ABSTRACT
Basic assumptions regarding the design of cracked concrete sections at the ULS are outlined in section 5.2. The tensile strength of the concrete is neglected and strains are evaluated on the assumption that plane sections before bending remain plane after bending. Reinforcement stresses are then derived from these strains on the basis of the design stress-strain curves shown on Table 3.6. The BS 5400 curve is the one that was used in CP 110 prior to that code being superseded by BS 8110. For the concrete stresses, alternative assumptions are permitted. The design stress-strain curve for concrete shown on Table 3.6 gives a stress distribution that is a combination of a parabola and rectangle. The form of the data governing the shape of the curve causes the relative proportions of the two parts to vary as the concrete strength changes. Thus the total compressive force provided by the concrete is not linearly related to f cu , and the position of the centroid of the stressblock changes with f cu.
