Statistical models for resistance of concrete components

The paper summarizes the resistance models for the reliability-based calibration of ACI 318 Code. Resistance is influenced by the three factors: material properties, fabrication (dimensions), and professional (analysis factor). The primary focus of this study is the analysis of material properties based on new material test data provided by the industry in 2004. The considered materials include three categories of concrete, ordinary concrete, high strength concrete, and light weight concrete (cast-in-place and plant-cast), a wide range of reinforcing steel bars, from 9.5 mm through 34.5 mm, as well as prestressing strands. In particular, considerable new data was recently obtained for the most common concrete grades, rebar sizes and prestressing strand diameters. For each type of material, the cumulative distribution functions are developed and plotted on the normal probability paper for an easier interpretation of the results. The basic parameters are the bias factor, or the ratio of mean strength to nominal value, and the coefficient of variation. It was observed that the quality of materials has been improved over the last 30 years and this is reflected in reduced coefficients of variation and increased bias factors. Other factors such as fabrication and professional are also considered and summarized based on the available literature. The developed resistance models served as a basis for the selection of resistance factors (strength reduction factors) for the new edition of ACI 318.