Replacement of FAARFIELD Tandem Factors with Cumulative Damage Factor Methodology

The United States Federal Aviation Administration (FAA) adopted FAA Rigid and Flexible Iterative Elastic Layered Design (FAARFIELD) as its standard thickness design procedure for airport pavements in September 2009. FAARFIELD includes a layered elastic analysis routine for flexible pavement design and a three-dimensional finite element structural analysis routine for rigid pavement design.

The current FAARFIELD design procedure for flexible pavements accounts for the effect of aircraft gears in tandem as part of the pass-to-coverage (P/C) ratio computation. The result is a two-part P/C ratio consisting of a wander-related factor multiplied by a tandem factor. The tandem factor is computed as a straight-line interpolation between the number of wheels in tandem (for shallow structures) and unity (for deep structures).

The objective of this report is to accompany the source code implementation of replacing the current method using a tandem factor with an alternative calculation, in which the cumulative damage factor (CDF) due to wheels in tandem is computed based on the subgrade linear elastic strain response.

The report contains a comparison of CDFs for flexible pavements under tandem axle gear loads (two dual-gear and three dual-gear configurations), as computed by the current method (FAARFIELD Version 1.4) and by the new method. The report also contains a comparison of CDF computed by the new method with the CDF computed for multiple wheel sets in tandem using the Alizé-Aircraft program, which was developed by the Institut français des sciences et technologies des transports, de l'aménagement et des réseaux - French Institute of Science and Technology for Transport, Spatial Planning, Development and Networks (IFSTTAR) and the French Directorate General for Civil Aviation (DGAC).