The required analytical method of flexible is FPS 21.

FPS 21 uses a systems approach centered upon deflection measurements from the falling weight deflectometer (FWD). These measurements are then used by the MODULUS computer program to estimate the material properties (modulus) of the pavement layers through a backcalculation process. The material properties of the pavement layers, traffic loading (cumulative ESALs), the change in the serviceability rating, and the desired level of reliability are then used to compute alternative structural designs using FPS 21.

It is mandatory to check the design derived by FPS 21 for subgrade shear strength adequacy using the Modified Texas Triaxial Class (TTC) design method contained within the FPS 21 software.

FPS-19W is the previous design program which has been replaced by FPS 21. FPS 21 has virtually the same input requirements and performance models, and the two programs result in the same solutions when equivalent inputs are used. The was developed jointly between the Texas A&M Transportation Institute and TxDOT and serves as a preliminary reference to the essential operational aspects of the program (TTI Research Report 1869-2 contains information on the models embedded in both FPS-19W and FPS 21). Additional guidance pertaining to recommended inputs and functionality is given in this section.