I am in the planning stages of a bridge that I intend to build over a creek at my hunting property, actually 1 of 2. This bridge will span 20' and be 10' wide. The use for this bridge will be ATV/UTV and tractor use. My tractor will be the heaviest item to use the bridge with a weight including tractor and implements approximately 11k pounds. I have placed a pipe in the creek and it has held up fairly well but the limbs, leaves and other items coming down the creek are clogging up the inlet side and I am looking for a better long term solution as I do not live on the property and do not get to visit as often as I would like.
Here is what I am thinking...I plan to use concrete on both sides where the beams will rest. The foundation for the beams will be poured into the banks on either side measuring approx. 18"x18"x12'. The beams will be anchored to these and will span the 20' across the creek. I plan to use 3 pieces of plate steel in between the beams to anchor them all together to prevent rolling or shifting. I plan to use approx. 35 4"x6"x12' pressure treated timbers laid horizontally spaced approx. 2" across the I beams. I will then place 3 2"x8"x20' PT boards across the bridge at the location of the tractor tires. All fasteners will be hot dipped galvanized.
The only item that I am unsure of is what size I beam do I use. I am considering 4" or 6". While I am no engineer, I would think that a bridge designed and built like this would hold more weight than what I need. It would also open up the creek under the bridge to prevent obstructions from stopping the flow of water. Any input or suggestions would be welcomed. I have attached a drawing of what this bridge would look like.