Does Strength Test Satisfying Code Requirement for Nominal Strength Justify Ignoring Size Effect in Shear?

Abstract

A recent University of Toronto test of a 925 mm (36.4 in.) deep beam without stirrups showed a shear strength Vc that is only slightly below the value Vc = 2."ff~ required by ACI 318-08, and comfortably above the value rjJ x 2."ff~ (where rjJ = 0.75 is the strength reduction factor, and Vc and f~ are in psi). On that basis, and in view of the safety provisions of the Code, it is often thought that the current shear strength provisions for beams up to 0.2 m (8 in.) deep, which neglects the size effect, are safe for beams up to 1 m (40 in.) deep. This is not true, however, for it must be expected that if numerous tests of 1 m (40 in.) deep beams with different shear spans and steel ratios, made of different concretes and under different hygro-thermal conditions, could be carried out, the beam strength would exhibit a similar statistical scatter, with approximately the same coefficient of variation (Co V), as the strength of beams up to 0.2 m (8 in.) deep, for which there are numerous test results in the database. Based on this expected scatter, it is shown that neglecting the size effect for beams up to 1 m (40 in.) deep is likely to increase the expected frequency of failures from approximately 1 in a million to approximately 1 in a thousand when the beam depth increases from 0.2 to 1 m (8 to 40 in.).

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Cite this paper

@inproceedings{ZdenekDoesST, title={Does Strength Test Satisfying Code Requirement for Nominal Strength Justify Ignoring Size Effect in Shear?}, author={Zdenek and Bazant and Qiang Yu} }