I am not following what you want to do, but a couple of comments.

Red oak is a very poor wood for exterior usage. White oak is much better. Often used for things like swings. But it not considered an exterior construction wood.

"rather sinple 4" x 4" I beams then 3x6 or 2x6 boards"

4x4 is very small beam. Most of them are multiple 2x10 or 2x12's, but it depends on the loads.

setting my steel I beams at 8' 1/4" apart on footing columns. then all you do is slide your boards in the track the beams make. This type of construction is usually done verticaly for drop shafts or retaining walls. but i need a board strog enough to cope with an 8 ft span. thank you