The real problem is with the I beams used. There was quite a bit of talk about the other components of the bridge, but it does not really matter. They are sitting on (4) 10 inch I beams only 3/8 thick. The thickness is not so much of a concern, as much as their depth. 10 inches is not that strong.
But the real, real, real issue is: you have a trucker that will not cross it, and I doubt that would change if you had built it out of 20 inch deep I beams. So, you really need a new truck driver.
Or, have him dump on this side of the bridge, and use your tractor and bucket to ferry it over the bridge, as long as the distance is not that far.