I am not an engineer, but I like to build and I prefer them to be safe, so what I do is apply a little logic to my designs.
In the case of round wood it is pretty easy to find strength formulas by converting the round logs to beams. Lets say you want to span a 12 area with a 10 inch diameter White Spruce log; is it up to the task?
Well if we were to look at the weakest point of the log, the small end, and draw a square, we would most likely be able to get a 8" square box. Now 8 inch timber load bearing tables are easy to find, and it shows that an 8 inch beam with multiple point load can hold 4500 pounds for a 12 foot span. That is quite a lot.
Now the real world test.
I know for instance, that my bulldozer weighs 10,200 pounds. Being on tracks it would be a multiple point load, and so while I probably could cut (2) 12 foot long 10" logs and safely cross a 12 foot stream, for extra safety I might reduce the stream crossing to only 10 feet, just to be sure.
That gives me a quick, real world determination of how much the logs I am using can support. From that I can reduce my spans, or increase my logs diameter. Now I used my bulldozer as an example, but it could be any known weight, like your ATV, your car, etc.
I know a structural engineer might balk at this, but they are paid to design structures to hold certain loads at the cheapest possible costs, where as we are just trying to ensure we build things safe
enough with the materials at hand. We don't have to over-think things here.