Excuse me Ubercrap for using your picture without asking ...
I am no metal expert, just remember from school (some 30 years ago) that stress can cause corrosion...if the sills have given up the load/tension has to increase somewhere else.
Oh, by all means please feel free to use any of my pictures for demonstration/explanation/reference/archive purposes! The only thing I would object to using them for is some sort of fraudulent activity.