AASHTO Direct Structural Capacity Method Error Analysis

Abstract

The AASHTO Direct Structural Capacity method utilizes an approximate composite pavement modulus to predict an effective structural number based on deflection measurements. An analysis of the errors associated with the approximation is presented. The modulus approximation is purported to predict a composite pavement's modulus to within 15% when its base course thickness is less than twice the applied load radius. However, when typical pavement deflections are derived using a multilayer elastic solution and then used to back‐calculate the composite modulus, errors significantly greater than 15% can occur in both the composite pavement modulus and the effective structural number. Engineers who use composite‐modulus‐based procedures should be aware that approximation error may be significant, especially when analyzing thick pavements.