Code Validation for Flexural Strength of PSC Members with High-Strength Strand

Abstract

A material model for the newly-developed high-strength strands is proposed, and by using this model, the flexural strength of prestressed concrete (PSC) members are calculated to validate the approximation equation of current code. The material model developed is based on the 95% lower bound of actual stress-strain curves provided by the strand manufacturer. A Modified Ramberg-Osgood model is used to mathematically represent the stress-strain relationship of prestressing strands, and then the material model was evaluated through performing curve fitting processes. The proposed material model allows the theoretical investigation of the flexural strength of PSC members under various conditions without any performance test. Nominal flexural strength is calculated by sectional analysis, based on simultaneous satisfaction of compatibility of strains and equilibrium of forces. In addition, various cross-sectional shapes, effective prestressing force, and area of strands are also taken into consideration as parameters. The values obtained from sectional analysis shows closer agreement with the values obtained from actual flexural behavior of PSC members. Nominal flexural strength, strand stress and strength reduction factor at nominal flexural strength are obtained from the approximate equation in current code, and the validation of the equation code is considered. The results indicate that for the I-type section, the nominal strand stress obtained by approximate equation over-estimated the actual stress of high-strength strands at compression-controlled section. However, the approximate nominal flexural strength derived from approximate value of strand stress, is well matched with the results from sectional analysis, even at compression-controlled section, because the over-estimated strand stress makes the tensile force increased and the moment arm decreased oppositely. The results also indicate that the values obtained from the approximate equation of current code under-estimates the design flexural strength significantly for the transition-sections. Therefore, a strength reduction factor and strain limit of design section need further study, in order to accurately evaluate design flexural strength with current approximate equation for the transition-sections.