Calibration of design code for buildings (ACI 318): Part 1 - Statistical models for resistance

This paper summarizes the resistance models for calibration of the ACI 318 Code. The reliability analysis and calculation of the resistance factors are presented in,another paper Resistance is influenced by. three factors: material properties, fabrication (dimensions), and professional (analysis factor). The primary focus of this study is the analysis of material properties based on material test data provided by the industry. The considered materials include: three categories of concrete-ordinary concrete, high-strength concrete, and lightweight concrete (cast-in-place and plant-cast); a wide range of reinforcing steel bars, from 9.5 to 34.5 mm (No. 3 to No. 11); and prestressing strands. 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.