Evaluation of the characteristic values of mechanical properties in five tropical wood species using different statistical methods

ABSTRACT Brazilian Code NBR 7190:2022 for timber structures design adopts a simplified approach for determining characteristic values from small samples, based on the Centered Estimator Method assuming data normality. However, there is a gap in literature regarding its accuracy compared to the widely used approach in interna-tional codes, which defines the characteristic value as the 5th percentile of a fitted statistical distribution. This study evaluated strength and stiffness properties in tension and compression parallel to grain of five Brazilian tropical species, comparing the characteristic values obtained by NBR 7190-3:2022 with those derived from the 5th percentile of normal, lognormal, two-parameter Weibull, three-parameter Weibull, Gamma, and Logistic distributions. Results indicated that the normal distribution was adequate in most cases, while three-parameter Weibull distribution demonstrated greater flexibility in fitting, in capturing skewness in strength/stiffness distributions but also resulted in less conservative characteristic values. Additionally, it was observed that NBR 7190-3:2022 method led to characteristic values of mechanical properties, on average, 7% to 29% higher than those obtained from the 5th percentile of the analyzed statistical distributions. These results suggest the need to revise the Centered Estimator Method in NBR 7190-3:2022, to improve estimating characteristic value reliability.