Keep in mind that the DC20 strength check doesn't necessarily imply that you're breaking the strands - you might just be trying to get "unglued". In fact, I'd be inclined to think the web is a lot stronger than that, considering that there's no theoretical limit (aside from the spell's 40' diameter) on the creature you can net with it... DC 20 is a check any average man can make given enough time - which would make it a lot weaker than a good rope. If it can, at least in theory, hold a dragon, it should have no problems holding up a bridge.
So if you wanted to apply that weight formula to it, you'd need to base it off of STR 10 rather than 20, because STR 10 will, given enough time (around 2 minutes) break the web - which'll really bring the numbers down some.
