Rational criteria for the structural design of pavements are emerging from classical theories equated to the observed behavior of real pavements. Pavement behavior is known to be affected by traffic, variations in soil support, and variations of component thicknesses. Considerable attention has been devoted to the mechanistic response of pavements to static and dynamic loads and to the development of theoretical design procedures, which rely, in part, on the computation of certain critical stresses, strains, and(or) deflections in the structure. A computer program (1) for the elastic analysis of multilayered pavement systems has enabled an extensive investigation of the effects of soil support properties, the materials used in the pavement structure, and component thicknesses. In this study, this computer program was used to determine the patterns of stresses, strains, and deflections of the pavement system. In the second portion of the study, attempts have been made to show the relationships between these stresses, strains, and deflections and current and proposed design curves using the fatigue concept (equivalent axleloads – EAL's, or equivalent wheel loads – EWL's).

From the mechanistic point of view, load deflection relationships outwardly portray the composite stiffness or rigidity of pavement systems. Contrary to general impressions, surface deflection is not a discrete, limiting parameter. Stresses and strains in the subgrade soil and in the extreme fibers of the bituminous concrete layers may (do) constitute overriding, fundamental limits. Therefore, thickness design criteria cannot be based directly upon deflection spectra. In other words, two different pavements having equal, 18-kip deflections are not necessarily equal designs unless all accompanying stresses and strains are also equal.

Report Date


Report Number

No. 269

Digital Object Identifier



The opinions, findings, and conclusions in this report are not necessarily those of the Department of Highways or the Bureau of Public Roads.