You probably let them grow too big. Don't think you will get delicious tender beans if you let them grow to the same size as you see them sold in the supermarkets where growers let them grow bigger in weight so they can get a better price for their crop. the secret is to pick them when they are no longer than about eight inches long.
If you pick them at this stage they should only need to be topped and tailed as the stringy side of the bean will not yet have developed.