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South African Pavement Engineering Manual
Chapter 10: Pavement Design
18 kip = 80 kN
Figure 19. Damage Factors for Various Axle Configurations as a Function of Total Load
4.1.3.2 Load Equivalency of Grouped Axle Load Data
The conversion from a mixed traffic spectrum to an E80 value requires using the load equivalency formula for the
lower and upper limits of each load interval, followed by the calculation of the average load equivalency. Equation
(2) is used to calculate the LEF for each load interval, using the lower and upper limits of the interval.
n n
P P
� l � + � u � (2)
LEF = 80 80
i
2
where LEF i = Load Equivalency Factor (E80) for the load interval i
P l = Lower limit of the axle load interval (kN)
P u = Upper limit of the axle load interval (kN)
80 = Reference axle load (80 kN)
n = Damage exponent
4.1.3.3 Using the Full Traffic Spectrum in Pavement Design
Modern pavement design methods are moving away from using load equivalency and standard axle loads by
incorporating the axle load histogram in the design analysis, using an incremental damage approach. The simplest
form of this utilises Miner’s Law with the current failure criteria, as defined in Section 7.1. A more detailed, and
theoretically correct, approach requires a recursive pavement design method.
To use Miner’s Law with existing failure criteria or transfer functions, the following process is followed for each layer:
• Determine the number of ultimate load repetitions of each axle group or vehicle type that can be
accommodated, denoted N 1 , max , N 2 , max … N j,max . For example, determine the number of fully loaded tandem axle
trucks can be accommodated before the asphalt layer fails in fatigue using the asphalt fatigue transfer function
Section 4: Design Traffic Estimation
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