Yamo, have you thought about basing encumberance on casual Strength instead of on normal strength? I mean, people historically did not take a trip down to the tavern or to the next town in 8PD armour.
(edit) After re.-reading this I thought that I needed to clarify.
Casual Strength is what you can carry and lift without thinking about it, which is perhaps, what a D&D fighter going around nearly all the time in his armour represents. We do not see real world soldiers wearing armour when not at war for lots of reasons already highlighted in this post.
Fighter-lad with 20 Str now has 10 str for the purposes of casual everyday-wear-armour. The armour that he wears "all the time". He could augment that when going to war and take the appropriate penalties inherent in heavy armour and having ten str instead of 20. Is that more clear?
Otherwise, you, as a GM, have to impose some of the problems of walking around 24-7 in full battle armour.