It is a well known fact that the (simple) Engineers’ Theory of Bending is based on the assumption of the Law of Proportionality, that is to say that the stress-strain characteristic of the material concerned is a straight line, and it accordingly follows that stress values determined by the formulæ: , for the stress at any station on the section, and M/Z for the maximum fibre stress can be correct only when the M/Z value lies within the limit of proportionality of the material.