WORLD FOR CIVIL

Structural Numbers

STRUCTURAL NUMBERS FOR FLEXIBLE PAVEMENTS

The design of a flexible pavement or surface treatment expected to carry more than 50,000 repetitions of equiva- lent single 18-kip axle load (SAI) requires identification of a structural number SN that is used as a measure of the ability of the pavement to withstand anticipated axle loads. In the AASHTO design method, the structural number is defined by

SN= SN₁+SN₂+SN₃

where
SN₁ structural number for the surface course=
a₁
D₁

a₁=
layer coefficient for the surface course

D₁=
actual thickness of the surface course, in
(mm)

SN₂= structural number for the base course=a₂ D₂ m₂

a2= layer coefficient for the base course

D2= actual thickness of the base course, in (mm)

m2= drainage coefficient for the base course

SN₃ =structural number for the subbase course=
a₃ D₃ m₃

a3= layer coefficient for the subbase course

D3= actual thickness of the subbase course, in
(mm)

m3= drainage coefficient for the subbase

Based on the above calculations the layer coefficients are assigned to convert structural numbers to actual thickness. Example the layer coefficients may be 0.44 for asphaltic-concrete surface course.