Flexural Strength of Prestressed Concrete Members

T he flexural strength theory of prestressed concrete members is well established. The assumptions of equivalent rectangular stress block and plane sections remaining plane after loading are commonly accepted. However, the flexural strength analysis of prestressed concrete sections is more complicated than for sections reinforced with mild bars because high strength prestressing steel does not exhibit a yield stress plateau, and thus cannot be modeled as an elasto-plastic material. In 1979, Mattock' presented a procedure for calculating the flexural strength of prestressed concrete sections on an HP-67/97 programmable calculator. His procedure consisted of the theoretically exact "strain compatibility" method and a power formula for modeling the stress-strain curve of prestressing steel. This power formula was originally reported in Ref. 2 and is capable of modeling actual stress-strain curves for all types of steel to within 1 percent. Prior to Mattock's paper, the strain compatibility method commonly required designers to use a graphical solution for the steel stress at a given strain. There are computer programs for strain compatibility analysis (see for example Refs. 3 and 4). However, these programs were developed on main