Load equivalency concepts. A mechanistic reappraisal

The history and evolution of load equivalency concepts are traced and discussed, in view of the fact that accommodation of mixed traffic consisting of multiwheel load assemblies is of cardinal importance in any pavement design. It is explained that the equivalent single-axle load concept is considerably different from the equivalent single-wheel load (ESWL) approach, inasmuch as the former is statistical-empirical and is based on the assumption of linear pavement damage accumulation, whereas the latter is soundly mechanistic, if relatively crude, and is merely an innovative means of computing responses under multiwheel gears. A third concept, namely, the equivalent single-axle radius (ESAR), is also mechanistic in nature and dispenses with the arbitrary constant pressure of constant radius assumptions of the ESWL approach. According to the ESAR concept, it is possible to determine with reasonable accuracy a primary structural response, such as the maximum bending stress that occurs in a concrete pavement system, through the use of available closed-form equations for a single-wheel load, into which an equivalent single-wheel radius of a multiple-wheel assembly is substituted. The ESAR concept is currently considered for incorporation in an improved mechanistic 'limit state' design procedure for concrete pavements. Because of the far-reaching implications of these proposals, rigorous engineering mechanics derivations are used to verify the applicability of the concept for both the dense liquid and the elastic solid foundation.