years it has endured many 13 yd. loads of concrete crossing it.

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That which is tolerated by the first generation is magnified in the next.

CIW