Analysis of size effect in shear transfer mechanisms and size effect suppression by transversal reinforcement - contributions to NBR 6118

Análise de efeito de escala nos mecanismos complementares de resistência ao cisalhamento e sua supressão por reforço transversal - contribuições para a NBR 6118

Abstract

Size effects are known to be relevant in the shear transfer mechanisms of quasi-brittle materials like concrete. Bažant proposed an asymptotic approximation between plasticity theory and Linear Elastic Fracture Mechanics (LEFM), showing a proportionality of concrete nominal resistance with $d^{-1/2}$, where $d$ is beam depth. Recently, the long-standing shear transfer mechanism expressions of ACI 318:2014 have been updated (ACI 318:2019), with introduction of a size effect factor. In Brazil, recent publications identified non-conservative trends in predictions of ABNT NBR 6118:2014 for larger beam depths; yet, the Brazilian code never considered size effects because they are suppressed by transverse reinforcement. Considering this background, in this manuscript we make a comprehensive analysis of NBR 6118:2014 shear strength predictions using as a reference the papers of ACI-ASCE DatStb 445-D database. The results exhibit strong tendencies in the model error regarding longitudinal reinforcement and effective depth for beams without transversal reinforcement. A two-step analysis is made herein to describe model errors: first, a nonlinear regression for longitudinal reinforcement is made; second, a linear regression is made for size effect. The reliability analysis corroborates that model error may be reduced by introducing size effect and longitudinal reinforcement factors. Next, for beams with transversal reinforcement, smoother tendencies regarding beam depth are noted, indicating a size effect suppression for the beams depths available in the database. However, as the analysis shows that the higher beam depths concentrate most of the results with unconservative model errors, further studies are necessary to accurately describe how transversal reinforcement suppress the size effect.

Keywords:
fracture mechanics; size effect; shear transfer mechanisms; size effect suppression; model error