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Structural Design Strategies

In this week’s edition of the RoadReady Newsletter, we discussed how pavement loads are estimated for newly constructed highways. Once these estimates are made, a number of design strategies are used to translate these figures into a pavement design. In this edition of the blog, we’re going to explore the two main categories of structural design methodology, and look at different examples of each.

Empirical Design

Empirical design is based on previous experience with roadway structures. This experience can either be a result of previous roadway designs and their performance in the field, or testing. Tests for structural design are carried out on special test tracks, which are subjected to controlled loads. Pavement response is measured through observation and instrumentation. Empirical methods do not seek to determine the mechanisms behind loadings and roadway failure, but rather develop reliable models of how roadways perform. As a result, these methods are used in situations where there is a great deal of experience with projects in similar conditions. Empirical design methodology is very common in practice. Notable procedures which use this approach are the AASHO Road Test developed in the 1960′s, and the widely used 1993 AASHTO Design equation.

Mechanistic-Empirical Design

Mechanistic-Empirical Design, or M-E Design, attempts to use the theories of mechanics to more wholly understand roadway behavior. Mechanistic-Empirical models use mathematical models to emulate the relationships between loads, stresses, strains, deflections, and damage in a pavement structure. However, as the name infers, there is still an empirical component to these types of models. Experience with previous designs and testing are commonly used to determine the levels of stress and deflection which cause failure. In this way, M-E designs are still grounded in the past experiences of transportation agencies. M-E designs can be based on techniques such as a layered elastic model, which models each layer in the pavement as a homogenous material and computes reactions to loads throughout its depth. In addition, M-E designs can be based on computer based finite element models, which analyze pavements by breaking them up into a large number of distinct smaller elements. Groups such as the National Cooperative Highway Research Program have developed design procedures based on M-E design.

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