re: Engineers of the OB needed (bridge advice)

A 8"X8" Southern Yellow Pine post has a compressive strength of about 15 tons in a laboratory. A 8 inch diameter pole is about 3/4s of the area of a 8 inch post so a post is probably somewhere in the neighborhood of 12 tons...figure a safety factor of 50% somewhere around 6 tons. The problem of course will be how the poles are fixed, braced and how the structure is attached to them. The lateral forces are where the devil lies...with only 2 posts any lateral movement due to anything being out of square would cause a leveraging effect from the load. A single 4x4 post would easily "support" a 6000 pound balanced load fixed in place...if the load started shifting any at all the whole damned thing will likely topple....it won't crush the post, it will simply push it out of the way until the load stops moving.