logarithmic curve on a graph with a linear scale on
the vertical axis. In this case, the vertical axis in
Figure 2 is the number of impacting wheels per
year, so if the rate of occurrence of such impacts
over the year of the study is a representative of
impacts over a longer period, then extrapolation of
that sloping dashed line will provide the frequency of
occurrence of impact forces greater than 310kN.
On that basis, it could be predicted that an
impact force of 380kN would occur at the rate of 0.1
axles per year, or once in every 10 years; an impact
of 450kN would occur on average once in every 100
years. This process naturally leads on to the concept
of a return period for impact force, which Murray
and Leong [12] developed to produce equation (2):
Impact Force (kN) = 53(5.8 + log R) (2)
where R is the return period in years of a given level
of impact.