I think "medium duty" is the most common term for a load bearing cover, but "medium level/grade" Could work. Manholes covers are different everywhere so I don't think there is a set international standard grading strength levels.

http://www.nationalmanholecovers.co.za/dwnld/product_range_A...

http://www.cotterillcivils.co.uk/products/manhole-covers-and...

http://www.kkindia.com/manholecovers.html