How much load can a 48" (wood) board support if it is supported in middle, but unsupported for 6 inches at ends? It is for a 600 lb mower that must go over a wooden bridge we will are adding width to. The tires of the mower are 45" wide. Could we use 1 x 6" pressure treated decking? The weight is more on the back 2 tires. The 600 lbs includes the person on the mower. Can we get away with 1" X 6"?
Assuming reasonably equal weight distribution, each wheel of a 600 lb. mower and a 200 lb person would carry 200 lbs. Any 2x4 or wider SPF lumber that's reasonably free from large knots will suffice.
The actual calculations are difficult due to the unknown weight distribution (front/rear, tire width, tire pressure, etc.), but experience tells me that you certainly will not break 2x4 or 2x6 (or larger) lumber in that application, especially if you were to run a sleeper under the ends to tie the boards together and distribute load. I consider that overkill, though, and I'd only bother to do it if you see substantial flex in the individual boards as you roll over them.