Whilst on a recent visit to that there London recently, we had a little walk round Hyde Park. (Was going to have a go on a Boris Bike for the novelty, but I think you had to be a member...) Anyway...
Riding down the road towards us was a very, very large gent on a mountain bike with front suspension . The frame was one of those with a large spring in the tube down from below the seat mountings to the bottom bracket.
This chap was going pretty quickly, and the bike, whilst flexing, was holding up quite well. This guy was cracking on well over 25 stone. (And I'm usually fairly accurate at weight guestimating).
Don't know if this helps at all, because I've obviously no idea if the bike was suffering from cumulative stress...