Load Distribution Factor:
The formula for deriving at the number of repetitions of the standard axle load during a design period of n years is:
N =365 × A × VDF × F [(1 + r )n - 1]/r
where A = Number of commercial vehicles per day initially (both directions),
VDF = Vehicle Damage Factor,
F = Load Distribution Factor,
r = Annual rate of growth of traffic (in decimals), and
n = Design period in years.
Thus, if a two lane road has a present commercial traffic of 4000 veh/day, and the design period is 15 years, the rate of growth of traffic is 7.5%, and the VDF is 3.5, the number of repetitions of standard axle loads during the design Period
N =365 × A × VDF × F [(1 + r )n - 1]/r
A = 4000
VDF = 3.5
F = 0.75 (Table 4.3)
r = 7.5% = 0.075
n = 15
Substituting, N = 365 × 4000 × .35 × 0.75 [1.07515 - 1]/0.075
= 365 × 4000 × 3.5 × 0.75 [2.96 - 1]/0.075
= 100 million
The million standard axles of repetitions is denoted by "msa".