Challenge 1

In Hognestad method concrete takes up a parabolic curve and then fails at the point where the strain is apporximately 0.003. But as per IS 456 the stress-strain curve takes a parabolic curve path and then proceeds to a linear path from the point where the strain takes up a value of 0.02. While this is the curve given in the book it also expects us to take for designing purpose a lesser value whereby we are not at the limit of the strain and hence gives us the value of 0.67*fck