Page 42 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

Equivalent Standard Axle Load Calculation: Example
A 100 kN axle load is equivalent to 2.4 single axle loads:
4
[100/80] = 2.4

Axle load (P)

=

4.1.3.1 Effect of Axle Configuration on a Standard Axle
The effect of tandem, tridem and other axle configurations on the LEF is generally not considered in South Africa.
The effect can be evaluated using Figure 19 (Havens et al, 1981). The standard 18 kips (80 kN) axle load applied as
a single axle has a damage factor of 1. Therefore, when six 18 kips axle loads are applied (108 kips total) as single
axles, the number of equivalent axle loads is 6 x 1 = 6. However, when the 108 kips are applied through a group of
six axles on a truck, the damage factor is 0.85 and the number of equivalent axles thus 6 x 0.85 = 5.1. It follows
that the number of equivalent axles are overestimated by 15 percent when the effect of multi-axles is not
considered. This overestimation is not considered significant, especially considering the wide limits of the design
traffic classes, especially at the higher end of the load spectrum.

Equivalent number
Frequency of standard axles

80
0 to 10 10 to 20 20 to 40 40 to 60 60 to 80 80 to 100 Standard axle (kN)

Axle load interval (kN)
Actual mixed traffic load spectrum Equivalent design traffic

Figure 18. Representative Axle Load Histogram for Mixed Traffic by Equivalent Standard
Axle Load Histogram

Section 4: Design Traffic Estimation
Page 31