Computer Applications to Airport Pavements

LEAF – A New Layered Elastic Computational Program for FAA Pavement Design and Evaluation Procedures

A new computer program implementation of the layered elastic response equations has been written for use in Federal Aviation Administration (FAA) airport pavement design and evaluation computer programs. The program is written in Visual Basic (VB) 6.0 for Microsoft Windows 95, or higher, as a Dynamic Link Library with a defined interface. It can therefore be executed from programs written in other languages compiled for execution under Windows. Major objectives in writing the new program were to improve the efficiency of the computation of linear elastic pavement responses in the LEDFAA thickness design program and to provide a well-documented methodology and implementation suitable for further development when necessary. Wheel loads are modeled as circular loads with constant vertical pressure. Efficiency has been improved by structuring the program loops so that redundant computations are eliminated for multiple aircraft on a multiple-layered structure. This makes the computation time only very weakly dependent on the number of layers and the number of aircraft when all of the structure and aircraft information is passed to the program before execution. A code fragment is presented to illustrate the structure of the program loops. The use of Gauss-Laguerre integration, with offset of the layer origins, and part inversion in the solution of the matrix equations also improve efficiency. The development environment for LEAF was a computer program for backcalculating the layer modulus values of pavement structures represented by linear elastic layers of infinite horizontal extent. The requirements for calling the DLL from an application are illustrated by code excerpts from the backcalculation program.

Author: Gordon F. Hayhoe