How is the allowable number of axle-load to failure equation actually used in the software? Is there any documentation regarding the calculation of the tensile strains?


The allowable number of axle-load repetitions to failure, $N_f$ is used to calculate the incremental fatigue damage using Miner's law for each analysis interval (which is equal to one month, or two 15-day intervals if there is a freezing cycle in the month). ME pavement design procedure considers the following inputs in the damage calculation process:

  • Truck traffic: Different FHWA truck classes, different axle loads, tire configurations (single, tandem, tridem and quad axles), hourly and monthly adjustment factors, etc.
  • Temperature variation within the pavement structure as a function of time. This is used to calculate asphalt mixture stiffness $E_{HMA}$ which is used to calculate $N_f$.

Pavement ME calculates the horizontal tensile strain at the bottom of AC layer (for bottom-up fatigue cracking cracking) and at the AC surface (for top-down fatigue or longitudinal cracking) using JULEA (linear elastic analysis procedure). Within each damage computation interval, the strains are calculated for each $i^{th}$ group of axle loads (axle passes, $n_i$) at critical locations that are defined separately for different axle types. These strains are then used to calculate allowable load repetitions $N_{f,i}$ for the $i^{th}$ load group, and damage $D_i = n_i / N_{f,i}$. Cumulative damage due to all traffic is the sum of incremental damage caused by each load group in all analysis intervals.


  1. National Cooperative Highway Research Program, “Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures, Part 3, Chapter 4 - Design of New & Reconstructed Flexible Pavements.” NCHRP Project 1-37A Final Report, Washington, D.C. (2004). [Contains definition of critical strain computation locations]
  2. American Association of State Highway and Transportation Officials, “Mechanistic-Empirical Pavement Design Guide - A Manual of Practice”, 2nd edition, Washington D.C. (2015)

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