Critique of Critical Shear Crack Theory (CSCT) for fib Model Code Articles on Shear Strength and Size Effect of RC Beams

TL;DR

This paper critically examines the Critical Shear Crack Theory (CSCT), demonstrating its fundamental flaws, and compares it with the energetic size effect law, supported by experimental and simulation evidence.

Contribution

It provides a detailed critique of CSCT's derivation, highlights its scientific shortcomings, and compares it with the verified energetic size effect law using simulations and experimental data.

Findings

CSCT closely matches the energetic size effect law asymptotically.

CSCT is based on six scientifically untenable hypotheses.

Finite element simulations support the critique and show incompatibility with modern models.

Abstract

The size effect of Muttoni et al.'s CSCT is shown to be very close and asymptotically identical to the energetic size effect law (SEL) extensively verified experimentally and theoretically (due to appear in the 2019 ACI Code for both beam shear and punching). However, the CSCT derivation and calculation procedure obfuscates the mechanics of failure. It is shown to rest on six scientifically untenable hypotheses, which makes CSCT untrustworthy outside the testing range. It would have to be taught to students as a dogma. These conclusions are supported by experimentally calibrated finite element simulations of crack path and width, stress distributions and localizations during failure, and strain energy release. The simulations also show the CSCT to be incompatible with the 'strut-and-tie' model modernized to include the size effect in the compression strut. Finally, further deficiencies are pointed out for the Modified Compression Field Theory (MCFT), currently embedded in the Model Code.