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

The ACI 318 Code has not adopted size effect provisions for beams of depths up to 1 m, invoking a recent test in which the strength of such a beam was almost equal to the nominal strength required by the Code and was much larger than the strength obtained after applying the strength reduction factor. This paper criticizes the use of this test to justify ignoring size effect in shear. 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, 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.). These findings indicate the need to introduce the size effect into the ACI 318 Code for all beam sizes.