load limit for a 6"x8", 8' span?
I'm not sure I understand where the posts and piers are in relation to the 6x8's. However, I can give you some info and maybe you can interpret. 

According to Western Woods Use Book by West Coast Lumbermen’s Association, 1973 edition, a 6x8 beam can support about 8,000 lbs., depending on species of wood, grade, etc. on an 8' span.