1  Introduction

The goal of structural design is to determine the number, material composition and thickness of the different layers within a pavement structure required to accommodate a given loading regime.  This includes the surface course as well as any underlying base or subbase layers.  This module is focused on the structural design of new pavement.  Structural design for rehabilitation is covered in Module 10, Section 3 (flexible pavements) and Section 5 (rigid pavements).

Design Life
Pavements are typically designed for a specified "design life".  Design life (or "design period") is the time from original construction to a terminal condition for a pavement structure.  Structural design is carried out so that the pavement structure is sufficient to withstand the traffic loading encountered over the pavement's design life.  It is recognized that intermittent maintenance and rehabilitation efforts may be needed to preserve a pavement's surface quality and ensure that the structure lasts through the design life.

For flexible pavements, structural design is mainly concerned with determining appropriate layer thickness and composition.  Calculations are chiefly concerned with traffic loading stresses; other environmentally related stresses (such as temperature) are accounted for in mix design asphalt binder selection.  The two principal methods of flexible pavement structural design in use today, empirical and mechanistic-empirical, are covered. 

For rigid pavements, structural design is mainly concerned with determining the appropriate slab thickness based on traffic loads and underlying material properties, and joint design.  This is done by considering a variety of stresses which affect rigid pavement performance: curling (temperature stresses), warping (moisture stresses), wheel load and shrinkage/expansion.  The two principal methods of rigid pavement structural design in use today, empirical and mechanistic-empirical, are covered. 

Overall, this section is only meant to provide a brief overview of the different structural design techniques as well as their assumptions, inputs and outputs.  Detailed analysis of the design methods presented here can be found in:

• Empirical Method: The 1993 AASHTO Guide for Design of Pavement Structures. 
• Mechanistic Method: The 2002 AASHTO Guide for Design of Pavement Structures, the WSDOT Pavement Guide (1998), and other state design procedures.